TOPIC 12.

THE ELEMENTS - the Periodic Table.

For millennia, humans have been discovering and extracting elements from nature and

using them either in their elemental state or in combination with others as compounds.

This process has accelerated over the past few centuries and in today’s highly technical

environment we are dependent on a continued supply of many elements, the names of

some of which were rarely mentioned or even recognised by chemists just a few

decades ago. Without an assured supply, much of the technology that is now widely

taken for granted would no longer be viable and potential future developments will be

hampered or rendered impossible. As an example, note how dependent current

technology has become on the supply of the so-called rare earth elements which

underpin many of the advances made in computing, communications and the many

applications in which powerful rare earth magnets are the basis. Other less exotic

elements are even more important for today’s living standards - consider how

dependent agriculture is on a continued supply of phosphorous in the form of

phosphate fertilisers, a supply which may be fated to be fully depleted in the future. In

these notes, a selection of elements which are among those that are essential

components of technology today are discussed in the context of the Periodic Table.

This system of classification of the elements is not only a convenient summary of the

chemistry associated with various families of elements in which the Table’s origin lies,

but in its modern form the Periodic Table provides the scaffold underlying the

electronic structures of the atoms and upon which their various properties and

reactions depend.

The following are a few illustrative examples to ponder concerning the elements which

are discussed in this Topic.

Nitrogen molecules in the atmosphere contain one of the most stable bonds yet about

half the nitrogen atoms in our bodies were extracted artificially from the air.

Salt is destroying vast areas of agricultural land in Australia and has undesirable

consequences if consumed in excess in our diet yet is a valuable source of essential

products.

The air surrounding us contains a large proportion of a highly corrosive gas that

originated as the most polluting ever and which reacts with almost all other elements

yet is indispensable to all animal life - oxygen.

The rechargeable batteries which power mobile phones and computers are based on

the extremely small size of the lithium ion.

XII - 1

XII - 2

Many nuclear diagnostic procedures rely on using artificially produced atoms that

originate from nuclear reactors such as that at Lucas Heights in Sydney.

An irreplaceable gas with vital applications in magnetic resonance imaging machines

is used and lost - filling party balloons!

An object made from some metals retains a memory of its initial shape and if distorted,

it will return to the original shape when heated.

Origin of the elements.

As discussed in Topic 1, there are 90 naturally occurring elements. In addition, there

are about 28 other elements which have been produced synthetically but some of these

exist only as very short-lived radioactive species which have been produced in

extremely small quantities using high energy particle accelerators. How did the 90

naturally occurring elements originate? The most widely accepted version of the

origin of all matter in the universe is the BIG BANG THEORY which proposes that

in an instant, all of space, energy and matter which had been confined to a volume the

size of a grain of sand at an infinitely high temperature underwent an explosive

expansion. Within 10 minutes of the big bang, nuclei of mostly hydrogen and helium

were formed from more basic particles and over the next million years these nuclei

cooled enough to capture electrons and form atoms. After about a billion years, the

gravitational attraction between atoms which were still mostly hydrogen and helium,

lead to clumps of matter which by gravitational attraction gradually increased in size.

With increasing size, the temperature of these clumps also increased and in some

regions of space the larger clumps became hot enough to initiate fusion reactions

between nuclei, forming stars which are in effect giant nuclear fusion reactors. Our

own sun, like all stars, converts hydrogen to helium with the concurrent release of

extremely large amounts of energy known as the BINDING ENERGY associated with

the strong nuclear force which was discussed briefly in Topic 2. Within a few billion

years vast numbers of new stars were formed and these in turn, through gravitational

attraction, clustered to create galaxies, each of which contains enormous numbers of

stars. By that point in time, the universe would have looked much as it does today. As

the hydrogen fuelling the fusion reactions in a star is consumed, other fusion reactions

can occur in which heavier elements form. Fusion reactions leading to new atoms of

elements as heavy as iron all release energy and can continue to fuel a star. Ultimately,

when a star has consumed most of its available fuel, it may simply cool and dim or in

some cases it may initially implode and then undergo an enormous explosion which

flings much of its constituent material and energy out into space in what is called a

SUPERNOVA EVENT. It is only during supernovae that elements heavier than iron

are formed. It is estimated that the present elemental composition of the universe is

92.7% hydrogen atoms, 7.2% helium atoms and just 0.1% atoms of all the other

elements. The shock waves of energy and material sent into space from supernovae

XII - 3

may interact with existing clouds of gas, ice and dust to eventually form new stars and

planets such as our solar system. Our sun was not one of the original stars in the

universe but is probably a second or third generation star, formed in part from the

energy and residues released by previous supernovae. Although already 5 billion

years old, it still contains 71% hydrogen and 27% helium, so the sun will burn for

several billion more years.

Thus on the basis of this theory, the elements which constitute all matter on earth apart

from hydrogen were originally formed from stars that existed before our sun and which

had consumed all their available hydrogen, converted it to helium and progressed to

other nuclear fusion reactions that created heavier elements and finally underwent a

supernova explosion in which more of the heavier elements were produced.

Discovery and isolation of the elements.

Most of the non-gaseous elements on earth are chemically combined with other

elements as compounds. Few non-gaseous elements are found in the free state. For

thousands of years gold, silver, copper, sulfur and carbon had been known because

they do occur in the free form, although they were not necessarily recognised as

elements - indeed the concept of an element as we know it today was not firmly

established until the 18th century through the visionary work of Lavoisier. While

some metals such as gold and silver are so unreactive that they can be found as free

elements, most elements occur as compounds in MINERALS. Extraction of

metals such as copper and tin from their ores by the process of SMELTING was

probably accidentally discovered when minerals were used as fireplaces. The

extraction process relied upon the use of bellows made from animal hides to

increase the heat obtained from a fire to the point where decomposition could occur

of rocks containing for example copper (malachite, a copper carbonate compound)

and tin (cassiterite, an oxide of tin). Charcoal in the fire reduced the copper and tin

compounds to the free elements. Later it was discovered that mixing about 9 parts

of copper and 1 part of tin together and melting them produced an alloy called

BRONZE which is much harder than either of its constituent elements, a feature

exploited in the bronze age from about 5000 years ago. Later, iron was isolated

from its ores by similar means - probably more than 3000 years ago. Prior to 1600,

the elements gold, silver, carbon, sulfur, copper, tin, lead, iron and mercury had

been discovered by persons unknown. However arsenic, isolated by Albertus

Magnus in 1250, is the first recorded instance of an attributed method of isolation

of an element.

The prehistoric technical advance of using crude furnaces to smelt ores set the

pattern for future discoveries of elements which resulted from newly devised

processes. The rate of isolation of the elements was a process that tended to occur in

steps where development of a new method or technology gave impetus to a flurry of

XII - 4

discoveries, often followed by a quieter period prior to another new method or

technology being devised. This is shown in the chart on page XII-5 which plots the

number of known elements against dates of discovery based on the data given in the

Tables on Pages XII-4 and 5. In the Table on Page XII-6, the dates of discovery of

those elements known in prehistoric times are simply listed as <1600 while in the chart

the prehistoric discoveries have been placed undated prior to 1600 along the axis.

Prior to 1600, the driver of isolation of elements such as copper, tin, lead and mercury

was augmentation of the use of fire with bellows to create a furnace. Isolation of

elements in the 18th century became increasingly rapid, boosted by several new

technologies. Methods for analysing minerals reached a high state of development,

particularly as a result of the advent of the blowpipe. This tool allows the ready

decomposition of minerals on a carbon block, a prerequisite for analysis of their

components. Elements discovered through the improved analytical techniques during

the 18th century were cobalt, bismuth, platinum, zinc, nickel, manganese, molybdenum,

tellurium, tungsten and chromium.

However, the greatest advance in new techniques during the 18th century was the

development of apparatus designed to handle gases. This not only allowed the isolation

of the elements hydrogen, nitrogen, oxygen and chlorine as well as gaseous compounds

such as carbon dioxide, but lead to our current understanding of the nature of chemical

processes and to fundamental laws such as the law of conservation of matter.

The development of the battery and its application to isolating elements via the method

of electrolysis was exploited, particularly by Humphry Davy, in the early years of the

19th century. He used large batteries made from copper and zinc to electrolyse molten

salts of elements from Groups 1 and 2, isolating potassium, sodium, calcium and

barium in rapid succession in 1807 - 1808.

Throughout the 19th century methods for analysing minerals continued to develop.

These methods included the use of the blowpipe to obtain high temperatures in

conjunction with acid digestion of minerals, selective precipitation of salts, gravimetric

analysis and fractional crystallization allowing separation of pure salts from mixtures.

Along with improved analytical techniques, the development of two new pieces of

apparatus, the Bunsen burner and the spectroscope by Bunsen and Kirchoff

respectively lead to discovery of elements initially by their atomic emission spectra.

The Bunsen burner allowed high enough temperatures to produce atomic emission

spectra from salts and the spectroscope allowed the unique pattern of spectral lines of

each element to be observed and recorded. In 1860 they discovered the element

caesium initially through its spectral lines which they found in samples extracted from

minerals containing other Group 1 elements. In 1878 holmium and in 1879 samarium

were first discovered by the presence of their spectral lines in extracts from ores. In

XII - 5

1868 Lockyer observed previously unknown spectral lines in sunlight and he attributed

them to an unknown element which he called helium. Subsequently in the 1890's it

was found that helium is present in our atmosphere along with the other noble gases

neon, argon, krypton and xenon. Their discovery was in part prompted by Ramsay’s

observation that there was room for gaseous elements at the end of each Period of the

Table. Removing all the known gases from air left a small, unreactive component

which was called argon.

However, it required yet another technological advance before neon, krypton and

xenon were isolated. This time the new technology was the ability to liquefy air.

Fractional distillation of liquid air after gases such as oxygen, nitrogen, argon and

carbon dioxide had been removed, left a small fraction which showed an unknown

spectrum - this element was krypton. Careful fractional distillation of this remaining

material revealed the presence of yet another gaseous element with an unknown

spectrum - neon. Xenon was finally isolated by repeated fractionation of liquid

krypton, again it was determined that they had obtained a new gaseous element by

observing its spectrum. Thus a completely new family of hitherto unsuspected

elements, the noble gases had been discovered as a result of combining spectroscopy

and fractional distillation of liquid air.

The development of sensitive apparatus for measuring radioactivity by the Curies was

central to their isolation of the elements polonium (1898) and radium (detected in 1898

but not isolated until 1903).

The chart shows a long gap until another rise in the rate of isolation of elements around

1940 when nuclear reactors were first invented and elements such as plutonium and

americium were isolated for the first time. More recently, the development of high

energy particle accelerators has been applied to production of synthetically produced

elements, generally in microscopic quantities.

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Isolation of the elements - chronological listing.

Prehistory Au, Ag, C, S, Cu, Sn, Pb, Fe, Sb

1250 As (but used earlier)

1669 P

1735 Co, Pt

1746 Zn (but used earlier)

1751 Ni

1753 Bi (but used earlier)

1755 Mg

1766 H

1772 O, N

1774 Mn, Cl

1778 Mo

1781 W

1782 Te

1789 U, Zr, Be

1790 Sr

1791 Ti

1794 Y

1797 Cr

1801 Nb

1802 Ta

1803 Os, Pd, Ce, Rh, Ir

1807 Na, K

1808 Ba, B, Ca

1811 I

1817 Li, Se, Cd

1823 Al, Si

1826 Br

1828 Th

1830 V

1839 La

1843 Tb, Nd, Er

1844 Ru

1860 Cs

1861 Tl, Rb

1863 In

1866 F

1875 Ga

1878 Yb, Ho

1879 Sm, Sc, Tm

1880 Gd

1885 Pr

1886 Dy, Ge

1894 Ar

1895 He

1896 Eu

1898 Po, Ra, Kr, Ne, Xe,

1899 Ac

1900 Rn

1907 Lu

1917 Pa

1923 Hf

1925 Re

1937 Tc

1939 Fr

1940 Np, Pu, At

1944 Cm

1945 Am

1947 Pm

1950 Bk, Cf

1952 Es

1953 Fm

1955 Md

1958 No

1961 Lr

1965 Rf

1970 Db

1974 Sg

1976 Bh

1982 Mt

1984 Hs

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XII - 8

DATES OF DISCOVERY OF THE ELEMENTS

1

H1766

2

He1895

3

Li1817

4

Be1798

5

B1808

6

C<1600

7

N1772

8

O1772

9

F1866

10

Ne1898

11

Na1807

12

Mg1755

13

Al1823

14

Si1823

15

P1669

16

S<1600

17

Cl1774

18

Ar1894

19

K1807

20

Ca1808

21

Sc1879

22

Ti1791

23

V1830

24

Cr1797

25

Mn1774

26

Fe<1600

27

Co1735

28

Ni1751

29

Cu<1600

30

Zn1746

31

Ga1875

32

Ge1886

33

As1250

34

Se1817

35

Br1826

36

Kr1898

37

Rb1861

38

Sr1790

39

Y1794

40

Zr1789

41

Nb1801

42

Mo1778

43

Tc1937

44

Ru1844

45

Rh1803

46

Pd1803

47

Ag<1600

48

Cd1817

49

In1863

50

Sn<1600

51

Sb<1600

52

Te1782

53

I1811

54

Xe1898

55

Cs

56

Ba

57-71 72

Hf

73

Ta

74

W

75

Re

76

Os77

Ir

78

Pt

79

Au

80

Hg81

Tl

82

Pb

83

Bi

84

Po

85

At

86

Rn

87

Fr1939

88

Ra1898

89-103 104

Rf1965

105

Db1970

106

Sg1974

107

Bh1976

108

Hs1984

109

Mt1982

LANTHANIDES

57

La

58

Ce

59

Pr

60

Nd

61

Pm

62

Sm

63

Eu64

Gd65

Tb

66

Dy

67

Ho

68

Er

69

Tm

70

Yb

71

Lu

ACTINIDES

89

Ac

1899

90

Th

1828

91

Pa

1917

92

U

1789

93

Np

1940

94

Pu

1940

95

Am

1945

96

Cm

1944

97

Bk

1950

98

Cf

1950

99

Es

1952

100

Fm

1953

101

Md

1955

102

No1958

103

Lr

1961

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The Periodic Table

As each element has its own characteristic properties, this implies that one would need

to be familiar with more than 100 different sets of chemical properties in order to

understand the chemistry of all the elements. However the elements actually consist of

families or groups, each of which contains a number of elements that all share many

similar properties. Thus by knowing the general properties of each group of elements,

the task is made much easier. Further, recognition of the existence of these groups has

led to an understanding of why various properties are associated with each of them.

The arrangement of the elements as chemical groups constitutes the PERIODIC

TABLE, one of the fundamental cornerstones of chemistry which not only embodies

the outward properties of elements, but also incorporates the inner atomic structure of

their atoms.

Development of the Periodic Table.

The earliest suggested grouping of elements was simply on the basis of very obvious

properties such as being shiny or malleable (classed as METALS) or not (classed as

NON-METALS). Metals were further grouped as COINAGE METALS (silver,

gold, copper) or as REACTIVE METALS. From this simple beginning, the modern

Periodic Table evolved.

One of the first classifications into families was by Dobereiner (1829) who noted that

there were often groups of three elements which shared similar properties, e.g.

Ca, Sr, Ba reactive metals

Li, Na, K very reactive soft metals

S, Se, Te foul smelling hydrides

Cl, Br, I highly corrosive non-metals

Fe, Co, Mn hard metals, coloured salts

In each case, the atomic weight of the middle member was approximately the

arithmetic mean of the other two. As chemical knowledge increased along with the

number of elements isolated, other bases for classification became possible. One basis

tried was to arrange the elements in order of increasing atomic weight. Newlands

(1864) observed that the chemical properties seemed to be repeated every 8 elements

when this order was used, leading to his law of octaves: "the span between repetitions

in chemically similar species is an octave". While Newlands's classification appeared

valid for the lower atomic weight elements, at higher atomic weights there were many

obvious absurdities with elements of very disparate properties being classed as a

family. This problem arose because many elements had not yet been isolated in 1864,

and no spaces had been left for them in his classification. Dobereiner’s suggestion of

chemical families being related to atomic weight apparently inspired Mendeleev in

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1868 to assemble a small fragment of a Periodic Table, based on more accurate atomic

weights and chemical properties.

element atomic

weight

element atomic

weight

element atomic

weight

Cl 35.5 K 39 Ca 40

Br 80 Rb 85 Sr 88

I 127 Cs 133 Ba 137

In 1869 Mendeleev proposed a classification of the then known 65 elements which

placed priority on allocating elements to each family on the basis of similar properties

with special emphasis on valence rather than atomic weight alone. He left blanks in

families where discrepancies would otherwise appear and repositioned some elements,

disregarding their accepted atomic weights and/or valencies in recognition that these

could be in error. In 1871 he produced an updated version of the Table which included

then unknown elements to which he gave names such as eka-aluminium (following

aluminium: now gallium) and eka-silicon (following silicon: now germanium). Using

this Table he predicted the properties of the missing elements based on the

corresponding properties of the preceding and following element in the Group. His

predictions were remarkably accurate when the missing elements were ultimately

isolated.

The Periodic Law proposed by Mendeleev states: “When arranged by atomic mass, the

elements exhibit a periodic recurrence of similar properties”.

The modern Periodic Table.

With the discovery of the sub-atomic particles and the subsequent knowledge of the

structure of atoms, the fundamental basis for the periodic classification was realised to

be arrangement in order of increasing atomic number (the number of protons in the

nucleus), rather than atomic weight (Moseley, 1913). The difference from atomic

weight order is due to the various isotopes that contribute to the atomic weight (the

weighted average of all naturally occurring isotopes of that element) so that an element

which exists as an abundant heavy isotope but a lower atomic number would be out of

order and appear in the wrong Group. There are only three instances where atomic

weight order departs from atomic number order arising from the proportions of the

different isotopes of each element. Atomic number is the number of protons in the

atom’s nucleus and as atoms are electrically neutral, is numerically the same as the

number of electrons in the atom. As the arrangement of electrons around the nucleus

depends on how many electrons are present, then it is the electronic structure of the

atom which determines the properties of each element. Thus what began as a

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classification solely on the basis of properties of elements was found to be a

classification based on the atomic structure of the elements. A copy of the modern

Periodic Table is given on the last page of this book.

The Periodic Table in Review.

(i) The periods.

Each horizontal row or PERIOD of the Table starts with a Group 1 element and ends

with a Group 18 element. [Note: There are several different Group numbering systems

in use. One uses Roman numerals and extends from Group I to VII plus 0 or VIII for

the noble gases while the current IUPAC system uses normal numbers and extends

from 1 to 18. The IUPAC system is used in this Topic.] In between, there are various

numbers of elements as follows:

1st period H, He 2 elements

2nd period Li v Ne 8 elements

3rd period Na v Ar 8 elements

4th period K v Kr 18 elements

5th period Rb v Xe 18 elements

6th period Cs v Rn 32 elements

7th period Fr v see “The Search Continues” on Page XII-20.

Elements in each period do not constitute a family and there is no value in specifically

committing to memory the members of each Period of the Table. Indeed, across each

Period there is a steady change in properties from those elements classed as metals (on

the left hand side) to those classed as non-metals (on the right hand side). Properties of

metals compared with non-metals have been mentioned in previous Topics, but the

following extended summary of physical and chemical properties of metals compared

with non-metals is appropriate at this point.

METALS NON-METALS

Good conductors of heat and electricity Poor conductors

Malleable, ductile Brittle, often powders or gases

Shiny appearance when freshly cut Dull appearance

React with non-metals to form cations in salts Form anions in salts

Dissolve in acids to form cations Not soluble in acids

Ionic halides Covalent halides

Have oxides which are ionic and dissolve Have covalent oxides which are

in acids to form salts (basic oxides) insoluble in acids but may

dissolve in bases to form salts

(acidic oxides).

The basis for the above properties and why they are associated with a metal or a non-

metal are now well understood in terms of the structure of each element’s atoms.

XII - 12

Variation of atomic properties across Periods

The sizes of the atoms (as measured by the atomic radius) decreases from left to right

across any Period of the table. [See diagram on page XII-10] This may seem strange at

first, given that additional electrons are being added to the structure of the atom as the

Group number increases. The explanation lies in the fact that as the atomic number

increases for each extra electron added to the structure, there is also an extra proton

present in the nucleus. For a given Period, the added electrons are all in the same orbit

and so all outer electrons experience the additional attraction of the increased number

of protons, leading to reduced atomic radius. At the end of each Period, the last

element is a noble gas after which there is no more room in the electron orbit that is

being filled to accommodate further electrons. The next element (a Group 1 element)

is the first in the following Period and its added electron must occupy a higher orbit,

further out from the nucleus. Each time a new orbit is occupied, the electron allocated

to it is partially SCREENED from the attraction of the nucleus and so its atomic

radius increases to become the largest for that next Period. Moving across this next

Period, the same process is repeated with atomic radius decreasing from left to right,

becoming smallest for the next noble gas, then again increasing for the subsequent

Group 1 atom as electrons occupy the next highest orbit, and so on throughout the

Table.

Consequently if one examines the attraction felt by outer electrons of the atoms from

left to right across any Period, it increases until reaching a maximum for the noble gas

element at the end of that Period. This can be expressed as the charge actually felt by

the outer electrons called the EFFECTIVE NUCLEAR CHARGE Sometimes this is

called the CORE CHARGE. Noble gases are unreactive because their outer electrons

experience a very large effective nuclear charge and too much energy is required for

them to be removed in a chemical reaction. Thesingle outer electron of atoms of Group

1 elements experiences a much smaller effective nuclear charge due to the screening of

that electron by all the other electrons and so only a relatively small amount of energy

is required to convert Group 1 atoms to 1+ ions. Having lost an electron to form the

1+ cation and thus become isoelectronic with a noble gas, a very large amount of

energy would then be needed to remove a second electron - the high effective nuclear

charge of the noble gas structure is reinforced by the excess 1+ charge on the ion - and

so a 2+ ion does not form for Group 1 elements.

For Group 2 elements, it is not until after two electrons have been removed that the

noble gas structure is attained with its large effective nuclear charge. While it does

require more energy to remove the second electron from a Group 2 atom than was

required to remove its first electron, stable 2+ ions still result for Group 2 elements

(apart from Be2+ due to the small radius of the Be atom).

XII - 13

XII - 14

Similarly stable 3+ ions are the norm for most elements in the Group starting with

boron, but not boron itself due to its extremely small atomic radius. From the Group

starting with carbon onwards across a Period, the large excess positive charge that

would result on the nucleus prevents stable cations forming.

The atoms of the Group just prior to the noble gas at the end of each Period (the

halogens) have an effective nuclear charge which is only slightly less than that of the

adjacent noble gas and they have room for one more electron in that outer energy level.

Consequently these atoms are readily able to form 1! anions by gaining an electron

and actually release some energy in the process. Hence all the halogen atoms are

readily converted to the corresponding halide ions carrying a 1! charge. However, in

order to gain a second electron and form a 2! ion, that second electron would have to

occupy the next outer atomic orbit and thus be screened from the nuclear charge by all

the other electrons, so reducing the effective nuclear charge it would experience. To

achieve this would require the input of too much energy to form a stable anion and so

no 2! ions of halogens exist.

Similarly, outer electrons of the atoms of the Group starting with oxygen which are all

just 2 electrons short of the structure of the noble gas at the end of the Period are

subject to a high effective nuclear charge and can form anions with a 2! charge, but

formation of a 3! ion is energetically too difficult. Thus it can be seen that the

transition from metal to non-metal from left to right across a period can in part be

attributed to the accompanying increase in effective nuclear charge of the atoms.

(ii) The Groups.

Each of the vertical columns of the Table headed Groups 1 through to 18 constitutes a

chemical family of elements. Each family has many chemical properties in common.

(You will recognise eight of these as the groups that you committed to memory at the

beginning of this course.) The chemical similarities within each Group are attributable

to there being the same number of electrons in the outer level of the atoms of elements

in that Group. For example, atoms of all Group 1 elements have 1 electron in their

outer level; all Group 2 atoms have 2 electrons in the outer level and so on. This is the

reason for there being common valencies within any given Group, a property to which

Mendeleev gave priority in devising his version of the Table.

While the elements of each Group have many properties in common, there are also

differences in properties within the Group. These differences typically are exhibited as

a trend from the first to the last element within each Group. The most notable is that

metallic properties (which are associated with the ease of removal of outer electrons)

increase down the Group. This is because the outer electrons are partially screened

from the nuclear charge by the inner electrons and thus attracted less strongly to it and

XII - 15

so are located at an increasing distance from the nucleus. The larger the atomic radius,

the weaker the outer electrons are held.

Probably the most striking instance of this transition is to be seen in the elements of

Group 15. At the top of this Group, the element nitrogen is totally non-metallic - it is a

gas, exists as covalently bonded N2 molecules and forms an anion with a 3! charge in

salts. The element bismuth at the bottom of Group 15 is a solid with metallic bonding

between atoms and forms 3+ cations in salts.

Another good example of how increasing atomic radius leads to greater reactivity as a

consequence of the outer electrons being further from the attraction of the nucleus is to

be found in the reactions of Group 1 elements with water to form hydrogen gas and

hydroxide ions. The element at the top of this Group, lithium, only reacts mildly with

water. The element at the bottom, caesium, reacts violently. The elements in between

become increasingly reactive to water down the Group.

The d-block and f-block elements.

In addition, there is a large block of elements, called the d-block, located towards the

middle of the table containing Groups 3 to 12. These elements are often called the

transition elements. You will also recognise some of these elements as those in the last

section of Table 2 from Topic 1.

Finally, there is another block of elements called the f-block located near the bottom of

the Periodic Table.

Properties of the Groups

The following notes provide an outline of the more important chemical properties of

each of the main Groups and also very briefly discusses the d-block and f-block

elements. Also a range of the elements are discussed in more detail with emphasis on

those properties which have significant useful applications.

GROUP 1. Li, Na, K, Rb, Cs, (Fr)

Group Overview.

Group 1 elements are known as the alkali metals because they react with water to form

hydroxides which are all soluble in water. They are all soft, very reactive metals

which can be cut with a knife and tarnish rapidly so must be kept under oil to

protect them from reacting with air. They all react with water to form hydrogen gas

and hydroxide ions, the reaction becoming increasingly violent down the Group.

This is a redox reaction in which the metal is oxidized and hydrogen atoms in water are

reduced.

2Na + 2H2O v 2Na+ + 2OH! + H2(g)

XII - 16

They form ionic compounds called "salts" with non-metals. For example, sodium and

chlorine combine in another redox reaction to form sodium chloride, a compound

known as common salt or table salt. The elements in such compounds are no longer

made up of electrically neutral atoms of free element, but instead are present as species

called ions - atoms which have gained or lost electrons so as to have an electrical

charge. When a salt forms, the Group 1 metal atoms have each had one electron

removed, leaving them as +1 charged cations. As all Group 1 elements have a single

outer electron, they all form only the M+ ion in their compounds because this

arrangement leaves the ions with the noble gas electron structure. In all compounds of

Group 1 elements, each atom shows a combining power or valence of 1 only. To form

cations with a 2+ or higher charge would require too much energy. The physical and

chemical properties of Group 1 elements can be clearly related to this aspect of their

atomic structure. The lone outer electron of Group 1 elements in the solid state can

move from atom to atom easily when an electrical voltage is applied, causing these

elements to be good conductors. As there is only the one outer electron available to

bind each atom of the metal to its neighbours in the solid state, the metallic bond

between the atoms is easily broken and causes the softness of Group 1 elements. In

later groups where there are more outer electrons available to participate in metallic

bonding, the elements become harder. The increased reactivity down the Group is a

consequence of the outer electron being further from the nucleus and therefore more

easily removed.

The atoms of a non-metal reacting with Group 1 elements gain the electrons that were

removed from the metal atoms when their cations formed and so the non-metal atoms

also become ions but with a negative charge (anions). The two oppositely charged ions

are then held together by electrostatic attraction to form an ionic compound. These are

characteristic reactions of all Group 1 elements.

For example, in the reaction of sodium with the non-metal chlorine, the process could

be shown as

2Na + Cl2 v 2Na+Cl–

However, the charges on the Na+ and Cl– in such compounds are not usually shown in

their formulas. It is a typical property of metals that they form ionic halides whereas

non-metals form covalent halides.

When a salt is melted or when it is dissolved in water, it shows electrical conduction.

This is because the ions are no longer held together in a solid crystal but instead, under

the influence of an electrical voltage, move to the electrode of opposite charge and

there undergo a reaction in which cations gain electrons and anions lose electrons to

form free elements again. This process is called electrolysis and is the reverse of the

reaction by which the original salt was formed. Electrolysis of molten sodium chloride

XII - 17

will produce the elements sodium and chlorine at the electrodes according to the

following equations.

2Na+(l) + 2e– v 2Na(l) and 2Cl–(l) v Cl2(g) + 2e–

Reactions of this type, redox reactions, were examined in more detail in Topic 11.

Sodium and potassium ions are important in the conduction of nerve impulses.

Lithium ions provide a treatment for schizophrenia. Common compounds of Group 1

include sodium hydroxide (caustic soda), used in oven cleaners and as a starting

material in many industrial processes and sodium hydrogencarbonate (NaHCO3) which

is used in cooking.

FOR THOSE WHO WANT TO KNOW MORE - lithium, sodium, caesium.

LITHIUM

Lithium is the third lightest atom and has the smallest radius of any metal. The Li+ ion

is the smallest stable metal cation, a property which makes it ideal for use in the

lithium ion battery. [See the table of radii on page XII-13].

Lithium is extracted from salt beds in the deserts of some South American countries.

Unlike other Group 1 elements, it has an insoluble carbonate which provides an easy

method of extraction and purification. Until about 1990, lithium had only minor uses,

the main one being as a treatment for bipolar disorder which is readily controlled with

lithium carbonate. Prior to the introduction of this treatment, psychiatric hospitals

housed large numbers of sufferers but following the introduction of lithium treatment,

they were emptied. The mode by which it works is not known but is assumed to

replace sodium ions in some of the functions of the brain in a way that calms the

depression and suicidal thoughts that are part of bipolar disorder.

At that time, apart from treating bipolar disorder, the other main application for lithium

compounds was in greases where lithium soaps are added to normal grease to make it

flow better.

Lithium aluminium alloys are now being used in aircraft construction.

Lithium batteries.

The market for lithium increased vastly when batteries based on lithium were devised,

especially the lithium ion battery. Lithium batteries now power most portable

electronic devices where the light weight and in the case of lithium ion batteries, the

ability to recharge them are unchallenged at present. They are also being used to

power electric vehicles and to store electricity from renewable sources to smooth out

the variation in supply. These batteries have the advantages of large charge density

and being rechargeable many times before they degrade.

XII - 18

There are two types of lithium battery:

(i) Non-rechargeable. These are typically the small button-sized batteries used in

small electronic devices. This type contains elemental lithium and the oxidation of Li

atoms to Li+ ions is the source of the electron flow from the negative electrode. The

positive electrode is typically MnO2. Because lithium is so reactive, this type of

battery is not suitable for large applications.

(ii) Rechargeable (lithium ion) battery..

The lithium ion battery is so named because the lithium is not present as lithium metal

but only as Li+ ions and there is no redox reaction involving them. The Li+ ions are so

small that they can move through solid graphite which constitutes the negative

electrode and also through the compounds of various metals such as CoO2 used for the

positive electrode. In a lithium ion battery the graphite anode and cathode which

contains a ‘mixed metal’ oxide, typically LiCoO2 are divided by a separator which

allows Li+ cations, but not electrons to flow through it in the same way that H+ ions

move through the familiar lead battery. When the battery is being charged, the

charging voltage causes electrons to be removed from the metal electrode which in the

case of cobalt undergoes an oxidation number increase to +IV. To preserve electrical

neutrality in this electrode, Li+ ions move out from the mixed metal electrode into the

electrolyte and through the barrier into the other half cell containing the graphite

electrode. At that electrode, the charging voltage forces electrons into the graphite

layers and the resulting excess negative charge is neutralised by the flow of Li+ which

squeeze into the spaces between the carbon layers. When fully charged, the battery has

the lithium ions packed between the layers of carbon atoms in the graphite anode.

When discharging, the reverse process occurs with the electrons flowing from the

graphite, through the device to be powered and into the mixed metal electrode where

the cobalt accepts the electrons and undergoes reduction from +IV to a lower oxidation

state. To preserve electrical neutrality in the battery, the lithium ions move from the

graphite electrode into the sponge like structure of the mixed metal oxide.

A typical lithium ion battery produces 3.7 V compared to 1.5 V of a standard dry cell.

There is intense research in this area to discover an electrode system that can work

with the larger, and much cheaper, sodium ion. Because the Na+ ion is too large to fit

between the layers of graphite, chemical reactions are being used to expand the gap

sufficiently. Also under investigation is the use of silicon, another Group 14 element,

to replace graphite.

The fully charged and discharging states of the lithium ion battery are represented in

the following diagram.

XII - 19

SODIUM

The symbol Na for sodium is from the Latin, natrium. Elemental sodium is produced

by electrolysis of molten sodium chloride. Large deposits of sodium chloride occur in

dried-out lake beds.

Elemental sodium is used in sodium vapour lamps in which the metal is heated by an

electric current under vacuum inside a glass envelope Sodium atoms are released into

the gas phase and electrons are excited to higher energy orbits before falling back to

the ground state and releasing the yellow light which is often used for street lighting.

Sodium vapour lights are much more efficient that incandescent lamps, converting

70% of the electrical energy to light.

Sodium compounds are so ubiquitous that they are the source of the yellow flashes

from their atomic spectrum that can be seen in gas flames and fires.

Sodium chloride.

The common name “salt” used for sodium chloride is derived from the Latin and is

incorporated into numerous English words such as salacious, salsa and salary - the

latter because Roman soldiers were paid in salt. Historically salt has played a major

role in commerce and social changes over millennia. The vital need for salt to preserve

food and to keep people and animals healthy meant that controlling the supply of salt

was most advantageous both commercially and militarily. Salt was of equal value to

gold the middle ages and was the basis for the existence and prosperity of cities such as

Venice which had a monopoly of salt supply in that region.

Capitalising on monopolies of salt through salt taxes were one of the underlying causes

of the French revolution and the American war of independence. The beginning of

XII - 20

civil disobedience in India was started by Ghandi’s march across the country to fight

the British monopoly of salt there. Lack of salt was one factor for Napoleon’s defeat

in Russia and the defeat of the confederate army in the American civil war. By the 20th

century, refrigeration and modern packaging reduced the need for salt to preserve food.

Sodium chloride is still used in some cases as a food preservative and additive but

before the advent of refrigeration, food such a meat was “salted” for storage and hence

the expression “to salt something away” meaning to conserve it.

Because it is readily accessible, salt is the ideal starting material for the manufacture of

industrial chemicals such as sodium hydroxide and chlorine.

In countries where snow falls, large amounts of sodium chloride are spread on roads to

melt snow and ice.

Sodium ions are an essential component of the nervous system because conduction of

currents through nerves is achieved by movement of Na+ ions through membranes

rather than movement of electrons as in metallic conduction. Sodium chloride also

plays an important role in regulating blood pressure. This is because the sodium and

chloride ions attract water molecules - in particular, each Na+ ion can attract more than

20 H2O molecules around it. The effect of excessive salt concentrations in the blood is

to increase water uptake causing the volume and thus blood pressure to increase.

However, lack of salt for example when suffering from diarrhea can be fatal. Lack of

circulating salt in the blood can cause cramps. The normal mass of sodium chloride in

the human body is about 250 g. Salt licks are used to attract cattle to various locations

in paddocks.

Sodium hydroxide.

Sodium hydroxide in water solution provides the strong base, OH!. Such strongly

basic solutions have many industrial applications, such as:

Paper manufacture - wood consists of cellulose fibres held together with lignin. In the

Kraft method of paper production, wood chips are boiled in a mixture of sodium

hydroxide and sodium sulfite which removes the lignin. Subsequent squeezing out of

the water solution and drying results in paper sheets,

Soap - soaps are salts of fatty acids which are the components of fat. Heating with

sodium hydroxide causes the fats to break down to the sodium salts of these

component acids. Soaps dissolve dirt and grease because their anions consist of a

negatively charged end and a long hydrocarbon end. The negative end of the anion

can interact with water through ion-dipole attractions while the hydrocarbon end can

interact with non-polar entities such as grease through dispersion forces (see

Supplementary Topic 4). As a result, particles of grease and dirt or oils can be

dispersed into water as very tine droplets.

XII - 21

Sodium hydrogencarbonate.

Also commonly known as sodium bicarbonate, NaHCO3, this compound is used in

cooking to make cakes rise. In water solution it has a pH of about 8 and is used where

a mildly alkaline environment is required and it is used in antacid tablets. Soft drinks

used to contain sodium bicarbonate which released bubbles of carbon dioxide by

reacting with acid in the drink and hence the name soda pop although now the carbon

dioxide is gas forced in under pressure. Heating causes carbon dioxide to be released

from sodium hydrogencarbonate which is why it is used in cooking where it is usually

mixed with an acidic ingredient. It is also utilised in solid state fire extinguishers.

Sodium carbonate.

Sodium carbonate, Na2CO3, is soluble (as are all sodium salts) and produces a basic

solution of pH about 10 . It is used in washing powders where it functions as a water

softener. Sodium carbonate is also an important component in the manufacture of

glass. Some toothpastes include sodium carbonate and sodium hydrogencarbonate to

counteract the acids in the mouth resulting from eating carbohydrates.

CAESIUM

Caesium, symbol Cs, has a very low melting point, 28oC. A vial of solid caesium will

melt if held in the hand. It is the softest of all the solid elements. Being at the bottom

of Group 1 of the Periodic Table, its outer electron is the most readily removed of any

element. Thus caesium is extremely reactive and must be protected from the

atmosphere but its compounds are very stable.

Caesium was the first element to be initially discovered by observing its spectrum in a

flame. This discovery was made by Robert Bunsen using his newly invented gas

burner in conjunction with Gustav Kirchoff, the inventor of the spectroscope.

Salts of caesium such as caesium formate are used in drilling fluids in the mining

industry where the relatively high density of the Cs+ ion causes less dense liquids such

as oil to be displaced.

The radioactive isotope 137Cs which is derived from nuclear reactors is used for various

medical applications.

Caesium clocks.

Extremely accurate methods of determining time are of increasing importance. High

precision is needed for GPS devices, electricity grid stability and telecommunications

amongst many others. Use is made of the frequency at which a particular electron

transition occurs in the 133Cs atom which can be measured with extreme accuracy.

Using this standard, an accuracy of 1 second in 20 million years is currently available

and using a different basis, it is anticipated that an accuracy of 1 second in 13 billion

XII - 22

years will be achieved, a greater time interval than the age of the universe. Because

the rotation of the earth is not constant and is slowing, in order to keep the time as set

by the atomic clock synchronised with “normal” time, a leap second is introduced

occasionally.

GROUP 2. Be, Mg, Ca, Sr, Ba, (Ra)

Group Overview.

These are also metals, but are harder than Group 1 elements and their reactions are

slower because more energy is required to remove the valence electrons.

Beryllium is hard enough to scratch glass but at the bottom of the Group, barium is

only slightly harder than lead. Increasing hardness compared with Group 1 elements

can be attributed to the doubling of the number of outer electrons and a resulting

increase in the strength of the metallic bonds. Apart from beryllium, they generally

form salts with non-metals, always showing a valence of 2 in their compounds due to

the presence of 2 outer electrons in all their atoms. Removal of the two outer electrons

leaves all Group 2 elements with the noble gas structure. Beryllium forms covalent

compounds with non-metals, a property more like that of a non-metal than a metal.

The reason for this is that the Be atom is very small and the energy required to remove

even one of its outer electrons is relatively high. Removal of both outer electrons from

subsequent atoms in the Group becomes easier due to the increased size of the atoms

and therefore less attraction between outer electrons and the nucleus and this explains

the increased reactivity observed down the Group.

Some typical reactions of Group 2 elements -

(i) burn Mg in air 2Mg + O2 v 2MgO

(ii) Ca in water Ca + 2H2O v Ca(OH)2 + H2

calcium hydroxide

Alloys containing beryllium are used in aerospace applications. Beryllium is used as

windows in X ray sources as it is transparent to X rays. It is also used in the nuclear

industry because it reflects neutrons. However beryllium dust can cause fatal lung

diseases. Mg is the central atom in chlorophyll. The compound calcium carbonate

(CaCO3) is present in shells and calcium phosphate (Ca3(PO4)2) is of particular

importance as a component of teeth and bones. Large amounts of calcium carbonate

(limestone) are used commercially in the manufacture of glass and cement. Radium

has been used in radiotherapy treatment of cancer.

XII - 23

FOR THOSE WHO WANT TO KNOW MORE - calcium.

CALCIUM

Calcium is found as various salts, mostly as calcium carbonate of which limestone is

constituted. Calcium compounds in the earth’s crust are slowly eroded and washed

into the oceans where calcium ions are combined with carbonate ions formed from

carbon dioxide which has dissolved in the water. Over long periods the insoluble

calcium carbonate settles to the ocean floor, joined by shells from microscopic

organisms. The usual geological processes result in compression and ultimately

uplifting to reveal limestone such as the huge cliffs that border the shores of much of

the Southern Ocean in Western Australia. Thus limestone represents a vast storage of

carbon dioxide, extracted from the atmosphere in previous eras. Shells of sea creatures

and exoskeletons of some marine organisms are made from calcium and carbonate ions

present in sea water. Carbon dioxide dissolved in water forms a small amount of

carbonic acid. The consequence of increasing CO2 in the atmosphere is an increase in

the acidity of the oceans. Because calcium carbonate, the main constituent of marine

invertebrates, dissolves in acids, the acidity of their environment is critical and much

marine life is threatened by the increased atmospheric CO2 concentration.

Cement.

The most widespread use of a calcium compound is for manufacturing cement which is

the vital ingredient of concrete. Limestone is heated strongly to drive the carbon

dioxide off leaving calcium oxide or lime. Adding water to lime produces slaked lime,

a strong waterproof bonding material. However, the process of driving the CO2 off the

limestone contributes significantly to the increased concentration of this greenhouse

gas in the atmosphere, not only by the heating needed but also the CO2 released from

the limestone. The production of 1000 kg of cement releases 800 kg of carbon dioxide

to the atmosphere and amounts to 5% of the world’s total annual CO2 release.

Calcium sulfate.

Calcium sulfate (gypsum) is another calcium compound found in large deposits and it

is used in the building trade for plastering and also for setting broken bones in which

application it is known as Plaster of Paris.

Teeth and bones.

Calcium compounds are a large proportion of the body’s weight as bones and teeth are

made of calcium phosphate. Bones form when special cells produce a scaffold made

from protein which gives the bone its tensile strength and then calcium and phosphate

ions form the solid bone which provides its compressive strength. Calcium ion intake

is required throughout life to maintain healthy bone.

Other physiological functions of calcium ions.

XII - 24

Apart from solid structures in the bodies of most living creatures, calcium ions are

also a vital component in numerous physiological processes. For example, calcium

ions are used in the transmission of nerve impulses, facilitate the operation of muscles

and serve as cofactors that helps various enzymes to operate in functions such as

stabilising blood pressure and blood clotting. In plants, calcium ions are vital in the

mechanism that causes the stomata to close when required as well as many other

functions.

GROUP 13. B, Al, Ga, In, Tl

Group Overview.

Boron is a non-metal, having a black powdery appearance and not having any of the

usual properties of metals. Boron is a non-conductor of electricity and has a very high

melting point (2040 oC), indicating covalent network bonding of the boron atoms in the

solid. It has an acidic oxide, typical of non-metals and it does not form ions when it

reacts to produce compounds, but instead bonds by sharing electrons with the bonded

atoms to form covalent compounds. This is the method by which non-metals bond to

each other. The other elements of Group 13 mostly form ionic compounds. The

Group 13 elements usually have a valence of 3 in their compounds, due to the presence

of 3 outer electrons in their atoms.

Aluminium, the third most abundant element in the earth’s crust, has considerable

commercial application due to its very high strength to weight ratio and also its being a

particularly good conductor of electricity. Aluminium is produced in large quantities

by electrolysis of aluminium oxide ores such as bauxite. Although aluminium is very

reactive, it forms an oxide layer (Al2O3) on its surface which protects the metal from

further corrosion, making it useful as a building material. However, at high

temperatures, aluminium burns vigorously.

This Group shows clearly what is a general trend whereby the elements display

increasing metallic properties down the Group. This trend is quite subtle in Group 1

where lithium does form a few covalent compounds and is more pronounced in Group

2 where beryllium forms compounds that are predominantly covalently bonded but

retains many other properties of a metal including metallic bonding in the solid

element. In Group 13, boron is essentially a non-metal in appearance and in physical

and chemical properties. Aluminium forms both ionic and covalent compounds as do

the other elements of this Group, but all exist as stable cations in solution. This trend

from non-metallic to metallic properties down a Group continues to be apparent in

Groups 14 - 16, attributable as discussed previously to the larger size of the lower

atoms in each Group resulting in weaker attraction between nucleus and electrons and

thus less energy being required to form cations.

XII - 25

FOR THOSE WHO WANT TO KNOW MORE - boron, aluminium, gallium,

indium.

BORON

Boron forms an oxide, B2O3, which dissolves in water to form a weak acid, boric acid.

Both compounds have widespread applications. Compounds of boron have some mild

anti-microbial properties. Boric acid is used to keep contact lenses free from bacteria

and as an eye wash. Its sodium salt is used as an insecticide which is harmless to

humans but effective against insects and it is typically the active ingredient of ant and

cockroach traps. It is also used in trace amounts as fertilizer for plants. The very small

size of the B atom allows boron atoms to fit into small spaces in metals’ crystal lattices

where they prevent sliding of planes of metal atoms and thus impart greater strength to

the crystal structure.

Boronsilicate glass.

Adding up to 15% of boron oxide to normal glass made from silicon dioxide (silica)

imparts the property of having minimal expansion on heating. This allows its use in

applications such as laboratory glassware and cooking utensils where heating would

cause cracking of normal glass. Consequently one can heat test tubes, beakers,

distillation flasks and other equipment made from boron-containing glass without

risking breakage. Varying the boron content of glass allows any coefficient of

expansion to be produced for a given task. Lenses made from normal glass suffer from

achromatic distortion whereby the lens fails to focus the various wavelengths (colours)

of light equally, leading to blurred images. Using boronsilicate glass eliminates this

difficulty and achromatic lenses can operate down to the limits imposed by the

wavelength of light.

Abrasives.

The compound boron carbide is one of the hardest materials known, third only to

diamond and another boron compound, boron nitride. It is used as an industrial

abrasive (carborundum) and also to make body armour. Because boron is also a very

light element, the weight of boron carbide armour is minimised, an important

consideration when wearing it.

Reactor control rods.

Nuclear reactors operate because of a flow of neutrons from decaying 235U atoms

impinging on other uranium atoms and continuing the fission process which produces

more neutrons in a chain reaction. If the flow of neutrons is not regulated, the reactor

would overheat and meltdown of the core would occur. The 10B atom’s nucleus has

the ability to capture neutrons. Control rods packed with boron oxide or boron nitride

are placed beside the fissile material in the core. By raising or lowering the rods, the

reaction can be turned on or off as required.

XII - 26

Boron neutron capture therapy.

This technique, still being developed, relies on the ability of the 10B nucleus to absorb a

neutron and convert to the unstable 11B atom which then decomposes to release an alfa

particle and a lithium atom - the same process that is used in nuclear reactors. In this

procedure, a suitable boron compound is injected and the tumour site is exposed to a

focussed source of neutrons of the appropriate energy. The release of alfa particles

that ensues damages the tissues that have absorbed the boron compound. To be

effective, the boron needs to be well distributed throughout the tumour.

ALUMINIUM.

Aluminium is the most abundant metal in the earth’s crust and occurs in easily

accessible deposits well distributed world-wide, so its supply is guaranteed. A lot of

electricity is required to liberate the free metal by electrolysis of its common ore,

bauxite, which is aluminium oxide or Al2O3. Consequently aluminium smelters are

usually located close to cheap sources of electricity such as hydroelectric generators.

Aluminium is very reactive but is stabilised by the formation of an oxide layer on its

surface. The low density of aluminium combined with mechanical strength and

resistance to corrosion are why it is used in so many industrial applications which

include motor vehicle, aeroplane and boat construction. Food and beverages are

ubiquitously contained in aluminium cans. Other application use the very shiny

surface attainable on aluminium metal. Thus it is used in insulation materials and

thermal blankets and also, if sprayed onto glass as a thin layer, it is used to make

mirrors,

Sapphire.

Aluminium oxide is one of the hardest substances known and is used as an abrasive

including sandpaper. Sapphires are crystals of Al2O3 which was subjected to very high

temperatures and pressures deep in the earth’s crust before ultimately being uplifted by

geological processes. Sapphires are used in various applications which take advantage

of their property of hardness. These include as bearings that resist wear such as in

mechanical clocks and watches and as components of various scientific instruments.

The colour of sapphires is derived from small amounts of impurities consisting of

atoms of elements of similar size to that of aluminium. Small amounts of iron atoms

results in the blue colour which is common. Other colours are attributed to addition of

atoms of titanium, copper, magnesium and chromium, all of which are a neat fit to

replace aluminium atoms in the crystal. Addition of chromium atoms produces the red

colour of ruby which is another gemstone of aluminium oxide. Artificial sapphires are

grown at very high temperatures around 2500oC

Recycling.

Aluminium is recycled very efficiently with typically about 50% being reused. This is

commercially viable because the amount of energy needed to reuse aluminium metal is

XII - 27

only 5% of that needed to produce it from its ore. There is no limit on how often

aluminium can be recycled. In theory, ultimately there would be no need for newly

smelted aluminium but given its expanding market, that would be a long time into the

future.

GALLIUM AND INDIUM.

These two consecutive elements in Group 13 are both soft metals with low melting

points. Indium is named not for the country but after the characteristic indigo spectral

line by which it was first discovered. The existence of gallium along with some of its

properties was predicted by Mendeleev when proposing his version of the Periodic

Table and he called the yet to be discovered element eka-aluminium for “following

aluminium”.

Gallium melts at 30oC and indium melts at 157oC . Both “wet” glass which means

that they adhere to glass which makes them ideal for soldering electrodes to glass

surfaces. They are both soft, shiny metals. Indium has a crystal structure that causes it

to emit a crying sound when bent, like tin, due to planes of their atoms moving relative

to each other. They do not occur in minerals in quantities worth mining but instead are

extracted as trace elements from ores of zinc (indium) or aluminium (gallium).

Compounds.

Alloys of gallium, indium and often tin are used for example as solders for special

applications. The compound indium tin oxide is a conductor of electricity and is used

to coat touch screens and liquid crystal displays. An alloy of gallium and indium with

tin (galinstan) is a liquid at room temperature and is used in medical thermometers.

With mercury being banned, its use in normal thermometers will provide a substitute.

Light emitting diodes.

Compounds of gallium and indium with various Group 15 elements are used in the

production of light emitting diodes (LEDs) and semi-conducting lasers which are the

type found in modern electronic devices. To manufacture LEDs, a substrate is used

onto which is deposited very thin layers of atoms of gallium, indium and various other

elements. For lighting LEDs, the substrate is a small sapphire but different materials

including silicon are also used for other applications. LEDs are much more efficient

than conventional globes as almost all the electricity used is converted to light whereas

an incandescent light globe converts 95% of the energy supplied to heat. As their cost

is reduced, LEDs are rapidly replacing other types of lighting because of their greater

efficiency and very long lifetimes. About 35% of worldwide electricity production is

used for lighting so widespread replacement with LEDs will make a significant

reduction in carbon dioxide emissions. Another rapidly growing application is the use

of LEDs in TV picture tubes.

XII - 28

The principle of LEDs can be used in reverse to convert light energy to electricity as

solar panels. Compounds based on these two elements have produced efficiencies of

40% in conversion of sunlight compared with 10% typically for the normal silicon

panels because unlike silicon, these compounds can utilise all visible frequencies. At

present the cost precludes their mass production but they are used in some special

environments. Using mirrors to focus sunlight onto a small area of such materials

overcomes their larger cost.

Solid state lasers.

The small blue-light lasers used in CD and DVD players are mostly made using

gallium(III) nitride, GaN, deposited onto saphire substrate. The process to do this

successfully was elusive for many years but finally was put on a commercial basis in

the late 1990s, a feat which earned its developers the 2014 Nobel Prize in Physics.

Blue light lasers lead to the development of high density DVD discs which allow much

more data to be stored because the blue light is of shorter wavelength than that used

previously.

Recycling.

Due to the lack of concentrated sources of ores containing gallium and indium,

significant amounts are obtained by recycling. Indium supplies are likely to be a

problem at some time in the future.

GROUP 14. C (non-metal) Si, Ge (intermediate) Sn, Pb (metals)

Group Overview.

Carbon occurs as graphite and diamond as well as an amorphous (non-crystalline) form

such as charcoal. Carbon also occurs as hollow, soccer-ball shaped arrangements of up

to 76 or more C atoms known as fullerenes. These are examples of allotropes -

different physical forms of the one element arising from different arrangements in the

way their atoms are bonded.

XII - 29

Diamond contains a large number of carbon atoms joined by network covalent

bonding. Each C atom is bonded to 4 other carbon atoms by covalent bonds which are

pointing to the corners of a tetrahedron. Graphite contains carbon atoms which are

bonded to 3 other carbon atoms, all in the same plane, and also has weak bonds to

carbon atoms in the plane above and below. It is these weak bonds which cause

graphite to easily peel off into flakes when it is used in "lead" pencils or in graphite

lubricants. Graphite is exceptional among non-metals in being a conductor of

electricity, also due to the mobility of the electrons that constitute the weak bonds

between the planes of carbon atoms.

Carbon has an acidic oxide, CO2, forms only covalent bonds in compounds including

its halides and apart from graphite, does not conduct electricity - typical properties of a

non-metal. Carbon is the fundamental element in living cells, and is recycled through

the carbon cycle. Many millions of carbon compounds exist as natural products or

through laboratory or commercial synthesis. Because carbon has the ability to form

molecules containing long chains of covalently bonded carbon atoms along with atoms

of many other elements, almost unlimited numbers of carbon compounds are yet to be

prepared. The chemistry of carbon compounds is called ORGANIC CHEMISTRY.

Important inorganic compounds of carbon include carbon dioxide, CO2, a product of

RESPIRATION and the main GREENHOUSE GAS from burning fossil fuels.

Carbon monoxide, CO, which results from the incomplete combustion of carbon

compounds is a powerfully toxic substance. Fortunately, carbon monoxide rapidly

converts to carbon dioxide in the atmosphere and does not accumulate to large

concentrations.

Silicon is the fundamental element of mineral chemistry, and it makes up the majority

of the earth's crust - usually in combination with oxygen as compounds called

SILICATES. For example, the white sand on beaches is substantially silicon dioxide

(silica), SiO2. About 87 % of the earth's crust is made up of SiO2 and related

compounds. Silicon does not form such extensive chains through bonding as does

carbon, and has nowhere near as many compounds. Apart from its role in mineral

chemistry, elemental silicon is used to make solid state electronic components

including photovoltaic cells for converting sunlight directly to electricity. Elemental

silicon displays network covalent bonding in the solid state. Do not confuse the

element silicon with the class of compounds called SILICONES which are polymers

containing silicon bonded to hydrocarbon groups in particular. Silicon and germanium

have some properties of both metals and non-metals so are often called SEMI-

METALS or METALLOIDS.

While carbon and silicon do not form cations, germanium, tin and lead are more like

metals in that they do form cations in compounds but they can also covalently bond.

Group 14 again clearly illustrates this trend observed in most Periodic Table Groups

XII - 30

towards an increase in metallic properties of the elements down each Group and both

tin and lead in the solid state exhibit metallic bonding. Tin and lead are both relatively

low melting metals used together to form solder. Tin is used to plate the ubiquitous

steel cans because, unlike iron, tin does not corrode readily. Lead has anti-fungal

properties which make it valuable as a component in paint, usually in the form of lead

oxides. Concerns about health risks through ingestion of paint have caused lead to be

deleted from most paints at present. Lead is also used in large amounts in the lead acid

accumulator (i.e. the common rechargeable battery used in cars). There, alternate

plates of lead metal and lead(IV) oxide, PbO2, are immersed in sulfuric acid, all

contained in a plastic case. Electricity is provided by the redox reactions which take

place at the electrodes. Redox reactions were discussed in detail in Topic 11.

Group 14 elements usually have valence = 4, but lead and tin in particular exist also in

compounds with valence = 2, often as salts containing the ions Pb2+ or Sn2+.

FOR THOSE WHO WANT TO KNOW MORE - carbon, silicon, tin, lead.

CARBON

Graphite.

The most common form of carbon is graphite and it can be converted to another form,

diamond, by application of heat and high pressure. The traditional use for graphite has

been as a dry lubricant. Oils eventually dry and form thick gums and thus cease

lubricating the surfaces that require it. Because the layers in graphite can peel apart

and slide over each other, they provide lubrication without these problems. Writing

with a “lead” pencil is in fact depositing thin layers of graphite on the written surface.

However, in recent years a number of revolutionary materials based on carbon atoms

has been developed and graphite has become an important starting material for their

production. Some of these materials are discussed below.

Carbon fibre.

CARBON FIBRE was the first and it consists of C atoms bonded to each other in

long strings. On a weight basis, carbon fibres are much stronger than metals as the

bond energy of the covalent bonds between carbon atoms greatly exceeds that of

typical metallic bonds. [Recall how the bonding in metals relies on mobile outer

electrons - see Topic 3.] The carbon fibres can be woven into mats and set with resin,

the fibres providing the tensile strength. Before that stage, the carbon mat can be

shaped to simple or complex designs as required, for example in components of

aircraft, especially where stress resistance is paramount. Apart from strength and the

shaping advantages of carbon mat construction, its significantly lighter weight makes it

ideal for applications such as aircraft and motor car bodies. The carbon fibres are

made from preexisting chains of carbon atoms in polymers such as rayon from which

all the non-carbon atoms have been removed.

XII - 31

Graphene.

A very recent development among carbon materials is the discovery of graphene.

Graphene was initially made by putting sticky tape onto graphite and pulling it off.

Adhering to the tape is a single layer of C atoms bonded to each other in the same

hexagonal pattern as in the layered graphite structure which has been likened to

chicken wire in appearance. The backing material can be dissolved to leave the free

graphene. On a weight basis, it is one of the strongest materials known and has the

ability to stretch as much as 20% of it length without damage. Potential uses for

graphene are being discovered regularly. It is an excellent conductor of heat and

electricity and has promise as a component of batteries and also photovoltaic cells.

The C atoms in the chicken wire-like structure of graphene provide an impermeable

surface through which even helium atoms cannot penetrate. No other material has this

property. Its flexibility allows it to be made into curved surfaces which holds promise

for making curved solar panels. Another use mooted is for the touch screens of mobile

phones due to its transparency and electrical conductivity as well as its ruggedness, all

properties required for that application. Graphene can also be used to make filters

which admit only a specific sized entity by putting the desired size hole in it so it could

be used as membranes in desalination plants for example.

Nanotubes.

Tubes of carbon atoms called NANOTUBES is another newly discovered carbon

material with much potential. These can be made by passing a large electric current

through a graphite electrode and sheets of the chickenwire structured atoms fly off and

spontaneously roll up to form the nanotubes, so named because they typically have a

diameter of 1 nanometre. The C/C bonds in nanotubes are stronger than those in

diamond and carbon nanotubes are hundreds of times stronger than any metal.

Applications to take advantage of the properties of carbon nanotubes are still being

explored but already they are being used in bulk, added to polymers to increase

strength. Individual nanotubes potentially could be used in biomedical applications,

as transistors and in conjunction with copper to make high capacity electricity

conductors.

Diamond.

The very regular tetrahedrally arranged structure of the C atoms in diamond coupled

with the strength of the C/C bonds gives diamond the property of being the hardest

known material. Consequently it is in widespread use for cutting and grinding.

Diamond also has another unique property in being the substance having the greatest

heat conductivity. As a non-metal, this might seem surprising because most non-

metals are poor conductors of heat. The explanation lies in the highly regular

arrangement of its atoms in the crystal. As described in Topic 3, heat conduction arises

by two mechanisms. One is the increased movement of electrons which bump into

atoms and other electrons and transfer energy along the material from the heat source.

XII - 32

Metals have mobile outer electrons associated with their atoms and so this mode of

conduction is exhibited by metals. The second is transfer of vibrational energy by

atoms moving in a synchronous manner under the influence of increasing temperature.

This mechanism requires very regular arrays of atoms with no impurities or

discontinuities in the solid. There are no free electrons to move in diamond unlike in

metals, but it does meet the other requirements for transmitting heat by vibrations of

the C atoms. The C atoms in diamonds are arranged to a high degree of symmetry and

few impurities are present in the diamond crystal. Diamond is four times better at

conducting heat than copper. Consequently diamond is used in the windows of high

powered infrared lasers for cutting steel for example, allowing ten times more energy

to pass through than glass. Diamond is also used as heat sinks for solid state

components on electronic circuit boards where heat dissipation is a significant

problem. Artificial diamonds of industrial grade can be made from graphite by

applying heat and extreme pressure. By this method, a lump of graphite can be

converted into a mass of microscopic diamonds in less than an hour. High quality

jewellery grade diamonds can be made more slowly.

SILICON.

The element silicon from the Latin silax, symbol Si, is not to be confused with

silicones which are polymers containing silicon atoms. Silicon and oxygen are the two

most abundant elements in the earth’s crust, frequently combined in minerals together.

Silicon is a grey coloured solid and is a semiconductor of electricity. Silicon and

oxygen are the two most abundant elements in the earth’s crust, frequently combined in

minerals together. Combined with oxygen, silicon forms a dioxide which unlike the

molecular covalently bonded CO2, is a network covalent solid named silica of formula

SiO2 which is ubiquitous as sand and many minerals. Like carbon, it forms four

covalent bonds in its compounds such as SiH4 (silane) and SiCl4 (silicon tetrachloride).

Glass.

Common glass is made from silica with some small amounts of additional substances

to improve its properties. Glass can be drawn into fibres and used for fibre optic

cables required for communications transmission. Chopped into small lengths and set

in resin, fibreglass is a common construction material.

Alloys.

Silicon is added to many different combinations of metals to form alloys with specific

properties. The most common alloy is a mixture of iron with silicon, accounting for

about 80% of total silicon production. Bronze is an alloy of copper (ca 90%) with tin

(ca 10%) which is much harder than copper and was the basis for bronze age

implements. Its properties can be enhanced by adding small amounts of other elements

including silicon. Silicon bronze is ideal where corrosion resistance under severe

conditions is required.

XII - 33

Semiconductors.

Silicon is one of a number of elements known as semiconductors which conduct

electricity less well than metals but better than non-metals. One of their properties is

the ability to pass electric current more easily in one direction than the other which can

be used to convert alternating current (AC) to direct current (DC). When purified to an

extreme degree, silicon forms the basis of transistors and integrated circuits which

underlie modern electronics.

Solar photovoltaic (PV) panels.

Silicon can also convert visible light to direct current electricity and is the most

common material for rooftop PV panels. The conversion efficiency is currently about

20% when crystalline silicon is used but technology is rapidly improving this

performance.

Silicones.

Silicones are polymers of Si atoms bonded to O atoms as a chain with organic side

groups bonded to the two remaining tetrahedral positions on the Si atom as shown

below.

Silicone chains can be cross linked to make three dimensional polymers which serve

for example as rubbers, sealants, greases and cookware. In dissolved form, silicones

are used as lubricants and water dispersant sprays, ingredients of hair conditioners and

fabric waterproofing agents as well as many other applications. Properties which make

silicones so valuable include their inertness, non-toxicity, electrical insulating capacity

and resistance to water and UV light.

TIN.

Tin was one of the earliest elements discovered, around 3000BC, presumably as a

result of tin-bearing rocks being used in fires. When mixed with copper which was

also discovered in ancient times, an alloy of the two metals called bronze results.

Bronze (12% tin) has superior properties to either of its component elements and gave

rise to the weapons and tools of the Bronze Age.

XII - 34

Pewter is another alloy of tin (85 - 90% tin) and has properties such as being resistant

to corrosion and non-toxic, thereby making it suitable for the manufacture of eating

and drinking utensils.

Tin is a soft, low melting point metal which resists corrosion due to the formation of a

protective oxide layer. It used to be combined with lead to make solder for electronics

circuits and plumbing, but the toxicity of lead has resulted in tin now being used

alone. The lack of lead in solder allows tin to develop problems known as tin whiskers

and tin pest to develop. Tin has a highly crystalline structure and bending a bar of tin

causes layers of atoms within the crystal to slide over each other leading to an audible

sound, sometimes called the “cry of tin”. Properties of tin have resulted in it being

used in organ pipes(50% with lead) to produce their characteristic mellow sound and

in bells (20% tin, 80% copper) from which long resonances are produced when struck.

However tin solder is not without its faults because the instability of the crystal can

result in small “whiskers” of tin erupting and leading to possible failures of printed

circuit boards. Tin pest is an eruption of pustules on the surface of tin arising from the

same cause. These problems can be reduced by adding small amounts of other

elements such as bismuth and antimony to tin. Tin plating of cans provides a corrosion

resistant surface. However this application is being superceded by polymer coatings or

using aluminium cans.

Tin has an important role through a revolution in the manufacture of sheet glass. Sheet

glass manufacture used to be a very hot and dangerous labour intensive occupation

until a process developed by the Pilkington company in the mid-20th century. The

basis for the Pilkington process is to float the molten glass sheet on top of a tank

containing molten tin. The tin is hot enough to stop the glass from solidifying as it

floats out to form a flat sheet which then moves along a continuous production line.

Compounds of tin with organic molecules find uses such as biocides and as stabilisers

in plastics such as PVC without which the polymer breaks down rapidly.

LEAD

Lead was one of the earliest elements discovered and isolated. Lead ores have been

mined in England from pre-historic times and at one period lead was exported to

Rome. The symbol for lead, Pb, is derived from its Latin name, plumbum, which has

also given rise to names of activities traditionally utilising lead such as plumbing and

devices such a the plumb bob. It is a soft, malleable metal which is shiny when freshly

cut but gradually tarnishes in air. Lead metal is very resistant to corrosion even by

strong acids such as sulfuric acid a property used in the lead acid battery. Many of the

applications in which lead or lead compounds were used have been largely

discontinued because of health concerns. Lead compounds when ingested or inhaled

cause damage to the nervous system and in particular, are associated with brain

XII - 35

damage. Some psychologists even claim evidence shows that leaded fuel caused

increased incidence of violent crime due to its effect on parts of the brain. Applications

now discontinued in most parts of the world include the following:

Lead solder - lead melts at 328oC and it wets copper and tin so it is an ideal solder for

electrical and plumbing applications, usually mixed with tin. Solder now contains no

lead.

Lead as a fuel additive - for many decades lead compounds were added to petrol to

improve its combustion properties and also to act as a lubricant for the engine’s valves.

Lead compounds in paints - these provide excellent antifungal properties but when old

paint was subsequently being burnt off, the inhaled vapours are dangerous. Flaking

paint also poses an ingestion hazard. The reaction against leaded paints has been so

strong that even artists’ paints cannot be legally sold in a some countries. White lead

carbonate is acknowledged as the best primer for oil painting as it does not crack and

imparts unrivalled visual properties to the paint overlaid on it but it too is banned from

sale in some regions.

Lead batteries.

Most (90%) of the lead mined today is used in the production of the ubiquitous lead

battery that is used in every motor vehicle and as standby power sources in the event of

power supply failure. These batteries are very reliable and simple and the materials

from which they are constructed can be recycled an unlimited number of times. These

include the sulfuric acid electrolyte, the lead plates that are part of the electrodes, the

lead dioxide and lead sulfate from the electrodes as well as the polypropylene battery

casing. Over 90% of all lead acid batteries sold are eventually recycled and plants that

once smelted lead ores are now largely involved in recycling. The main drawbacks of

lead batteries are their weight and limited lifetime.

Lead sheeting.

Lead sheeting is the ideal material for sealing tiled roofs and in similar building

applications because of its flexibility which allows moulding to the required shape and

its low melting point which makes it easy to seal. Lead is extremely resistant to

corrosion and so lead sheeting is used to protect electrical cables, especially from sea

water.

Ballast.

The large number of protons and neutrons in the nucleus of lead atoms endow this

element with exceptionally large density which, combined with its resistance to

corrosion, makes it ideal to use as ballast. It especially is useful in marine applications.

XII - 36

Radiation shielding.

Again because of its nuclear structure, lead can block radiation from sources such as

X-ray machines and nuclear radiation. When undergoing body scans, lead covering is

used to protect organs from dangerous radiation and limit exposure to the region under

examination.

GROUP 15. N, P, (non-metals) As, Sb, (intermediate) Bi (metal)

Group Overview.

Again, a transition from non-metal to metal is observed down this Group. Arsenic and

antimony are usually regarded as metalloids. Nitrogen is one of the few elements to

occur as a gas at room temperature and pressure. The other elements of Group 15 are

solids. The elements nitrogen and phosphorus exhibit molecular covalent bonding,

arsenic and antimony have network covalent bonding but bismuth has metallic bonding

in the solid state as expected of a metal. Elements of this group commonly have

valencies of 5 or 3 in their compounds. Compounds of nitrogen and phosphorus are of

vital importance in biological systems - nitrogen as a component of all proteins and

both nitrogen and phosphorus as components of DNA. Phosphate groups are an

essential component of the system by which cells store and use energy. Elemental

nitrogen is rather unreactive because it occurs as the highly stable N2 molecule which

constitutes 78 % by volume of the atmosphere.

Simple compounds of nitrogen found in the natural environment as part of the nitrogen

cycle include ammonia (NH3), ammonium salts (containing the NH4+ ion), nitrate salts

(containing the NO3– ion) and nitrite salts (NO2

– ion). The process whereby

atmospheric nitrogen is converted to such salts requires considerable energy input and

is called NITROGEN FIXATION. It is accomplished by lightning strikes and by

various microorganisms. Today, about 50 % of all nitrogen fixation is man-made

through synthesis of ammonia from nitrogen and hydrogen gases.

Phosphorus occurs in four different allotropic forms, all of which react vigorously with

oxygen in air, and thus must be stored under water. Phosphorus occurs most

commonly in the natural environment as phosphates which contain the PO43– ion.

Washing powders often contain phosphates.

Arsenic, antimony and bismuth occur less frequently and are of less importance in

biological systems.

Being mostly non-metals, many compounds of Group 15 are covalently bonded,

although compounds containing ions such as As3+, Sb3+, Bi3+ and N3– commonly occur.

Bismuth forms the usual salts containing the Bi3+ ion as expected of a metal.

XII - 37

FOR THOSE WHO WANT TO KNOW MORE - nitrogen, phosphorus.

NITROGEN

The element nitrogen occurs as a diatomic gas, N2, at atmospheric conditions but can

be liquefied by using high pressures and lowering its temperature. Liquid nitrogen

boils at –176 oC at atmospheric pressure. Nitrogen constitutes almost 80 % of the

atmosphere but is quite unreactive due to the very strong triple bond joining the two N

atoms of the N2 molecule. The bond in the nitrogen molecule is the strongest of any

molecule. The nitride ion, N3–, does occur in some ionic compounds and the black

tarnish that appears rapidly on the surface of freshly-cut lithium metal is the compound

lithium nitride. Being easy to produce, liquid nitrogen finds many uses such as

freezing biological specimens for future use and freezing water pipes to allow

disconnection for maintenance. Every doctor’s surgery has a container of liquid

nitrogen used to freeze and remove skin cancers. Many applications where strong

magnetic fields are required make use of the low temperature attainable from liquid

nitrogen. This allows greatly enhanced magnetic fields to be produced because

electrical resistance in metals which carry the electrical current through the

electromagnets decreases at low temperatures.

Gaseous nitrogen is used as an inert atmosphere in food packaging whereby excluding

the reactive element oxygen prolongs the storage life of perishable items.

Breaking the triple bond

Nitrogen atoms are one of the essential components of all the vital molecules involved

in living systems. For example the basic molecules of living cells - the enzymes, DNA

and RNA - all contain N atoms in their structures in combination with atoms of other

elements, predominantly H, C and O.

Both DNA and proteins contain N atoms within their structures where one of their

roles involves both intermolecular and intramolecular hydrogen bonding. The all-

important shapes of proteins are largely maintained by intramolecular hydrogen bonds

between the amino acids which constitute the protein chain. The opening and rejoining

of the links between the double strands of DNA are dependent on hydrogen bonds

breaking and reforming between N atoms and some O atoms on the two strands.

However, extracting the N atoms from the molecular form of nitrogen, N2, requires the

input of a lot of energy, 944 kJ per mole of N2 molecules split. Although nitrogen

molecules are abundant in the atmosphere, this large energy requirement to convert

them to a more accessible form is a limiting factor for all living systems. The triple

bonds in the elemental N2 molecule must be replaced preferably by single bonds in

nitrogen compounds. This feat is achieved in nature partly by lightning strikes which

produce vast amounts of energy and can split N2 and O2 molecules and the atoms

XII - 38

released may combine to form oxides of nitrogen. The main source of converting

atmospheric nitrogen to more accessible compounds is done by some plants called

legumes which host bacteria that can break the triple bond and incorporate the N atoms

into compounds that the plant can use. Subsequently animals eat the plants and gain

the desired nitrogen compounds. The process whereby nitrogen molecules are

converted to compounds in this way is called nitrogen fixation. The food chain in

which animals eat the plants to derive their source of nitrogen and other animals eat

those animals then continues the nitrogen cycle. In due course, the death of the

nitrogen fixing plants and all the other forms of life return the fixed nitrogen to the soil

or the ocean. Until a method of artificially fixing nitrogen was devised in the early 20th

century, this vital element was mainly available by recycling using plant mulch and

manure from animals for example. One problem encountered by the First Fleet when

establishing the settlement in Sydney was lack of manure due to the shortage of

animals available.

Due to the very strong triple bond in N2, nitrogen availability for living creatures

including humans was once very limiting for food production. As world population

increased and farming expanded in the 19th century, the traditional means of recycling

nitrogen in fields as mulch and manure became inadequate. Significant deposits of

nitrates existed in Chile in particular and also use was made of guano (bird faeces)

which have accumulated for centuries in some South American locations. This

material is rich in nitrogen compounds and was ideal to use as a fertilizer. So valuable

was this source that wars were fought over it in the latter part of the 19th century in

South America where large deposits occurred.

The ionic compound potassium nitrate occurs in deposits where it has crystallised from

water as the water evaporated, usually in very dry environments such as the lakes in the

high regions of inland South America. However the natural and recycled sources of

fixed nitrogen were insufficient to feed an increasing world population and it is only as

a result of the production of synthetic ammonia that it is possible to feed 7 billion

people today. The driving force to develop the process used known as the Haber

Bosch synthesis of ammonia was the cornering of the guano supplies from Chile by

Britain. The German chemist Fritz Haber succeeded in combining hydrogen and

nitrogen gases over an iron catalyst at high temperatures and pressures in the lab in

1905 and the process was developed commercially in conjunction with Carl Bosch.

The Haber Bosch synthesis requires vast amounts of energy due to the high

temperatures and pressures needed and about 2 % of the total energy used world wide

is consumed by this process. About half of all the nitrogen now fixed on the planet is

produced by this process and about 40 % of the nitrogen atoms in each of us was fixed

by the Haber synthesis. Without the Haber Bosch synthesis of ammonia, mass

starvation would occur - it is estimated that the world’s population would be about 3.5

billion today instead of 7 billion. Following the commercialisation of the process,

XII - 39

nitrate production in Chile fell from 2.5 million tons annually to 800,000 tons within a

decade. Today annual production of ammonia by the process is more than 100 million

tons. Haber was awarded the Nobel Prize for chemistry in 1918 for his discovery.

This was a controversial decision by the Nobel Prize committee as Haber had also

developed the war gases used in the First World War and others that would be used in

concentration camps during the second world war. Despite his services to Germany,

Haber who was Jewish had to flee and died of a heart attack on his way to Israel in

1934.

Biofixation of nitrogen.

The mass use of ammonia fertilizers has had severe detrimental environmental

consequences. Fertilizers are sprayed on fields in far greater quantities than actually

used by crops and much runs off to contaminate waterways and to change acidity of

soils. It is estimated that only about 20 % of the fertilizer deployed is actually used with

the bulk being lost by various means. To avoid these undesirable consequences, much

research is in progress trying to harness the methods that some plants already use to

derive their nitrogen requirements. These methods utilise an enzyme called

nitrogenase which is able to split the N/N triple bond to make compounds that can be

taken up by the plant which typically hosts the bacteria in root nodules. These nodules

protect the bacteria from oxygen in which nitrogenase is ineffective. Legumes have

this mechanism but not the cereal crops such as wheat and the research aims to equip

cereal plants with the ability to host nitrogen fixing bacteria.

Explosives.

The majority of the ammonia synthesised is not used for food production but as the

basis for making explosives in which nitrogen compounds find considerable

application, replacing earlier products such as dynamite. One such is the compound

potassium nitrate which, mixed with charcoal and sulfur, produces gunpowder which is

believed to have originated in China in the 9th century AD. Potassium nitrate is not an

explosive on its own but another compound of nitrogen, ammonium nitrate, NH4NO3,

has widespread use in mining as a cheap and easily detonated explosive. If heated or

mixed with fuel oil, it explodes violently to form nitrous oxide and oxygen. The

hazzards associated with this compound were highlighted in the great explosion that

occurred in Texas Port in 1947 when a shipload of ammonium nitrate fertilizer caught

fire and detonated, causing massive destruction and the death of at least 581 people,

the injury of 5000 more and the destruction of the sea port. Ammonium nitrate is made

from ammonia and nitric acid which is itself made via the Haber Bosch process and is

an important industrial oxidizing acid.

PHOSPHORUS.

Phosphorus compounds like those of nitrogen play a vital part in all living systems.

Bones and teeth contain phosphate compounds, but far more important is the role of

XII - 40

phosphate groups in DNA molecules and in the energy supply systems for cells based

on the compound adenosine triphosphate, abbreviated as ATP.

Phosphate groups in DNA.

The structure of the backbone of each strand of DNA consists of a series of sugar

groups each of which is bonded to one of four bases, the so-called “letters” of the

genetic code designated as A, C, G and T. Each of these sugar/base groups is joined to

the next one in the DNA strand by a phosphate group. This sequence extends along the

entire strand. In the cell, two complementary strands of DNA form a double helix in

which the two strands are joined to each other through hydrogen bonding between N

atoms on the bases. This is illustrated by the following block diagram.

The strands open by

breaking the hydrogen

bonds to allow the

genetic code to be

transcribed to a molecule,

RNA, which is similar to

DNA but which uses a

different sugar. RNA is

single stranded and also

uses phosphate groups to

join the component

sugar/base groups.

Phosphate groups in the energy supply system, ATP.

All cellular processes need energy to drive them. A molecule containing a large

amount of accessible energy in its bonds is adenosine triphosphate.

XII - 41

The diagram above shows this molecule consists of a sugar molecule bonded to a base

(adenine) at one end and to a chain of three phosphate groups covalently bonded to

each other at the other end. When energy is required, the ATP loses a phosphate group

to become adenosine diphosphate, ADP and releases 30.5 kJ of energy per mole of

ATP reacting. A further reaction can also take place in which a second phosphate

group cleaves off to form adenosine monophosphate and releases 61 kJ of energy per

mole of ADP reacting.

The full sequence can be shown as

ATP 6 ADP + PO43! + 30.5 kJ 6 AMP + PO43

! + 61 kJ

The sequence can be reversed to rebuild the ATP molecules from AMP and phosphate

groups using energy supplied from other processes.

Sources of phosphorus compounds.

Normally phosphorus compounds are recycled when living systems die and break

down but by eating plants and animals which consume them, the phosphorus content of

soils diminishes and severely limits plant growth. The lost phosphorus content can be

replaced as for nitrogen previously discussed, by using mulch or manure but for high

intensity farming which is essential now, additional phosphate must be added to the

soil. Sources of phosphate were again bird droppings and when these were exhausted,

phosphate rich rocks were mined and converted to phosphate fertilizers. Initially

suitable sources were to be found on some Pacific islands such as Nauru but now the

main source of economically viable phosphate is from Morocco which controls 75 %

of the worlds known stock. Unlike nitrogen which can be extracted from air, albeit

with a huge energy requirement, there is no alternative source of phosphorous

compounds. There is concern about how agriculture will deal with the ultimate

depletion of the currently available stocks. One approach now under way in England is

to recycle the phosphates which are excreted into sewage. A side benefit of this is to

reduce phosphates entering waterways where they stimulate algal growth which then

leads to excessive bacterial growth for the amount of oxygen in the water, and killing

other marine life. While recycling cannot totally replace the need for additional

phosphate from mining, it can help to reduce that need.

In summary, Group 15 contains two of the most vital elements to life as we know it.

Both constitute a limitation on how many people can be supported on earth. The

Haber process has overcome the problem of lack of suitable nitrogen compounds but

the only long term solution for the limited phosphorous availability seems to be

different farming practices and recycling.

XII - 42

GROUP 16. O, S, Se (non-metals) Te (intermediate) (Po - metal)

Group Overview.

All except the rare and radioactive element, polonium, are non-metals or metalloids.

They exist as 2– charged anions in compounds with metals and form covalently

bonded compounds with other non-metals. Oxygen is the only member to occur as a

gas, the others are solids at room conditions. Oxygen also occurs in small amounts in

the atmosphere as the allotrope called OZONE. Ozone is dangerous to health when

inhaled but plays a vital role in the upper atmosphere where it absorbs much of the

harmful ultraviolet light that would otherwise impinge on the earth’s surface.

Tellurium has network covalent bonding in the solid state while elemental oxygen,

sulfur and selenium are molecular covalently bonded. The common valencies are 6

and 2, but oxygen has the valence of 2 only.

Oxygen is one of the most reactive non-metals, forming oxides with most elements.

The term OXIDATION originally referred to the reaction of substances with oxygen.

Apart from occurring as the diatomic element O2 (20.9 % by volume in air), oxygen

also occurs extensively as part of numerous compounds in the earth's crust and in

water. All aerobic organisms require elemental oxygen to survive as part of the

process of respiration. Compounds of oxygen may be ionic, containing the oxide ion

(O2–), or covalent (e.g. H2O, CO2). Oxygen also occurs as the peroxide ion, O22–, in

ionic compounds or covalently bonded to non-metals such as in the antiseptic,

hydrogen peroxide, H2O2, which has the structural formula H!O!O!H.

Sulfur occurs free in large deposits as the element and in compounds containing, for

example, sulfide (S2– ) and sulfate (SO42–) ions. Covalently bonded sulfur atoms are an

important component of proteins, helping to maintain the required shape of enzymes.

The compound hydrogen sulfide (rotten egg gas), H2S, is a highly toxic gas which is

generated by anaerobic bacteria. It is produced naturally by such bacteria in marshes

and in sewage holding tanks where oxygen is excluded and can sometimes be detected

in the exhaust fumes of modern motor cars.

Selenium and tellurium are very similar to sulfur in their properties and their hydrides,

H2Se and H2Te, have an even more repulsive odour. Selenium is used commonly as a

rectifier for converting AC to DC. Foods containing selenium compounds are in

vogue at present because of the belief that they remove cancer-forming free radicals

from the body.

All Group 16 elements form covalent compounds with other non-metals (e.g. H!O!H)

and anions in ionic compounds with metals (e.g. Mg2+O2–).

XII - 43

FOR THOSE WHO WANT TO KNOW MORE - oxygen, sulfur.

OXYGEN

Oxygen is the third most abundant element in the universe and in combination with the

most abundant element, hydrogen, it forms water which is the most abundant

compound. On earth, apart from atmospheric O2 gas, much more oxygen is present in

compounds including water and minerals, especially with silicon in silicon dioxide.

Two thirds of the mass of the human body is made up of oxygen atoms in compounds,

especially water.

When earth was formed about 4.5 billion years ago, its atmosphere contained no

elemental oxygen. Microbial life evolved in the absence of oxygen (anaerobic) but

after about 1 billion years, microbial species which contained organelles called

chloroplasts within their cells developed. Chloroplasts use light energy

(photosynthesis) to split water molecules and release O2 gas in the process which is

carried out by plants and some types of algae. At that stage in earth’s development

there were huge quantities of iron present as dissolved salts and as the oxygen was

released, iron oxides formed and precipitated to form the vast beds which today are the

ore bodies being exploited in places such as Western Australia. Ultimately the freely

available iron was depleted so instead the oxygen gas was released into the atmosphere

where its present composition is 21 %. This change had a drastic effect on the earth’s

future. Much of the preexisting microbial life was in effect poisoned by the new

pollutant and died out. Anaerobic life continued in places shielded from the

atmosphere such as swamps.

Oxygen is one of the most powerful oxidizing agents after fluorine. It reacts with

almost all metals to form oxides, many of which come away from the surface of the

metal which disintegrates. While oxygen gas is essential for aerobic life including

animals, within the body extremely reactive oxygen atoms rather than O2 molecules

damage cells. The body has defence mechanisms devoted to mopping up such oxygen

free radicals as they are called. Damage to cells by oxygen free radicals may

contribute to aging and be the cause of other deleterious effects. Anaerobic organisms

generally cannot survive in the presence of oxygen gas.

Oxygen in the atmosphere is inherently so reactive that constant replenishment from

plants and algae is needed to maintain its concentration. Due to the burning of fossil

fuels, the percentage of oxygen gas in the atmosphere is reducing.

Water.

In space, water can exist as a gas but mostly as ice in small grains or lumps and on

asteroids. Current theory holds that when the sun ignited about 4.5 billion years ago,

water along with much other old material swirling around the sun was vaporised and

XII - 44

condensed further out, possibly where Jupiter is now. Comets which formed far out

may have been the vehicle by which water came to be on earth is such large amounts.

There must also have been some special circumstances on earth that allowed it to retain

its water whereas other planets such as Mars which 3 billion years ago had oceans but

now has none. Recently a comet lander has analysed comet-borne water and found

that it has a H:D ratio different from water on earth so comet delivery is not confirmed

as a source yet.

SULFUR

Sulfur is found as the free element near volcanoes and hot springs. It also occurs as

strata buried during previous geologic eras. The stable form of sulfur is the familiar

yellow coloured powder which contains 8-membered rings of S atoms covalently

bonded to each other. If heated, these rings of S atoms break apart and form long

chains of a treacle-like appearance. If this form of sulfur is cooled, it solidifies into a

less stable allotrope called plastic sulfur.

Sulfur burns with a blue flame forming an acidic oxide, SO2, sulfur dioxide, which is

soluble in water and forms sulfurous acid, H2SO3. In the mid-20th century, burning of

large amounts of sulfur-containing coal and petroleum lead to sulfur-containing acids

contaminating forests and waterways, falling as acid rain and killing plant and animal

life. In the 1970's the countries of North America instituted the first and very

successful emissions trading scheme in which polluters paid for the amount of SO2

they emitted. Like the schemes currently used in some parts of the world to abate CO2,

any unused permits could be sold to other polluters who could not meet the mandated

targets. Reduced levels of acid rain soon followed as it became uneconomic to burn

“sour” coal and oil so that coal mines producing high sulfur content coal closed and oil

refineries installed equipment to remove sulfur before it was incorporated in the end

products. Now so much elemental sulfur is extracted from oil that there is a world glut

of it unused despite its being the starting material for the manufacture of sulfuric acid,

a vital and widely used industrial material.

Elemental sulfur and slow release compounds of sulfur are now added to fertilizer as

sulfur is an essential nutrient for plant growth. Ironically this addition of sulfur to

fertilizer has become necessary as a result of the reduction of sulfur compounds being

emitted industrially.

Compounds.

Cysteine.

Sulfur containing compounds are essential components of living organisms, mainly as

atoms incorporated into amino acids that are the building blocks of proteins. The

enzymes which catalyse all metabolic reactions are proteins and they function because

XII - 45

of the fixed 3-dimensional shape which the long chain of amino acids maintains (see

Topic 13). To keep their required shapes, amino acids at different locations in the

chain intramolecularly hydrogen bond via N–H bonds but also S–S bonds form

between S the atoms of the amino acid which contains them, cysteine. The

combination of the hydrogen bonding and the S/S linkages provide the rigidity needed

for the enzyme to operate as a catalyst. The human body contains about 0.25 % by

mass sulfur in compounds. The S–S bonds can be broken and reformed relatively

easily. Hair contains strands of protein with S/S linkages between them which

provides hair’s strength. These cross linking bonds are broken and reformed when

hairdressers straighten or permanent wave hair. The unpleasant smell from burning

hair is due to sulfur compounds being formed.

The formation of S/S linkages is the basis for producing rubber from latex which is a

liquid but which contains polymers with S atoms by mixing elemental sulfur with latex

and heating it to convert it into the stretchable material, a process called vulcanising.

The first vulcanised rubbers were developed by Dunlop in the 19th century enabling the

production pneumatic tyres.

Hydrogen sulfide.

Hydrogen sulfide is a highly toxic gas at room conditions. One might expect it to be a

liquid given that H2O from the period above boils at 100 oC but there is no hydrogen

bonding associated with S atoms due to their larger size compared with O atoms, (See

Supplementary Topic 4). It is well known as rotten egg gas because some is formed

from the sulfur-containing amino acids in eggs. Anaerobic conditions in swamps for

example, make use of the redox reaction in which the S atoms in compounds undergo

the oxidation state change which in aerobic conditions would be fulfilled by O atoms.

Unpleasant odours can be detected when microbial life is using this method of redox.

Deep oceans sometimes contain vents which release mineral-rich steam from beneath

the surface. Surprisingly these vents teem with living creatures despite their being too

deep to receive any sunlight which is normally the driver for life. Instead the heat from

the vent and the oxidation of hydrogen sulfide are the basis for the deep water life

forms. It is thought that the earliest forms of life may have started around such vents.

Sulfuric acid.

Sulfuric acid is the most widely used commercial acid due to its low cost. Large

amounts are used in many industrial processes including the production of detergents.

Detergents contain molecules which consist of a hydrocarbon group at one end which

is attracted to dirt and grease by dispersion forces (see Supplementary Topic 4) and an

ionically charged group at the other end which is attracted to polar solvent such as

water. This structure allows molecules of detergent to surround dirt particles and carry

them into water. The most commonly used ionic group used is derived from sulfuric

acid.

XII - 46

Sulfuric acid is used to extract phosphate from calcium phosphate which is mined and

the other end product is calcium sulfate, also known as gypsum, which is used to make

plasterboard. Sulfuric acid is also used to extract metals from their ores by leaching

and then reducing the leachate to form the free metal - copper and nickel in particular

are extracted this way.

Sulfuric acid is made by converting sulfur dioxide over a catalyst to sulfur trioxide

which is dissolved in water.

Sulfur dioxide.

Sulfur dioxide derived from burning sulfur in air or roasting sulfides of iron is the

starting material for the production of sulfuric acid. Sulfur dioxide was used to

sterilize wooden wine barrels between vintages - sulfur was burnt in the barrel as the

SO2 gas kills bacteria. Today sulfites are still used as preservatives in wine as one can

read on he labels.

Dimethyl sulfide.

Most sulfur compounds have foul smells - one of them, dimethyl sulfide, is added to

natural gas which is odourless so that gas leaks are detected. Dimethyl sulfide is

released from the decomposition of marine organisms including seaweeds. It is then

oxidized in the air to other compounds including sulfuric acid as an aerosol which can

act as nuclei for the formation of drops of water which may become clouds. As vast

amounts of dimethyl sulfide are released over the oceans, it is thought that this process

may be significant in controlling the earth’s climate.

GROUP 17. F, Cl, Br, I, (At) also known as the halogens.

Group Overview.

Fluorine and chlorine are diatomic gases at room conditions, bromine is a liquid and

iodine is a low melting solid. All exhibit molecular covalent bonding. The halogens

are very reactive non-metals which can combine with metals to form ionic compounds

containing the halide ion (F–, Cl–, Br– or I–). Species which can enter into such

reactions are called OXIDIZING AGENTS, and all the halogens are therefore good

OXIDANTS. Because of its oxidizing power, chlorine gas is introduced into the

drinking water supply to oxidize organic contaminants such as bacteria and viruses

which could be harmful, as well as to remove algae. Chlorine solutions can also be

obtained for household use from the compound sodium hypochlorite (NaOCl) which is

sold as bleach and also as an agent for maintaining clean swimming pool water. Iodine

dissolved in alcohol (tincture of iodine) is used as a disinfectant for cuts of the skin,

and is used to sterilise dairy equipment. Lack of iodide in food is one of the most

common deficiency disorders in Australia where once it was mandatory to add iodide

to table salt but this practice has ceased.

XII - 47

Some compounds of Group 17 elements are covalent (e.g. HCl) while others are ionic

(e.g. Na+Cl–). They occur as anions in the latter case. The only ionic valence shown is

1 but, apart from fluorine, they have numerous other valencies when covalently bonded

e.g. as the polyatomic anions in the salts NaClO, NaClO2, NaClO3, NaClO4.

Important compounds of halogens include hydrogen chloride (HCl) which is a gas, but

when dissolved in water it breaks down (ionizes) to form a solution of H+ and Cl–

called hydrochloric acid. The halide ion Cl– is an essential component of the nervous

system and the fluoride ion, F– plays an important role in developing strong enamel on

teeth. As natural levels of F– in drinking water are often too low, the salt sodium

fluoride, NaF, is commonly added to drinking water to prevent tooth decay as a public

health measure.

FOR THOSE WHO WANT TO KNOW MORE - fluorine, chlorine, bromine.

FLUORINE

Fluorine is the most reactive element because of its small size and proximity to the

noble gas neon so consequently it has the largest effective nuclear charge. Conversely,

having reacted with other atoms, the resulting compounds of fluorine are usually

extremely stable given the large bond strength of covalent bonds to F atoms.

Fluorine is so reactive that it is difficult to store and handle and extremely hazardous if

contact is made with living organisms. It is an even stronger oxidant the chlorine and

oxygen and it reacts with all the other elements except a few of the noble gases.

Fluorine is mined as the mineral fluorite which is essentially calcium fluoride. The

mineral can be crushed and used directly as flux in steel making. Fluxes lower the

temperature at which a metal melts. Elemental fluorine is extracted from fluorite by

heating it with sulfuric acid.

Fluorine is unusual in that it only exists in a single isotopic form, a property made use

of in the separation of isotopes of uranium - see uranium hexafluoride below.

Hydrogen fluoride.

Hydrogen fluoride dissolves in water to form hydrofluoric acid which is analogous to

hydrochloric acid but unlike the other hydrogen halides, hydrogen fluoride is a weak

acid. Even so, if contact results in it penetrating beneath the skin, hydrofluoric acid

reacts with any calcium ions which control vital functions of the body and converts

them to solid calcium fluoride thereby disrupting those functions.

Hydrogen fluoride is produced from fluorite by mixing it with sulfuric acid.

Hydrofluoric acid must be stored in plastic containers and cannot be stored in glass

which it dissolves. Use is made of its ability to attack glass when glass etching is

XII - 48

required for example, the security coding on car windows. Hydrofluoric acid is

essential in many industrial processes, for example in the smelting of ores of

aluminium by electrolysis and production of refrigerants.

Sulfur hexafluoride.

While fluorine is extremely reactive, it follows that its compounds are extremely

stable. One of many extremely stable covalent fluorine compounds is sulfur

hexafluoride, SF6, which has an application as a spark retardant in high voltage

electricity transformer stations. The extreme stability of SF6 which is a gas prevents

any spark from ionizing the air and thus being propagated and damaging the electrical

equipment. However sulfur hexafluoride is an extremely potent greenhouse gas with a

lifetime of 3000 years so its use must be carefully monitored.

Fluorocarbon refrigerants.

Compounds of carbon with fluorine have been used as refrigerants for decades but it

has been discovered that when released they destroy ozone molecules in the upper

atmosphere, opening the way for UV light to penetrate to lower levels with serious

health and environmental implications. Refrigerants are compounds that are gases at

room conditions and which can be liquefied by compressing and cooling externally.

The liquid refrigerant is then passed through a small hole in a sealed unit and it

converts back to the gaseous state with the absorption of heat. This is the basis for the

operation of all refrigerators. A desirable refrigerant needs to undergo these phase

conversions readily and to be non-toxic in case of leakage. One of the early

refrigerants was ammonia but it fails the toxicity test. Hydrocarbons in which the H

atoms have been replaced by F and Cl atoms are ideal refrigerants but since the

discovery of the damage they were causing to the ozone layer, the original versions

have been banned worldwide. The Montreal protocol devised in 1987 brought an

international agreement to phase out their production. The replacements are

chlorofluorohydrocarbon compounds which are similar but retain one or two H atoms

on each of the carbon atoms. These compounds usually break down too rapidly to

reach the upper atmosphere but if they do, they are found to be extremely potent

greenhouse gases with very long lifetimes. It is estimated that 15 % of the total

greenhouse gases released by humans is made up of such compounds.

Polymers of fluorocarbons.

One of the most versatile polymers is teflon, a polymer of tetrafluoroethylene which is

an ethylene molecule in which all four H atoms have been replaced by F atoms. In the

polymer both C atoms of each ethylene molecule join to other tetrafluoroethylene

molecules. The double bond is broken in this process and the electrons used to form

the two new bonds. Thus the structure of the polymer is an unbroken chain of C

atoms, each of which is bonded to two F atoms and its formula could be represented as

!(CF2)n! where n is a very large number. The chemical name for the polymer is

XII - 49

polytetrafluoroethylene or PTFE. Teflon is a very versatile polymer. It is extremely

resistant to chemical attack and can be machined to any shape. Thin teflon tape is used

to seal joints in plumbing. It has self-lubricating properties which find application as a

material for bearings and in a powdered form, as an enhancement with other lubricants.

One of its best known applications is the surface of non-stick saucepans.

Fluoride in teeth.

Unlike the element, having achieved the noble gas configuration of neon, fluoride ions

are very stable. The compound calcium fluoride, CaF2, is insoluble. The enamel on

the surface of teeth can be given a protective coating of glass-like calcium fluoride by

bringing F!ions into contact with teeth which consist of calcium compounds. The

fluoride content of most natural water supplies is insufficient to achieve this so most

public water utilities add additional fluoride ion as sodium fluoride. Dentists also

typically finish an examination by coating the teeth with a fluoride-rich gel.

Uranium hexafluoride.

Uranium hexafluoride, (UF6), is a stable gas above 57oC. It is vital in the separation

processes used to separate the uranium isotope U235 which undergoes fission, from the

non-fissile U238 which occur as a mixture in uranium ores. By converting all the

uranium to UF6 and either using gas diffusion or gas centrifugation , the slightly lighter

isotope can be separated based on the mass difference. In gas diffusion, the U235F6 will

move faster through a membrane than U238F6 and by setting up many gas diffusion

chambers in series, gradually the diffused gas becomes richer in the desired compound.

Gas centrifugation uses centrifuges to spin the gas samples at high speed and the

heavier 238UF6 is concentrated lower in the container and mixture richer in the lighter

compound can be drawn off from the top. Again, a large number of the centrifuges

must be used in sequence to achieve the desired level of enrichment. Both processes

can only work because the F atom exists as just one isotope so the mass difference

between the 235UF6 and 238UF6 is solely attributable to the difference in the masses of

the two U isotopes.

CHLORINE

Chlorine was known but not recognised as an element prior to it being named by

Humphrey Davy in 1810 - the name is derived from the Greek meaning “green” on

account of the pale yellow green colour of gaseous chlorine. It is produced by

electrolysis of concentrated sodium chloride solution in a cell in which the anode and

cathode are separated by a membrane which allows electrical neutrality of the

electrolyte to be maintained but prevents the gaseous products formed at the electrodes

from mixing. At the positive electrode, chlorine gas forms and at the negative

electrode hydrogen gas and hydroxide ions form, both from the water. Metallic

sodium cannot form by electrolysis of an aqueous solution, only by electrolysis of

molten sodium chloride.

XII - 50

Chlorine like all the halogens is a diatomic element and it is a powerful oxidising

agent. Because the element is so reactive, the compounds it forms are consequently

generally stable. This is especially useful with regard to organic chloro compounds in

which Cl atoms replace H atoms. Chlorine atoms are incorporated into hundreds of

compounds used for example in the building industry and in the manufacture of

pharmaceuticals. Apart from such compounds where Cl atoms are part of the final

product, chlorine is used in many processes to form intermediate compounds without

being part of the final product. Two common examples are the polymers nylon and

polyurethane, neither of which contain Cl atoms but synthesis of both requires

chlorine. The following discusses a few of the many compounds which incorporate

chlorine atoms.

Sodium chloride.

Sodium chloride is easily obtained from underground deposits or by evaporation of salt

water. It is the basic feedstock for both elemental sodium and chlorine and the

manufacture of their compounds. Because chlorine is very reactive, the Cl! ion formed

when the Cl atom has gained an electron is very stable and along with sodium ions is

an essential component for maintaining the correct water balance in the blood stream.

[See sodium chloride under Group 1 elements.]

Sodium hypochlorite.

This compound, NaClO, is sold as common household bleach and is also used in water

treatment, especially swimming pools. It is made by reacting chlorine gas with sodium

hydroxide solution, the chlorine being released in solution as it is used. The Cl2

molecules kill microbial life because of their strong oxidizing properties. Chlorine gas

is a very toxic substance if inhaled and it has been used as a war gas - it damages the

linings of the lungs and causes a fatal build up of fluid. Consequently for most

disinfection purposes it is much safer to use a solution of sodium hypochlorite rather

than elemental chlorine gas.

Polyvinylchloride.

An organic compound of chlorine, vinyl chloride is very stable and forms the polymer

polyvinyl chloride, commonly known at PVC.

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Because of its extreme stability, PVC is used in many building products such as

drainage pipes, guttering and window frames. Production of PVC is the largest single

use for chlorine.

Hydrogen chloride.

The hydride of chlorine, hydrogen chloride, HCl, reacts vigorously with water to form

the strong acid hydrochloric acid in solution. Hydrochloric acid is one of the most

widely used in industrial applications.

Stomach acid.

Hydrochloric acid is secreted in the stomach as part of the digestion of food. It plays a

role in enabling the enzymes to break down amino acids and when food is being

digested, the pH of the stomach juices falls to the range 1 - 2. After a meal has been

digested, the pH increases to its normal value of 4 - 5.

DDT (dichlorodiphenyltrichloroethane).

DDT is a potent insecticide widely used in agriculture until the 1970's by which time

its propensity to accumulate in fat cells in living creatures lead to its disuse in most

situations. However in some locations, DDT is still used as a means of controlling

mosquitoes that carry malaria parasites.Apart from DDT, a number of other

organochloro compounds have been used as very effective insecticides but again, due

to their long lifetimes and unwanted indiscriminate effects, most have been banned

from sale.

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BROMINE.

Bromine, symbol Br, is a red-brown liquid at room conditions, boiling at 59oC. Like

the other halogens, it is a corrosive oxidizing agent and its reactivity lies between that

of chlorine and iodine. The halogen Group illustrates how the strength of

intermolecular attractions through dispersion forces increases with the number of

electrons (see Supplementary Topic 4) as reflected in the boiling points. The main

forces acting between the molecules in these non-polar diatomic elements are

dispersion forces. Thus at room conditions fluorine and chlorine are both gases,

bromine is a liquid and iodine is a solid.

Like chlorine and iodine, bromine is used as a disinfecting agent. Many of the uses

made of bromine in the past have been discontinued recently on health and

environmental grounds - for example some fire extinguishers once contained organic

bromo compounds but their ozone depleting properties have caused this use to cease.

Organic bromo compounds.

The main application for bromine is as a fire retardant. Organic molecules containing

Br atoms replacing some H atoms, when heated, release Br atoms which have the

property of extinguishing a fire. Organic bromo compounds are incorporated into

plastics which normally will burn readily if heated, for example in TV sets and laptop

computers if a fault develops and overheating occurs. Also bromo organic compounds

can be soaked into materials to reduce flammability. In these roles, bromine

compounds have undoubtedly saved many lives and much damage.

Drilling fluids.

When drilling oil wells and similar, the pressure outside the well must be maintained to

match that of fluids or gas inside the well. This is achieved using concentrated

solutions of bromide salts which are very dense because of the large size and mass of

Br! ions.

Mercury scavenger.

Bromide salts such as calcium bromide are sprayed onto coal just prior to its being

burnt in coal fired power stations. These salts remove the mercury which is always

present in coal and normally is released through the flu gases unless captured.

Photography.

Classical black and white photography using film is based on the ability of light to

reduce the silver ions in silver salts to silver metal. This is why silver salts and their

solutions are kept stored in dark containers to reduce light exposure. The most

favoured silver compound for photography is silver bromide, a cream, insoluble salt.

When light is focussed by the camera lens onto film which contains silver bromide in a

solid emulsion attached to a transparent base, the Br! ions are converted to Br atoms

XII - 53

and release an electron which the Ag+ ion accepts and converts to an Ag atom. The

more light striking a given spot on the film, the more Ag atoms are produced.

Developing processes amplify the effect and fixing removes any unreacted silver

bromide from the film. The image is reversed so the areas that would be white appear

as black on the negative produced while areas that received no light from the lens

appear transparent. To obtain a positive print, the process is used again, shining light

through the negative to expose silver bromide layered onto paper and developing the

print in the same way as was done for the negative. The Br atoms released in these

processes are removed in a layer containing a bromine acceptor on the top of the

emulsion . The exposed silver appears as black rather than shiny because the particle

size of the individual grains of silver is very small.

GROUP 18. He, Ne, Ar, Kr, Xe, Rn - the noble gases or inert gases.

Group Overview.

All occur as monatomic gases at room conditions - the only monatomic elements. They

have virtually no reactions due to the very stable atomic structure which has the

maximum effective nuclear charge for that period and which is therefore associated

with very large energy requirements for the gain or loss of electrons required for

reactions to occur.

Helium is found trapped in oil wells along with gas or crude oil. It occurs there

because helium is a product of the radioactive decay of large, unstable nuclei such as

uranium. Helium is an extremely useful substance because it has the lowest boiling

point of any element. It constitutes a significant part of the mass of the sun where the

nuclear fusion reaction of hydrogen to form helium provides most of the energy

released.

The gases neon, argon and krypton are all present in the atmosphere in small amounts -

argon to the extent of 0.9 %. Neon and argon are used in situations where an inert

atmosphere is required, such as in neon lighting tubes and in argon arc welding. The

element radon is a powerful carcinogen which is ubiquitous in all minerals where it too

arises from radioactive decay processes. Radon presents a particular hazard for miners

involved in uranium mining, but is also released from the burning of coal and even

from the clay of house bricks.

FOR THOSE WHO WANT TO KNOW MORE - helium.

HELIUM.

Helium is named after Helios, the sun god because it was first discovered by

spectroscopic analysis of sunlight in which previously unknown spectral lines were

observed. Later it was recognised as a gas released from some radioactive minerals

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where it originates from alfa particles expelled when unstable nuclei decompose. Alfa

particles contain two protons and two neutrons but no electrons so is in effect a helium

nucleus. The required electrons are picked up rapidly in air to form He atoms, atomic

number = 2 and mass number = 4. This nuclear arrangement of 2 p + 2 n has a

particular stability which accounts for why so much helium was formed in the short

interval following the hypothesised big bang. Helium is rare on earth because it is so

light that it escapes gravity and is only found trapped in the impermeable formations

along with natural gas and oil from which it is commercially separated. Helium is used

to dilute pure oxygen breathed by deep sea divers as it is non-toxic. When inhaled, it

causes one’s voice to take on a squeaky Donald Duck sound due to the much greater

speed of sound in the lighter atmosphere as the timbre of the voice is very dependent

on the speed of the sound waves. Its most obvious although trivial presence is as the

gas used to fill party balloons and blimps but it has a common industrial use as a gas to

protect metals from reacting with oxygen or nitrogen while being welded. Because of

its inertness, helium is used to provide an inert atmosphere when growing and handling

crystals of semiconductors such as silicon.

Cryogenic applications.

Helium has the lowest boiling point of any substance because of its small atomic size

and extremely weak intermolecular interactions. It liquefies at !269oC, a property

essential in superconducting magnets such as those used for magnetic resonance

imaging (MRI). At such low temperatures, electricity travels through copper wires

with no resistance allowing the generation of powerful magnetic fields that then can

interact with hydrogen atoms from which images of bodily structures and organs can

be obtained unobtrusively and without tissue damage. Likewise, many instruments

used for scientific research rely on the low temperatures achievable with liquid helium.

Although helium is one of the two most common elements in the universe, it is rare on

earth and there is concern that supplies of helium are not being husbanded carefully

enough.

The "d-block" elements.

Block Overview.

This is a block of elements located between Groups 2 and 13 following Ca, Sr and Ba.

In the current IUPAC system of Group numbering, they are numbered as Groups 3 to

12. They first appear after Ca in the fourth period of the Table, and there are ten d-

block elements in each period where they occur. Note that there are no d-block

elements in Periods 1, 2 or 3. They are all metals, most being hard with high melting

points, although mercury is a liquid at room temperature. Well known elements from

this block include chromium, iron, manganese, nickel, cobalt, copper, zinc, cadmium,

tungsten, platinum, silver, gold and mercury. Some of the special characteristics of

these elements include the following:

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(i) They all form cations in salts (e.g. MnCl2) but also can form covalent bonds to non-

metals such as oxygen (e.g. Mn in the polyatomic ion MnO4–).

(ii) They often have more than one ionic valence state (eg Fe occurs as the ions Fe2+

and Fe3+ in its compounds) as well as a number of valence states in covalent

compounds.

(iii) Their compounds are frequently coloured (eg copper(II) sulfate crystals - blue)

whereas compounds of other metals are usually white.

FOR THOSE WHO WANT TO KNOW MORE - vanadium, chromium,

manganese, nickel, technetium, tungsten, gold, mercury.

SCANDIUM (Z = 21) and YTTRIUM (Z = 39)

See “rare earths” in f-block

VANADIUM (Z = 23).

Vanadium is a silver-grey metal located in the first row of the d-block, between

titanium and chromium. Consequently it has many similarities to these two elements.

Vanadium added in very small amounts to steel forms alloys with vastly improved

strength and heat resistance, the latter being an essential for high speed drills.

Oxidation states and colours.

Vanadium has four common oxidation states, +II, +III, +IV and +V. Vanadium ions in

solution in each of these oxidation states have their own characteristic colours as

below:

+II lilac

+III green

+IV blue

+V yellow

The range of colours of vanadium compounds lead to its being named after the

goddess of beauty, Vanadis. Coloured compounds are characteristic of d-block

elements and result from the large number of electrons and transitions between orbitals

that are available in their outer levels. This is in contrast to s and p-block elements

which rarely form coloured compounds.

Vanadium flow battery.

Transitions between the various oxidation states of the V atom is the basis for the

vanadium flow battery. In the usual rechargeable or non-rechargeable batteries one or

both of the electrode materials undergoes a redox reaction. These redox reactions

involving the electrodes proceed in one direction when discharging and in the reverse

direction when charging. In the vanadium flow battery, the electrodes are inert and

merely act as part of the external electron transport. The redox reactions take place

entirely through vanadium ions in the electrolyte in each half cell changing their

XII - 56

oxidation states. Thus it is oxidation and reduction of the electrolyte rather than the

electrodes that provides the electron flow.

The following diagram illustrates a vanadium battery.

When discharging, in the negative half cell vanadium ions undergo the change from

the +IV (VO2+) to the +V (VO2+) states with the release of electrons through the

external circuit where they move to the positive electrode. There another vanadium

solution undergoes the change from the +III state to the +II state. The electrodes made

of an inert material play no part in the operation of the cell which is maintained by

pumping electrolytes of each type through the relevant half cell. For this reason, it is

known as a flow battery. To reverse the reaction and recharge the cell, the spent

electrolyte solutions are pumped back through their half cells while the voltage needed

to reverse the reaction is applied to the electrodes. The charge balance within the cell

is achieved by H+ ions migrating through a suitable membrane that keeps the vanadium

ions from intermixing. This type of cell has many advantages - it can be reused an

unlimited amount of times and can hold/supply large amounts of electricity, limited

only by the size of the electrolyte tanks. The main drawbacks are related to the bulk

which limits their use to fixed applications such as back up for the grid rather than for

transport.

Sulfuric acid manufacture.

One of the main uses for vanadium is for the oxide, vanadium pentoxide, which is used

as a catalyst for the conversion of SO2 to SO3 in the contact process by which sulfuric

acid is produced. In this process, sulfur is burnt or sulfide compounds are heated to

release sulfur dioxide gas which needs to be further oxidized to sulfur trioxide before

reacting with water to form sulfuric acid. Without the catalyst the process is too slow

to be commercially viable.

XII - 57

CHROMIUM (Z = 24).

Chromium is a shiny metallic element found in ores often in company with iron. The

name comes from the Greek, “chroma”, meaning “colour”. This relates to the many

chromium compounds which are brightly coloured and are used as pigments in paints

and elsewhere. More recently these uses have diminished due to health concerns about

ingested chromium compounds.

When chromium is mixed with iron, stainless steel results. First developed in 1913,

stainless steel largely resists rusting unlike normal steel because the chromium atoms

form an extremely hard oxide on the surface and this prevents the attack on the Fe

atoms by water and oxygen. Up to 30 % of chromium is mixed with iron to make

stainless steel and the higher the chromium content, the more resistant it is to

corrosion. Unlike iron, chromium is not magnetic and high quality stainless steel can

be detected by its inability to be attracted to a magnet. Alloys of chromium and iron,

sometimes with other metals, are used for high temperature environments such as the

valves in car engines. The property of extreme hardness coupled with the ability to

resist corrosion is the reason that chromium plating is much used for both protection

and appearance. When a thin layer of chromium atoms is plated onto normal steel and

polished, a bright shiny surface results giving the familiar chrome plating used to

decorate and protect many types of metal objects and notably, on motor cars from

previous eras.

Chromium also is used in the tanning of leather. Chromium ions seep into the hide and

cross link the collagen polymers of the skin, making it more pliable and water resistant

and able to absorb coloured dyes.

Rubies and emeralds are crystals of aluminium oxide and they owe their colours to Cr

atoms occupying places within the crystals. This is possible because Al and Cr atoms

are of similar size so a chromium ion can easily replace an aluminium ion in a crystal

lattice. Ruby crystals were used in the first lasers where the properties of the imbedded

Cr ions allow the production of the coherent light which lasers produce.

However chromium in one of its many oxidation states is very toxic and can cause a

range of medical problems including potentially cancer. Chromium in oxidation state

VI also known as hexavalent chromium as for example in dichromate ions, Cr2O72–,

was shown to be associated with widespread illnesses among the population of the

town of Hinkley in California by Erin Brockovitch. In a landmark class action in 1996,

a fine of $333M was awarded against a gas company which had been dumping

polluted water into the groundwater which was later drunk by local well users. Her

work which was also featured in the film bearing her name as the title, has lead to

stringent limits on the amount of hexavalent chromium allowed to be released into the

XII - 58

environment worldwide. One consequence has been a much reduced use of chrome

plating which, along with tanning, used to be responsible for some of that release.

MANGANESE (Z = 25).

Manganese, symbol Mn, shows all the characteristic properties of d-block elements

especially the wide range of colours of its compounds and the range of valence states

(oxidation numbers). The element has a silvery grey appearance and occurs as

compounds in minerals, the dioxide MnO2 being the most common. Manganese is a

vital atom in the photosynthetic processes used by all plants to generate oxygen. Small

amounts are also essential for human health, playing a vital part in development,

metabolism and antioxidant processes.

Applications.

Most of the manganese mined is used in the production of steel and as a component of

alloys with iron and aluminium. Manganese dioxide is used as one component in the

standard zinc dry cell. The brown staining liquid from leaking dry cells is this oxide

material.

Oxidation states.

Manganese atoms can exhibit oxidation states from !III through to +VII. The most

common and examples of their occurrence are listed below.

+II MnCl2 manganese(II) chloride pink

+IV MnO2 manganese(IV) oxide or manganese dioxide brown

+VI K2MnO4 potassium manganate green

+VII KMnO4 potassium permanganate purple

The range of oxidation states available to Mn atoms in compounds is the basis for its

use as an oxidant, especially as the permanganate ion and as manganese dioxide. The

oxidizing power of these compounds increases as the number of O atoms bonded to the

Mn atom increases.

Coloured compounds.

The d-block elements are remarkable for having many coloured compounds as distinct

from the compounds of the s and p blocks. This property is a consequence of the large

number of outer electrons and available orbitals. The colours of solutions of the above

compounds is listed on their right.

NICKEL (Z = 28).

Nickel is another metal that resists corrosion because a protective oxide layer forms on

its surface. Nickel plating is similar to chromium plating in that it affords a corrosive

metal such a iron a shiny and protective surface. Nickel is used as one component of

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the common rechargeable batteries known as NICAD BATTERIES (for

nickel/cadmium) batteries as well as other types. While on its own, nickel has many

applications, its most widespread use is as a component of many alloys (mixtures of

metals), especially stainless steel which is an alloy of nickel, iron and chromium. There

are over 3000 different alloys of nickel mixed with other metals, each tailored for

specific purposes. A particularly valuable alloy of mostly nickel with copper is monel

which is very strong and highly resistant to corrosion. It has particular uses in marine

environments where it is very resistant to corrosion in salt water. An alloy of nickel

with iron, aluminium and cobalt known as ALNICO can be magnetised to form

stronger and more lasting permanent magnets which have only recently been

superceded by rare earth magnets. Some alloys of nickel are known as super alloys

because of their strength and extreme tolerance of heat. The turbine blades of jet

engines make use of these alloys which contain as many as ten other elements apart

from nickel and are able to resist the extreme temperature and high speeds of rotation

experienced. Modern jet engines would not be possible without various nickel alloys

used in their construction. The compositions of each alloy is designed for a specific

purpose in the engine.

Catalysts.

Apart from its role in alloys, nickel in a finely powdered form is used as a catalyst for

many chemical reactions which would be too slow without catalytic participation. One

common application as a catalyst is in the hydrogenation of vegetable fats (which are

usually liquids at room temperature) to form solid margerine as a substitute for butter.

Shape memory alloys.

Some alloys of various metals have a shape memory whereby if the metal is distorted

from its initial shape such as a straight wire to a twisted or bent shape and then warmed

to a specific temperature, the wire returns to its previous exact state. One such alloy

contains a mixture of about 55 % nickel and the remainder is titanium, so it is known as

nitinol. This self-expanding property of nitinol lends it to a particularly useful

application as stents to expand blocked arteries in the heart. The stent can be made to

the desired state at body temperature and then rolled into a suitable volume in order to

be inserted through an artery in the groin and into the desired location in the heart.

Once warmed to the original body temperature at which the stent was made, it opens up

to the required shape in situ. This type of stent is replacing the older balloon

expandable type previously used.

TECHNETIUM (Z= 43).

The existence of element 43 was predicted by Mendeleev in his Periodic Table (1871).

He named it as ekamanganese using his system for undiscovered elements, in this case

because its position is one place below manganese in the Table. However technetium is

not to be found in the normal manner of the elements because it has no stable isotopes,

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the longest lived being 98Tc of half life 4.2 × 106 years. Instead, it can only be obtained

from decay products of fuel rods in nuclear reactors which gives it the name technetium

and symbol Tc. There are numerous other isotopes of Tc each with its own

characteristic half life. Due to its relatively short half life compared to the age of the

earth, no significant amounts of technetium can be found naturally occurring but bulk

quantities are extracted from spent fuel rods of nuclear reactors.

Nuclear medicine applications.

One isotope of technetium, 99Tc, can exist is what is called a meta stable state where the

nucleus has a higher energy and it is designated as 99mTc. This isotope has a very short

half life of 6 hours and when it decays to the more stable form, 99Tc, it emits a gamma

ray. Gamma rays (ã rays) are high energy electromagnetic radiation like visible light

but of much higher energy. Gamma rays can pass through significant thicknesses of

materials without being absorbed and in the case of tissues, without doing significant

damage to cells. Use is made of this property to scan organs such as the heart by

injecting a solution of a salt of 99mTc into appropriate blood vessels and detecting the ã-

rays as they emerge with a ã ray camera. The resulting pictures show areas where for

example, plaque has built up in arteries or damage has occurred to the heart muscle.

Having such a short half life means the radioactive isotope is quickly cleared from the

body so it does little ongoing damage. This procedure is so widely used that millions of

scans of this type are conducted world wide ever year. The problem with using 99mTc

for diagnostic radiation testing is its half life is so short that it must be used quickly

after preparation. To overcome this to a certain extent, it is made in situ from a

radioactive isotope of the element molybdenum, 99Mo, which decays to form 99mTc.

The molybdenum isotope is mostly made in older style nuclear reactors and it has a half

life of 2.75 days so it can supply the required 99mTc for about a week before a new

supply is required. There is concern that there may be world wide shortages of supply

of 99mTc because most of the reactors that are used to produce the molybdenum isotope

are at the end of their operating lives. In Australia, the Opal reactor at ANSTO in

Sydney produces the molybdenum isotope for distribution throughout the country and

to nearby Pacific nations.

TUNGSTEN (Z = 74).

The symbol for tungsten, W, comes from the name wolfram by which it was known in

some European countries. Tungsten is one of the hardest elements and it has the

highest melting point of any metal, 3422oC. Its density is almost twice that of lead and

nearly twenty times greater than water as a result of the large number of protons and

neutrons in its atoms. Once a method of drawing tungsten into wires was developed, it

found a widespread use as the filament in incandescent electric light globes, replacing

tantalum which had been used earlier. The globe contains argon to prevent any

tungsten oxide forming which darkens the glass. Because of the very high melting

point of tungsten, an electric current passed through it causes the filament to glow

XII - 61

without melting. However, in the process, 95 % of the electrical energy is converted

to heat rather than light so this type of globe is now rapidly being displaced by more

efficient sources. It is still used in X-ray tubes in which electrons from a heated

tungsten filament hit a target, itself often made from tungsten, and X-radiation is

released. Tungsten is mainly used in various alloys which benefit from the increased

hardness and heat resistance, for example in high speed steel which contains as much as

20 % of tungsten.

Tungsten carbide.

This compound of formula WC is made by heating a mixture of tungsten and carbon to

temperatures up to 2000oC. It too has a very high melting point, 2870oC, and is

extremely hard. These properties make tungsten carbide an ideal material for industrial

grinding and cutting. Drill bits and saw blades are often tipped with tungsten carbide.

It is also used in armoury such as bullets and protective sheeting for military vehicles

such as tanks. The ball tip in the end of ball point pens is often made from tungsten

carbide.

GOLD (Z = 79).

Gold is one of the few elements that occur as the free element in nature and its symbol,

Au, is from the Latin, aurum. It is a soft, dense metal and along with copper, one of the

two coloured metals. Gold occurs as nuggets in surface layers of the ground and also

as inclusions in rocks such as quartze. It also occurs free in large veins underground -

the largest piece found was about 1.5 m long discovered at Hill End. Its limited supply

and demand for jewelry and as a currency hedge and industrial usage have all

contributed to the high price gold commands. Gold is extremely unreactive and does

not corrode in air or water. It is an excellent conductor of electricity and is used in

electronics where reliable contacts are essential. All mobile phones contain tiny

amounts of gold. Because of this and other valuable metals present, mobile phones are

crushed and several elements reclaimed. Despite the small amount of gold present,

crushed phones contain a much greater concentration than in the original ore from

which they were extracted.

Medical applications.

Because gold is so inert, it has uses in medicine. Gold nanoparticles have the ability to

bind to antibodies and drugs and therapeutic agents and deliver them to the desired part

of the body, evading possible defence mechanisms. In cancer therapy, gold

nanoparticles have been found to be taken up more by tumours than healthy tissue so

anti-cancer agents can be concentrated near to the site where they are needed. In some

applications, adsorbed gold nanoparticles at the tumour sites can be irradiated with near

infra red light which then is converted to heat of sufficient intensity to destroy

malignant tissues.

Gold nanoparticles are also finding applications in the new area of gene therapy.

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MERCURY (Z = 80).

Mercury, named after the god of speed, has the symbol which is derived from the Greek

name, hydragyrum. It has been known for thousands of years as it is easily obtained by

heating the ore cinnabar which is mercury(II) sulfide and which occurs in highly

concentrated deposits. Mercury is the only metal that exists as a liquid at room

conditions. Its shiny appearance is well known as are some of its common uses such as

in thermometers and blood pressure monitors. Less well known is that the largest use

of mercury today is for the extraction of gold by small scale miners who mix mercury

with low concentrations of gold such as from panning. Mercury can dissolve a number

of metals including gold to form an amalgam and then be removed by heating to leave

the concentrated gold, a process aided by mercury’s relatively low boiling point of 630oC. This process was also used by large scale miners in the past but now combination

of gold with cyanide is used commercially. In past eras, mercury and its compounds

were used as medicines and as cosmetics. Mercury is used in tiny amounts in all

fluorescent light tubes and the compact fluorescent light bulbs which have replaced

tungsten filament globes. The topical antiseptic, mercurochrome, is found in most

home medicine kits. Until a few decades ago, the most common material used for

dental fillings was an amalgam of mercury with silver, tin and copper. About 50 % of

the filling is mercury. Small amounts of mercury metal do leach out of these fillings so

people sometimes opt to have them replaced using modern materials which adhere to

the tooth rather than being wedged in as was the case with amalgams.

Health concerns.

Mercury ingested or inhaled is a deadly nerve toxin and in recent years, most of its

former uses have been supplanted by safer alternatives. Currently an effort is being

made to ban many of its traditional applications and to ultimately discontinue mining it.

To reduce pollution, fluorescent light tubes are recycled to remove the mercury content.

Coal fired power stations release mercury into the atmosphere and efforts are now made

to remove this source of pollution. Even thermometers containing mercury are being

replaced by alternatives.

Organo compounds of mercury such as methyl mercury are the most dangerous. This

danger was made very evident in Japan in 1956 when residents of a town called

Minamata fell ill in large numbers. The cause was a mystery but food poisoning of

some type was suspected as even local cats were dying. Ultimately it was traced to the

release of methyl mercury from an industrial plant into the nearby bay. The mercury

compound was concentrated by small aquatic organisms which in turn were eaten by

larger creatures up the food chain to fish and shellfish which contained dangerous

levels. Many thousands of people died or were severely affected by eating the

contaminated seafood. Despite the cause being known, the dumping of methyl mercury

into the bay there continued until 1968. Mercury poisoning by organomercury

compounds has subsequently been named as Minamata disease.

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The f-block elements.

Block Overview.

The f-block is located near the bottom of the Table and contains 14 elements in each of

Periods 6 and 7 using the 4f and 5f orbitals respectively. The 14 elements using the 4f

orbitals following lanthanum are known as the lanthanides and those using the 5f

orbitals following actinium are known as the actinides although neither lanthanum nor

actinium are f-block elements. They are all metals and many of the actinides have

unstable nuclei. Particularly important actinides are the elements uranium (U) and

plutonium (Pu) because of their relevance to the nuclear fuel cycle. The many outer

level electrons in the lanthanides impart unique magnetic properties to these elements

and colours to their compounds . The lanthanides are known as the RARE EARTHS,

a grouping which is frequently expanded to include the elements scandium and yttrium

from Group 3. The rare earths mostly are not rare but they generally do not occur in

sufficiently large concentrations to be mined profitably. Mining and purifying these

elements is a very polluting operation which is now almost exclusively done in China

where environmental controls are weak. However, some of these rare earth elements

have taken on great importance in modern technology and have become indispensable.

For example, rare earth elements are vital in the production of lasers and tiny magnets

used in computer drives as well as the larger powerful magnets used in electric motors

and generators.

(a) The lanthanides.

Lanthanum (La) - not strictly an f-block element as it is in the d-block in Group 3;

cerium (Ce); promethium (Pm) - not mined but synthesised in nuclear reactors;

praseodymium (Pr); neodymium (Nd); samarium (Sm); europium (Eu); gadolinium

(Gd); terbium (Tb); dysprosium (Dy); holmium (Ho); erbium (Er); thulium (Tm);

ytterbium (Yt); lutetium (Lu). Among the many useful properties of these elements and

their compounds are the following:

Coloured compounds.

All of the elements in this row of the f-block have coloured compounds and when their

ions are excited they emit light of very specific wavelengths in their atomic spectra.

Erbium is particularly useful as an amplifier in the fibre optic cables used for internet

transmission. Transmission of light along fibre optic cables requires the signal to be

amplified at regular intervals and erbium compounds are the ideal materials to do this.

Erbium ions are incorporated into the walls of the cable at intervals and are energised to

an excited state with laser light. Light travelling along the fibre stimulates the ions to

release the stored energy which is of exactly the required wavelength to boost the

signal.

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Small crystals of neodymium compounds are used to produce the green light from

lasers which for example are used in the familiar green pointers. Various mixtures of

rare earth elements and their compounds are the essential ingredients of phosphors used

in screens - for example phosphors based on terbium provide the yellow-green colours

while europium in conjunction with the rare earth from Group 3, yttrium, provides the

red phosphor. The amounts of these elements and their compounds used in the screens

are tiny but the quality of colour TV prior to their inclusion was very inferior. Terbium

compounds mixed with the element europium are also used in the long life fluorescent

light globes to provide the more favoured warm coloured light.

Magnetism.

A mixture of the elements neodymium, boron and iron allows the manufacture of

magnets which are 12 times stronger than those made from of the same weight of iron.

Consequently magnets made from neodymium can be much smaller than comparable

iron magnets and are essential for making the tiny motors used in the hard drives of

laptops. More significantly, the power of such magnets allows the efficient production

of electricity by wind turbines and has made electric vehicles practical. The new

generation of wind turbines is able to dispense with the gear boxes which used to be

needed to synchronise the blade revolutions with the requirements of the electrical

generators they drive by using 648 neodymium and dysprosium magnets set in a

doughnut shape directly on the rotating axle of the blade. Dysprosium in small amounts

when added to neodymium alloy magnets prevents the loss of magnetism which occurs

when the temperature exceeds 300 oC. Magnets made from samarium and cobalt resist

demagnetising when exposed to nuclear radiation which is particularly important in

nuclear power stations. Holmium is another 4f element which is used in high strength

magnets.

Other applications.

Cerium is used for fine polishing of liquid crystal displays and as a catalyst in the

catalytic converters fitted to the exhaust systems of motor cars.

Gadolinium compounds are used in magnetic resonance imaging as a contrast dye.

Lanthanum and cerium mixed are used as one of the electrodes in the nickel metal

hydride rechargeable battery commonly used to power electric cars.

Yttrium is incorporated into high temperature resistant ceramics that are essential in jet

engine turbine blades and rocket exhaust systems and similar applications.

Promethium is used in atomic batteries for specialised purposes.

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(b) The actinides.

Actinium (Ac); (like lanthanum, this element is not strictly a member of the f-block and

belongs to Group 3 in the d-block but is usually grouped with the adjacent f-block

elements); thorium (Th); protactinium (Pa); uranium (U); neptunium (Np); plutonium

(Pu); americium (Am); curium (Cu); berkelium (Bk); californium (Cf); einsteinium

(Es); fermium (Fm); mendelevium (Md); nobelium(No); lawrencium (Lr).

All the actinides are radioactive. Only uranium and thorium occur naturally in large

quantities so the others require nuclear reactors or particle accelerators to produce them

from smaller atoms. Plutonium is one such element produced in nuclear reactors and it

like uranium is used in both power stations and nuclear weapons. Thorium is currently

being used in a new prototype nuclear power plant where it would have many

advantages over the conventional fuels, especially from safety aspects. Thorium

reactors do not produce plutonium which was required for nuclear bombs and it seems

that this is the main reason thorium reactors were not the first choice for power

generation. Americium is used as the ionizing source in most household smoke

detectors.

FOR THOSE WHO WANT TO KNOW MORE - uranium

URANIUM

Uranium occurs as two common isotopes, 235U and 238U. Both are slightly radioactive

but only 235U is fissile - its nucleus when subjected to impacting neutrons breaks into

two smaller nuclei such as 92Ba and 141Kr and releases 3 neutrons per decaying nucleus.

Accompanying this fission process is the release of much energy as heat. The fission

process occurs naturally in the ore but normally only at a very slow rate. This property

is harnessed in nuclear reactors and also atomic bombs. In order to produce significant

fission, it is necessary for neutrons to be available to travel through the 235U nuclei in

sufficient quantities for the reaction to be self sustaining. Remember that neutrons

carry no electrical charge so can penetrate the positively charged nucleus without

experiencing electrostatic repulsions. When a 235U nucleus undergoes fission the three

neutrons released can, if the uranium is packed suitably, cause more nuclei to break

apart and start a self-sustaining chain reaction. In a nuclear reactor, the process is

controlled by surrounding the fuel with a moderator such as water to slow the neutrons

to a suitable speed lest they travel through nuclei without setting off fission. To stop

the reaction, control rods of material such as boron or rare earths which absorbs

neutrons readily, are suspended above the fuel rods and they can be lowered as needed.

In an atomic bomb, two sub-critical pieces of 235U are held well apart. To set off the

reaction, they are rammed together to form a critical mass in which the extra neutrons

are enough to provide an uncontrolled fission reaction. In a nuclear power station, the

heat from the fission reaction is captured and used to convert water to steam at high

XII - 66

pressure and this turns a generator to produce electricity. One kilogram of uranium fuel

can produce the same amount of electricity as 1500 tonnes of coal.

The other common isotope, 238U, sometimes called depleted uranium, is not fissile but

has some applications resulting from its extremely high density which is 70% greater

than that of lead. Military uses include providing armour for tanks and casing

explosive shells.

Separating uranium isotopes.

Uranium ore contains 99.3 % 238U and just 0.7 % 235U which must be enriched in order

to have suitably concentrated fuel. Being isotopes, they are chemically identical so

physical methods are the only way to do this. The physical difference between the

isotopes which is exploited is their differing masses. Although only small, by repeating

mass separating processes many times, a suitable degree of enrichment is obtained.

The first method used was gas diffusion in which the uranium is converted to gaseous

UF6 and the slightly more rapid diffusion of 235UF6 over many repeated diffusion steps

is able to be enriched to 3 %. For weapons grade, a higher concentration is needed.

More recently, repeated sequential centrifuging of uranium compounds has been

developed as a more efficient method.

Dating.

The very long half life of 238U, 4.5 × 109 years, makes it ideal for dating old rocks as

uranium is reasonably prevalent in the earth’s crust. The isotope 235U has a half life of

704 million years likewise is suitable for dating rocks of lesser age.

Hydrogen

Hydrogen does not fit into any Group of the Table due to its unique property of having

just one electron. Like Group 1 elements, it forms the +1 ion, but only in association

with a molecule such as water because H+ would be a naked proton, incapable of free

existence. Hydrogen is also similar to Group 17 elements (halogens), existing as an H–

ion in ionic compounds, e.g. sodium hydride (NaH). However, ionic hydrides are

unstable, reacting with water to form hydrogen gas and OH– ions.

Metals vs non-metals - distribution throughout the table.

The metallic elements are located on the left hand section of the Table and the non-

metals are on the right. Although the Periodic Table was originally devised purely on

the basis of macroscopic properties, the true underlying basis for it is now understood

in terms of the structure of the atoms of the elements. Metallic properties increase

down each Group, even amongst those on the extreme left, because the atomic radius

increases down each Group and outer electrons are more easily removed the further

they are from the nucleus of the atom. The non-metals are on the right because they

XII - 67

have more electrons in the same outer orbit which in turn are held more tightly by the

increasing nuclear attraction from the greater number of protons, causing the atomic

radius to decrease from left to right contrary to what one might have expected. Thus

atoms of non-metals are more able to gain electrons to form anions rather than lose

them to form cations. At the conclusion of each Period of the Table, the noble gases

represent the most stable outer electron arrangement as the next element has its last

electron located in a new energy level further from the nucleus and held less strongly,

allowing that element to take on the properties of a metal again. This sequence of metal

v non-metal is repeated for each Period of the Table. The following table gives a

guide to the general distribution of the metals compared with the non-metals in the

Periodic Table. Between the metals and non-metals, towards the middle of the Table,

some of the elements show properties of both and are called metalloids or semi-metals.

The predictive power of the Periodic Table.

When Mendeleyev devised his version of the Periodic Table, he was able to use it to

predict that a number of then unknown elements existed and further, was able to

make accurate predictions about the properties of those elements and some of their

compounds. However, the predictive power of the Periodic Table is by no means

limited to those initial triumphs. It has guided much successful chemical research

and still does today. The following is an example of where researchers have

employed a knowledge of the Periodic Table to guide their work.

The Nobel prize winner of 2010 in chemistry, Ei-ichi Negishi, won his prize for

work on palladium catalysis in such applications as the anti-cancer drug Taxol. In an

interview with New Scientist (16 October 2010) he said: “Years ago when I started

in chemistry, I was awed by the row of transition metals [containing palladium].

They are the most gifted bunch in the periodic table. I work with the Periodic Table

in front of me at all times and approach all challenges in terms of three particles:

positively charged protons, negatively charged electrons and neutrally charged

neutrons. That’s science.”

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DISTRIBUTION OF METALS AND NON-METALS WITHIN THE PERIODIC TABLE

H

FFnon-metal

Group

18

Group 1 Group 2 Group

13

Group

14

Group

15

Group

16

Group

17

He

Fnon-metal

Li

Gmetal

Be

Gmetal

B

:::non-metal

C

:::non-metal

N

FFnon-metal

O

FFnon-metal

F

FFnon-metal

Ne

Fnon-metal

Na

Gmetal

Mg

Gmetal

Al

Gmetal

Si

:::metalloid

P

FFnon-metal

S

FFnon-metal

Cl

FFnon-metal

Ar

Fnon-metal

K

Gmetal

Ca

Gmetal

Ga

Gmetal

Ge

:::metalloid

As

:::metalloid

Se

FFnon-metal

Br

FFnon-metal

Kr

Fnon-metal

Rb

Gmetal

Sr

Gmetal

In

Gmetal

Sn

Gmetal

Sb

:::metalloid

Te

:::metalloid

I

FFnon-metal

Xe

Fnon-metal

Cs

Gmetal

Ba

Gmetal

Tl

Gmetal

Pb

Gmetal

Bi

Gmetal

Po

Gmetal

Rn

Fnon-metal

Fr

Gmetal

Ra

Gmetal

Bond type in the element

G metallic ::: network covalent

FF molecular covalent F monatomic (noble gases only)

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Objectives of this Topic.

When you have completed this Topic, including the tutorial questions, you should have

achieved the following goals:

1. Understand that the Periodic Table was originally devised on the basis of

families of elements with similar properties, arranged in order of increasing

atomic weight (but subsequently found to be increasing atomic number).

2. Know the electronic basis for the similarities and trends within Groups of the

Periodic Table.

3. Commit to memory the elements of Groups 1, 2, 13-18.

4. Know the general distribution of metals and non-metals within the Table.

5. Have some familiarity with the main chemical properties of each Group.

Further resources.

Video

The video shown in the lecture can be viewed at

1. http://www.youtube.com/watch?v=M-lnauoORdA

On YouTube where there are many other videos relating to the Periodic Table also

available for viewing.

Audio

The BBC series titled “Elements” is available from their website as Elements Podcasts.

Recommended follow up chemcal modules:

Section: Properties of Atoms

Module: Atomic Properties

Topics covered: The concept of core charge and its relationship to fundamental atomic

properties.

Module: Electronic Structure of Atoms and Ions

Topics covered: Trends in atomic properties in relation to the Periodic Table

Section: Properties of Molecules

Module: Electronegativity and Polar Molecules

Topics covered: Electronegativity in relation to periodic table; polar and non-polar

molecules.

XII - 70

SUMMARY

The classification of elements into groups with similar chemical and physical properties

was begun prior to any knowledge of the structure of atoms and when the existence of

undiscovered elements was overlooked. Purely on these bases, it was apparent that

some elements having similar properties could be grouped together as families. With

the measurement of atomic weights of elements came attempts to find correlations of

properties with atomic weight order, but these were hampered by the many elements

that had not been isolated at that time and also by inaccurate atomic weight

determinations. Mendeleev recognised these deficiencies and turned them to

advantage, still using atomic weight order but giving primacy to assigning elements

with similar properties to the same Group. He left gaps in the arrangement of the

elements where needed and predicted which elements were still to be discovered and

their likely properties. This arrangement of the elements, called the Periodic Table, was

further refined when the structure of the atom was elucidated and it was then realised

that the order of the elements in the Periodic Table should be atomic number order

rather than atomic weight order. From today’s knowledge of atomic structure, the

reason for similarities of chemical properties within any Group of elements has been

clearly established as a consequence of each element in any Group having the same

outer-shell electron arrangement. However, gradations in properties within a Group are

normally observed, with the most metallic elements being at the bottom. Within any

row (Period) of the Table, the elements on the left are metals and on the right are non-

metals, a gradual increase in non-metallic properties from left to right being exhibited.

Some elements towards the middle of the Table show properties of both metals and

non-metals. These trends can also be related to aspects of atomic structure, in

particular the decreasing effective nuclear charge of atoms down each Group which

leads to less energy being required to remove electrons and thus form cations - a

property of metals. Elements to the right of the Table are non-metals because they have

more electrons contained within the same outer level of the atom and are accompanied

by a corresponding increase in the number of protons in the nucleus and thus an

increasing effective nuclear charge, resulting in all the electrons being held more

tightly. Thus more energy is required to remove electrons from non-metals - instead,

formation of anions or covalent bonding are their energetically preferred options. The

least reactive of all elements - the noble gases - have the largest effective nuclear

charge which explains their near complete lack of reactivity.

The Periodic Table is an elegant example of how observations and the collection of

data, development of hypotheses, predictions made from them and testing of each

hypothesis followed by discarding or refining it leads to increased understanding in

science.

Before commencing the questions associated with Topic 12, complete any

remaining questions from Topics 11, 7, 8, 9 and 10 in that order.

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TUTORIAL QUESTIONS - TOPIC 12

1. List the physical properties of metals compared with non-metals.

2. How do the chemical properties differ for metals compared with non-metals with

regard to the following:

(a) Type of ion formed in salts (b) Reaction with acids

3. Disregarding hydrogen and helium, for the Periodic Table Groups 1, 2, 13 - 18, what

electronic structural feature do all the atoms in any given Group have in common?

4. (a) Write the names and symbols for the elements of Group 1

(b) List some properties of Group 1 elements which indicate they are all metals.

(c) What valence do all Group 1 elements exhibit in their compounds?

5. (a) Write the names and symbols of the elements of Group 2.

(b) What properties would indicate that beryllium could in some respects be

more appropriately regarded as a non-metal?

(c) Aside from beryllium, list properties of the other members of Group 2

which indicate they are metals.

(d) What valence do all Group 2 elements show in their compounds?

6. (a) Write the names and symbols of the elements of Group 13.

(b) Is boron a metal or a non-metal? Give reasons for your answer.

(c) Why is aluminium a useful structural material even though it reacts readily

with water and oxygen?

(d) What is the usual valence shown by elements of Group 13 in compounds?

7. (a) Write the names and symbols of the elements of Group 14.

(b) Which elements of Group 14 could be best regarded as metals rather than

non-metals? Give evidence to support your answer.

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(c) Diamond and graphite are both forms of pure carbon. Compare their

physical properties and explain the differences in terms of arrangement of their

atoms.

8. (a) Write the names and symbols of the elements of Group 15.

(b) What type of bonding is present in elemental nitrogen?

(c) Give the names and formulas for some species to which nitrogen is converted

in the process of nitrogen fixation.

(d) Summarise the metallic/non-metallic properties of the Group 15 elements.

(e) In what way does bismuth differ from the other members of Group 15?

9. (a) Write the names and symbols for the elements of Group 16.

(b) Summarise the metallic/non-metallic properties of Group 16. List properties

which support your answer.

10. (a) Write the names and symbols for Group 17 elements (halogens).

(b) Explain why the halogens all have low melting and boiling points.

(c) Why are halogens strong oxidising agents?

(d) List all the elements that occur naturally as diatomic molecules.

(e) What properties confirm that Group 17 elements are all non-metals?

11. (a) Write the names and symbols for Group18 elements (noble gases).

(b) Why do all the elements of Group 18, the noble gases, occur as monatomic

gases in nature?

12. (a) Where in the Table do the elements of the d-block appear?

(b) While the d-block elements are all metals, these elements show some

properties in general that differ from the metals of Groups 1 and 2. What are

these different properties?

13. Summarise the occurrence of metals and non-metals in the Periodic Table.

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14. Where in the Periodic Table are those elements which are (a) most easily

oxidised and (b) those most easily reduced located? Explain why this is so.

15. Define the terms “effective nuclear charge” and “screening” in the context of

atomic structure. How does each relate to the properties of the elements as

arranged in the Periodic Table?

ANSWERS TO TUTORIAL TOPIC 12

1. Metals: shiny when freshly cut, malleable, ductile, good conductors of heat and

electricity

Non-metals: dull solids, powders or gases, brittle, poor conductors of heat and

electricity

2. (a) Metals form cations in reactions that produce salts while non-metals form

anions in those reactions.

(b) Many metals react with acids forming cations as part of a salt while non-

metals do not react with acids.

3. All the elements in any Periodic Table Group have the same arrangement of

electrons in their outer level.

4. (a) lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs)

(b) They all have the physical properties listed for metals above; they all form

cations in reactions with acids to produce salts.

(c) Group 1 elements always have a valency of 1 in their compounds.

5. (a) beryllium (Be) magnesium (Mg) calcium (Ca) strontium (Sr) barium (Ba)

(b) Beryllium does not form cations in its compounds, instead it is usually

covalently bonded.

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(c) They all display the physical properties of metals given above; they all form

cations in reactions with acids to produce salts.

(d) Group2 elements always have a valency of 2 in their compounds.

6. (a) boron (B), aluminium (Al), gallium (Ga), indium (In), thallium (Tl)

(b) Boron is a non-metal as it is a black powder, does not form salts with acids

and is only covalently bonded in its compounds.

(c) Aluminium reacts with oxygen in the air to produce an oxide which adheres

strongly to the surface of the metal and protects it from further corrosion in the

atmosphere.

(d) Group 13 elements mostly have a valency of 3 in their compounds.

7. (a) carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb)

(b) tin and lead behave more like metals than the rest of Group 14 because they

have the physical properties of metals and form cations in some of their

compounds with non-metals.

(c) Carbon atoms in diamond are arranged in a very stable tetrahedral structure

with each C atom bonded to four other C atoms. The very stable structure of

diamond imparts the properties of considerable hardness and non-conduction of

electricity. In graphite, each C atom is bonded to just three other C atoms in a

planar arrangement, leaving one unused valence electron on each atom. These

unused electrons form weak partial bonds to C atoms in the planes above and

below. Consequently graphite is soft and the planes of C atoms are easily peeled

apart (as in lead pencils). Also graphite can conduct electricity because the

electrons between the planes are so weakly held that they are mobile under the

influence of an electrical voltage.

8. (a) nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi)

(b) Elemental nitrogen consists of molecules consisting of two N atoms

covalently bonded by a triple bond. This is termed molecular covalent bonding.

XII - 75

(c) ammonia (NH3), ammonium ion (NH4+), nitrogen dioxide (NO2), nitrate ion

(NO3–), nitrite ion (NO2

–).

(d) Nitrogen and phosphorus are clearly non-metals. Arsenic and antimony

have properties of both metals and non-metals. Bismuth is more metallic than

non-metallic.

(e) Bismuth forms many ionic compounds containing the Bi3+ cation and it has

metallic bonding in the elemental state.

9. (a) oxygen (O), sulfur (S), selenium (Se), tellurium (Te).

(b) All the elements of Group 16 are non-metals. They have the usual physical

properties of non-metals, do not dissolve in acids and they form anions when in

compounds with metals. Their compounds with other non-metals are covalently

bonded.

10. (a) fluorine (F), chlorine (Cl), bromine (Br), iodine (I).

(b) The halogens occur as diatomic molecules which only have weak forces of

attraction between their molecules. Consequently it requires relatively little

energy (thus lower temperature) for the molecules to separate into the liquid

phase from the solid (melt) or from the liquid phase into the gas (boil).

(c) The atoms of halogens only need to gain one extra electron to become

isoelectronic with a noble gas. Species which readily gain electrons are good

oxidants.

(d) H2, N2, O2, F2, Cl2, Br2, I2.

(e) Apart from their physical properties, the halogens all form anions in salts

and covalent compounds with other non-metals.

11. (a) helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn).

(b) The outer electron level of the atoms of all the noble gases is filled with 8

electrons (or 2 electrons for helium), the arrangement that gives the noble gases

their stability. To form diatomic molecules for example, more electrons would

XII - 76

have to enter this level in forming covalent bonds between the atoms and this is

not possible as the level is already filled.

12. (a) The d-block elements are located in the Periodic Table between Group 2 and

Group 13. Where they occur, there are 10 d-block elements in each Period.

(b) Among the properties that distinguish d-block elements from the metals of

Groups 1 and 2 are that they frequently show a range of valencies and often have

coloured compounds.

13. Metals are located from the left hand side starting with Group 1 and extending to

include the d-block. The non-metals are located on the right hand side of the

table extending to boron in Group 13 and carbon in Group 14. Metallic

properties increase down each Group of the Table.

14. (a) The most easily oxidised (i.e. the strongest reductants) are located in the

bottom, left hand part of the Table where the outer electrons of elements are

further out from the nucleus and not held so tightly by attraction to it.

Consequently relatively small amounts of energy are required for an oxidant to

remove an electron. Group 1 elements are the most easily oxidised as they only

need to lose one electron to become isoelectronic with the nearest noble gas.

(b) The most easily reduced (i.e. the strongest oxidants) are located in the top,

right hand corner of the Periodic Table, excluding the noble gases. Fluorine is

the strongest oxidant followed by oxygen and chlorine. Elements in this region

of the table are only one or two electrons short of having the stable structure of

the nearest noble gas, so energy is released when the atom captures an electron

in the process of oxidizing another atom and itself being reduced in the process.

15. Effective nuclear charge is the actual force experienced by electrons in the outer

energy level of an atom. Outer electrons are said to be screened from the full

attraction they would otherwise feel from the positively charged nucleus because

of those electrons which occupy orbits closer to the nucleus.

Effective nuclear charge increases from left to right across each period of the

Table because, as electrons are added progressively to the outer orbit, all the

outer electrons experience increased attraction to the larger number of protons in

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the nuclei. This results in a steadily reducing atomic radius and increasing energy

requirements to remove electrons from atoms to form cations. Thus cations with greater

that +3 charge are rarely formed. In each Period, the last element (a noble gas) has the

maximum effective nuclear charge and so reactions in which electrons are removed

from noble gas atoms require a prohibitively large energy input.

The outer electron from elements containing one more electron than a noble gas

(always Group 1) must have that electron in the next highest energy orbit which

is now screened from the nucleus by the inner electrons. The consequence of this

is not only a larger atomic radius but also much less energy being required to

remove that electron to form a +1 cation.

Effective nuclear charge decreases down each Group due to increased screening

by the ever larger number of inner electrons and so electrons can be removed

from atoms more readily, i.e. elements become more metallic the lower they are

in a Group.

The large effective nuclear charge on atoms to the right of each Period also

explains why elements such as the halogens form anions so easily, actually

releasing energy in the process. Once the noble gas structure is attained, any

additional electrons would have to occupy the next outer orbit and be screened

from the nucleus to such an extent that they would be unstable. This is also the

reason that noble gases do not form anions.

8....... Improvements in analytical ....... 8 methods

Electrolysis

9

PrehistoricUse of furnacesto smelt ores

9

8Liquid air

8High energyparticle accelerators

Nuclearreactors

9Apparatus to m e a s u r eradioactivity

9

Bunsen burnerandspectroscope

9

Apparatus to handle gases

9

8..... Methods for analysing minerals - ... 8 especially use of the blowpipe

DISCOVERY OF THE ELEMENTS - NUMBER OF KNOWN ELEMENTS vs

YEAR

this file won’t print the labels - use the file called 04bridgingcourse append1.