Group 13 metals - KSU Facultyfac.ksu.edu.sa/sites/default/files/p-block_elements-_iiia.pdf ·...

P-block elements

III A elements (Group 13)

General Trend

The p-block elements The outermost electron enters one of the p-orbitals.

There are six groups of p-block elements (Groups 13, 14, 15, 16, 17 and 18).

The general outer electronic configuration is ns2 np1-6.

The covalent radii and metallic character increase on moving down the

group and decrease on moving across a period.

The ionization enthalpy, electronegativity and oxidizing power increase

across a period and decrease down the group.

Unlike the s-block elements, which are all reactive metals, the p- block

elements comprise of both metals and non-metals.

P-block elements- III A


General Trend

** Since the chemical behaviour of metals and non-metals vary, a regular

gradation of properties is not observed in p-block elements. Nevertheless

some generalizations may be drawn.


Difference in Chemical Behaviour of the First Element

The first member of each group differs in many respects from the other

members.

These differences are quite striking in Groups 13-16.

The effects of small size, high electro-negativity and non-availability of d-

orbitals for the first member are responsible for these differences.

Due to non-availability of d-orbitals, the first member can display a

maximum coordination number of 4, whereas the others can display higher

coordination numbers.


Difference in Chemical Behaviour of the First Element

Hence we come across species like [SiF6]2-, PCl5, PF5, SF6, but analogous

species for carbon, nitrogen and oxygen are not known.

The first member, having small size and high electronegativity, can form

pπ - pπ bonds with itself or other elements

e.g. C = C, N = N, C = O, C = N, N = O etc.

The heavier members do not display pπ - pπ multiple bonding but can

show pπ - dπ bonding.


Inert Pair Effect

The p-block elements display two oxidation states.

the s-block elements display only one oxidation state, the group number.

The higher oxidation state is equal to the group number minus 10

(i.e. number of s and p electrons in the valence shell)

The lower one is two units less than the group number

(i.e. number of p-electron in the valence shell).

Example: Al has 13 electron, the higher oxidation state is 13-10= 3;

13Al: [Ne]10 3s23p1, the lower oxidation is 1.


Inert Pair Effect

The higher oxidation state is displayed only when both the ns and np

electrons are involved in bond-formation.

The lower oxidation state is observed when only the np electron(s)

participate in bond formation.

On moving down the group the ns electrons tend to remain inert and do not

participate in bond formation.

This unwillingness of the outermost s orbital electron pair to participate in

bond formation is called inert pair effect.


Inert Pair Effect

The reason for this effect is explained in terms of bond energy.

Eenergy is needed to uncouple the s-electrons and on the other hand energy

is released during bond formation.

If the energy released is sufficient to unpair the s-electrons, then they

participate in bond formation, otherwise they do not.

The bond energy decreases down the group and hence inert pair effect is

prominent for the lower members.

The lower oxidation state becomes more stable on descending the group (the

inert pair effect).


Group-13

The elements in this group are:

boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl).

The general electronic configuration is ns2np1.

Atoms in this group have 3 valence electrons

a full s orbital and one electron in the p orbital

This group includes a metalloid (boron), and the rest are metals

This family includes the most abundant metal in the earth’s crust

(aluminum)


Group-13

The Group III elements are the first to really distinguish non-metallic and

metallic character in the group.

Boron, with an electronegativity of 2.0, very much forms covalent bonds.

And given its odd number of electrons and inability to form four bonds to

achieve an octet configuration, is involved in forming some remarkable

compounds.

Al is by far the most important of the elements in the row for two reasons,

i. it is third only to Si and O in the earth’s crust, and

ii. it is a smallest non-reactive metal, which makes it important in

manufacturing.


Oxidation states and Bond Type

The common oxidation states are;

+3 and +1.

The +3 oxidation states are favorable

except for the heavier elements.

Heavier elements such as Tl prefers

+1 oxidation state due to its stability.

The inert pair effect

The stability of the +1 oxidation

state, increases down the group.


Oxidation states and Bond Type

Compounds of Ga (I), In (I) and Tl (I) are known.

Tl (I) compounds are more stable than Tl (III) which are oxidizing in nature.

Ga (I) compounds are reducing indicating that Ga (III) is more stable.

The higher oxidation state is generally covalent.

Boron is always covalent and does not form B3+ ion

The M3+ ions are associated with high hydration energies and hydrated

cations are known.

However, some compounds of Al and Ga like AlCl3 and GaCl3 are covalent

in the anhydrous state.


Physical properties

The elements of Group 13 have; 1) smaller atomic radii and 2) higher

electronegativities

compared to s-block elements.

However, these properties do not vary in a regular manner.

The atomic radius of Ga (135 pm) is slightly less than that of Al (143 pm).

The electronegativity and ionization energy consequently are higher than

expected.

Ga contains ten ‘d’electrons

Similarly, the inclusion of fourteen ƒ’ electrons on the inner core affect the

size and ionization energy of Tl.


Physical properties

The elements at top of the group are hard , covalent materials.

those at bottom are soft metals, as reflected in their enthalpies of

atomization and their melting points.

Also, they are hard and soft in terms of their Lewis acidity in the same order

(elements at top are hard and those at bottom are soft ).

The latter is correlated with polarizability of the atomic orbitals.

The First I.E. decreases down the group, but there is a minor hiccup at Ga.

Similarly, the electronegativities are do not decrease smoothly.


for explaining stability of compound.

'Hard' applies to species which are small, have high charge states, and

are weakly polarizable.

'Soft' applies to species which are big, have low charge states and are

strongly polarizable.

The soft acids react faster and form stronger bonds with soft bases,

whereas

hard acids react faster and form stronger bonds with hard bases

hard and soft in terms of Lewis acid and base

Comparing tendencies of hard acids and bases vs. soft acids and bases

Property Hard acids and bases Soft acids and bases

atomic/ionic radius small large

Oxidation state high low or zero

Polarization low high

affinity Ionic bonding Covalent bonding

hard and soft in terms of Lewis acid and base

Physical properties

Some important physical constants of the Group 13 elements are shown in table

below:


Chemical Properties

Elements of Group 13 are quite reactive

Much of the important chemistry of the group 13 elements can be understood

on the basis of their electronic structure.

Since the elements have a [core]ns2 np1 electron configuration, neutral group

13 compounds can form up to three bonds.

This only provides for 6 electrons (not a complete octet) around the group 13

atom so such compounds are called “electron-deficient”.


Chemical Properties

Boron’s chemistry is so different from that of the other elements.

Chemically boron is a non-metal, it has a tendency to form covalent bonds

and displays similarities with silicon, which will be discussed later.

Boron combines with many metals to form borides e.g. MgB2, and Fe2B

where it displays negative oxidation state.

All elements except Tl, when treated with halogens, oxygen or sulphur form

halides (MX3) oxides (M2O3) and sulphides (M2S3).

Thallium forms TlX, Tl2O and Tl2S. ( as Tl+1)


Chemical Properties

B and Al form nitrides by direct combination with nitrogen at very high

temperature.

B and Al form carbides on heating with carbon.Aluminium carbide (Al4C3)

on hydrolysis given methane.

Al4C3 + 12H2O 4Al(OH) 3+ 3 CH4

• Boron carbide (B12C3) is a hard, high melting, inert compound.

Al has a very high affinity for oxygen

(enthalpy of formation of Al2O3 is – 1676 KJ mol-1) and is used to remove

oxygen from other metal oxides.

• This forms the basis of the Thermite process for extracting many metals from

their oxides. 3 MnO2 + 4Al 2Al2O3 + 3Mn

Fe2O3+ 2Al Al2O3 + 2Fe


Chemical Properties

The reactions of the elements with acids differ.

Boron reacts only with oxidizing acids to form boric acid

2B + 3H2SO4 2H3BO3 + 3SO2

B + 3HNO3 H3BO3 + 3 NO2

Boric acid is better represented as B(OH)3 and does not contain replaceable

hydrogen.

The other elements react with dilute mineral acids to evolve hydrogen

2M + 6HCl 2MCl3 + 3H2

Al is render passive with concentrated nitric acid.


Chemical Properties

Boron liberates hydrogen when fused with alkali.

2B + 6NaOH 2Na3BO3 + 3H2

Al and Ga dissolve in alkali to form tetrahydroxoaluminate (III) and

tetrahydroxogallate (III) respectively.

M + 4NaOH Na[M(OH) 4] + 2H2


Because of their electron-deficient nature, group 13 compounds containing

the element (M) in the (+3) oxidation state have a formally vacant npz orbital

and usually act as Lewis acids (electron acceptors).

This is probably the most important feature of group 13 reagents and they

are used in organic synthesis (e.g. Friedel-Crafts alkylation) and as catalysts or

co-catalysts for many different kinds of chemical processes.

R M

R

R

+Base Base M

R

RR

Base M

R

RR

or

ZrMe

Me + B(C6F5)3 Zr Me [MeB(C6F5)3]+C2H4

polyethylene


Chemical Properties

X M

R

R

X M

R

R

= F N

R

RO

RXWhere, for example,

group 13 compounds can also form “partial” multiple bonds with terminal

atoms that contain lone pairs of electrons.

The extent to which this happens depends on the energies of the AO’s involved

(the empty npz orbital and those of the lone pairs) and as you would expect

from MO theory it happens mostly for boron.


Chemical Properties

For the heavier elements, “bridging” is often observed if there are no other

electron donors to provide electron density to the vacant orbital.

If the substituents contain lone pairs of electrons, the bridges can be formed

from two -electron donor-acceptor bonds:

X

MRR

M

XRR


Chemical Properties

H

BB

HH H

H H

Diborane

Instead of using pure AO’s , MO two sp3 hybrids from each

B and the two 1s AO’s for the bridging H atoms.


When the substituents do not have any lone pairs of electrons, the bridges can

be formed from three-center-two-electron bonds Such bonds are readily

explained by MO theory or a combination of VB and MO theory:

Chemical Properties

Boron and Aluminum

There are a variety if allotropes of boron, the most famous being B12

All of them are formed in odd ways to achieve an octet configuration.

Boron is actually extracted from the earth as Na2B4O7 (borax).

The red color of the material is due to an iron contaminant.


Al is present in thousands of minearals,

however, the only important ore for Al is

bauxite, which contains hydrated oxides such as

Al2O3.H2O.

Bauxite is usually reddish-brown, but can

also be white, tan, and yellow, depending on the

type and concentration of iron minerals present.

Applications of B and Al

B(OH)3 is a Lewis acid, boric acid “organic” way to kill pests.

Boron also forms some very interesting compounds with nitrogen called

boron nitrides,

- have the same electronic configuration as graphite and C60 and

consequently have prompted a lot of interest in them for new materials.

2B + 2NH3 2BN + 3 H2

Al is used in many manufacturing processes, most famously, the production

of paper. Aluminum sulfate is known as papermaker’s alum.

Al2O3 + H2SO4 Al2 (SO4)3 + 3H2O



Applications of B and Al

Al are well known as it is widely used in every day life,

for example; Al foil, cooking pans, Al window sashes, etc.

Al metal usually exceeds 99% purity, and the metal itself and its alloys are

widely used.

Aluminum (Al)

Third most abundant element in the earth’s crust, which is found in

compounds with oxygen and often with silicon.

Al exists as aluminosilicates in the Earth’s crust.

Relatively weak metal so it is often alloyed with Cu, Mg, Mn, etc. to

produce a strong, but light materials (high strength to weight ratio)

Easily oxidized.

The thin, tough oxide layer produced provides a protective barrier.

Al is used to produce silver and white flames and sparks.

• It is a common component of sparklers and is often alloyed with Mg

into magnalium for extra-bright fireworks.



Production of Al

Initially, pure Al was very expensive because

of the difficulty of extracting the metal from its

oxide.

Al metal is obtained by electrolysis of bauxite

(Al2O3) in Cryolite Na3AlF6.

Al is recovered by the electrochemical Hall-

Héroult process (relatively inexpensive way).

The electrochemical process following overall

reaction : 4Al+3 + 6O-2 + 3C 4Al + 3CO2

Production of Al metal is Still very energy

intensive, requiring large amounts of electricity.

Chemistry of Al

Al metal dissolves in mineral acids, except concentrated nitric acid.

Al metal dissolves in aqueous solutions of alkali metal hydroxides evolving

hydrogen.

Al forms compounds with most non-metallic elements and shows a rich

chemistry, but unlike boron, no cluster hydrides are known.

As oxide and halides, Al have already been described.

Organo-aluminum compounds.


The trace metals added to get the color: Cr+3 Fe+3 and Ti+4 Fe+3


Compounds of Aluminum

Al2O3 (Aluminum oxide commonly know as alumina)

Variety of crystal structures

Many forms are important ceramic materials

Some impure forms of alumina are ruby (Cr+3), sapphire (Fe+3 and Ti+4),

and topaz (Fe+3 )

Amphoteric

Organoaluminum compounds

Organo-Al compounds are used in large quantities for olefin polymerization.

Olefin polymerization are industrially manufactured from Al metal,

hydrogen, and an olefin as follows.

2 Al + 3H2 + 6 CH2 CHR →Al2(CH2CH2R)6

They are dimers except those with bulky hydrocarbyl groups.

For example, trimethylaluminum, Al2(CH3)6, is a dimer in which methyl

groups bridge aluminum atoms by electron deficient bonds.


Organoaluminum compounds

The Ziegler-Natta catalyst is olefin polymerization catalyst

devolped 1950s and the Nobel prize award in 1963.

Consists of an organoaluminium compound and a metal compound.

Organnoaluminum compounds are very reactive and burn spontaneously in

air. They react violently with water and form saturated hydrocarbons, with

aluminium changing to aluminium hydroxide as follows;

Al(CH2CH2)3 + 3H2O 3C2H6 +Al(OH)3

Therefore, they should be handled in the laboratory under a perfectly inert

atmosphere.


Gallium (Ga)

The anomalous position of Ga affect its chemistry, and is a consequence of

the Scandide contraction (d- block contraction).

** Scandide contraction the effect of having full d orbitals (d10) on the period 4

elements (Ga, Ge, As, Se and Br).

This is reflected in the electron configuration of the element:

It is the first in the group to have a set of filled d orbitals preceding the

valence p orbitals.

The very poor shielding of the d electrons results in a higher-than-expected

effective nuclear charge on the valence electrons of Ga, and hence its

anomalous behaviour.

We have already seen that Ga2H6 is more stable than the corresponding

Al hydride, and in some ways Ga can act closer to boron than Al does.


Thallium (Tl)

Thallium, the first element in Group 13 ( A) to have a filled f orbital

preceding the valence orbitals. This is called the Lanthanide contraction.

A similar effect as the Scandide contraction

Another important factor, the primary influence of which is to greatly

stabilize the +1 oxidation state of Tl compared to the group oxidation state of

+3, is the inert-pair effect.

This is caused by the greater separation in the energy of the ns and np

orbitals on going down the group.

The inert pair effect operates for all the heavy p-block elements, whereas the

scandide and lanthanide contractions lose their importance with increasing

group number (i.e. with the addition of more valence electrons within the p-

subshell).



The composition of fireworks

The main part of fireworks is

concerned with pyrotechnics

– it is a mixture of substances

– produce an effect by heat, light,

sound, gas, smoke or a combination of

these

– exothermic reactions that do not rely

on oxygen from external sources.


5 basic ingredients

1. A fuel typically based on metal or

metalloid powders, or black powder;

2. An oxidiser that produces oxygen to

support the combustion of the fuel.

perchlorates (ClO4 - ), chlorates (ClO3 - ) or nitrates (NO3 - );

3. Colourants, usually chloride salts of

suitable metals such as Sr, Na or Cu;

4. A binder that holds the pellet together;

5. A chlorine donor to react with the

colour-imparting metals, which will enhance

the colour intensity.

Group 13 metals - KSU Facultyfac.ksu.edu.sa/sites/default/files/p-block_elements-_iiia.pdf ·...

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