Lecture 4. Chapter 2. Structure of the Atom (Contd.)

Lecture 4.

Exercise 2.1. A US penny is 19 mm across. How many Ag atoms could span this if diameter Ag atom is 2.88 Å? Note 1 Ag atom per 2.88 Å.

Conversion factors: 1 = 10-3 m/1 mm and

1 = 10-10 m/ 1 Å.

Number = 19 mm x 10-3 m x 1 Ag atom x 1 Å

1 mm 2.88 Å 10-10 m

= 6.6 x 107 Ag atoms

which is sixty six million Ag atoms.

Diameters of Atomic Nuclei.

These are on the order of 10-4 Å. So if we draw an atom on the board with a diameter of 1m, then the nucleus will be 0.1 mm across!

.

Diameter= 1 meter

diameter of nucleus = 0.1 mm

Atomic numbers, mass numbers, and Isotopes. (p. 45).

The chemical properties of the elements are controlled by the numbers of electrons and protons. The number of protons (= no. of electrons) is the Atomic Number (Z) of the

element. Z = 1 6 8 13

H C O Al

protons

Mass NumberThe Mass Number is the sum of the number of protons

plus neutrons. Atoms of a given element can have different numbers of neutrons. Thus, C (carbon) always has Z = 6, i.e. it has 6 protons, since C determines its chemistry, but can have different numbers of neutrons.

carbon-12 carbon-13 carbon-14

all have six protons (red)

six neutrons (blue)

eightneutrons

Nuclei ofCarbon atoms:

Isotopes• Different isotopes of the same element

have different mass numbers. Thus, 12C and 14C both have 6 protons (Atomic Number 6), but 12C has 6 neutrons, while 14C has 8.

Mass number: 14Atomic number: 6C

‘carbon-14’

Usually, only the mass number is indicated, and referred to as ‘carbon-14’.

Isotopes of Hydrogen (Z = 1)

Hydrogen Deuterium Tritium 99.9844% 0.0156% radioactive, unstable

one proton one proton, one proton in nucleus one neutron two neutrons

H H H1 2 31 1 1

The 12C atom:

nucleus –contains 6protons (Z =6) and 6 neutrons

6 electrons

2.4. Atomic weights.

These are expressed in amu. The mass of the 12C isotope has been defined as exactly 12 amu. 1 amu = 1.66054 x 10-24 g, or 1/6.022 x 1023 g. In these units, the 1H isotope has a mass of 1.0078 amu.

The mass of the12C isotope is exactly 12 amu.

Carbon-12

Atomic weights of the elements:

Elements usually consist of mixtures of isotopes. The Atomic Weights for the elements are thus the average masses of these mixtures, expressed in amu. Thus, naturally occurring C consists of 98.93% 12C and 1.07% 13C. The Atomic Weight of the mixture is thus given by:

Atomic weight = 98.93 x 12 + 1.07 x 13.0335 = 100 100

12.01 amu.Divide by 100because ‘%’= ‘per one hundred’

Average Atomic Mass = Atomic Weight

EXAMPLE: Si naturally occurs in three isotopes:

92.2% of 28Si (27.98 amu)4.7% of 29Si (28.98 amu)3.1% of 30Si (29.97 amu)

what is the atomic weight of Si?92.2100

4.7100

3.1100

0.922 x 27.98 amu

0.047 x 28.98 amu

0.031 x 29.97 amu

= 28.09 amu

addtogether

2.5. The Periodic Table.

The Periodic Table is the most important tool that chemists use for organizing and remembering chemical facts. The chemical properties show a periodic pattern, hence ‘periodic table’. Mendeleev in 1869 came up with the arrangement we now essentially use, possibly from the arrangement of cards in ‘solitaire’.

Dmitri Ivanovich Mendeleev (1834-1907)

Solitaire:

Periodic Table

Elements are sorted according to increasing atomic number

C6

12.01

Atomic Number

Atomic weight

Groups in the periodic table:

Group 1

Group 7Group 5

Group 3

Group 1B

Groups:

The vertical columns in the periodic table are called GROUPS. Properties repeat down a group, such as Alkali metals (Group 1) are all soft silvery reactive metals that form basic hydroxides, while Noble gases (group 8) are all chemically unreactive gases, halogens (group 7) all exist as X2 molecules, and form salts where they are the anion X-.

Group 1B, the coinage metals:

Copper penny American silver South African Eagle Gold Krugerrand

Cu Ag Au

They are all relatively inert, corrosion-resistant metals which have been used for minting coins. They are all excellent conductors of electricity. They all can form chlorides of thecomposition MCl (M = metal), such as CuCl, AgCl, and AuCl.

Rows or Periods in the Periodic Table:(these run across the periodic table)

First period contains only H and He

third period

5th

period

NumbersDown sideIndicateperiod

Periodic Table

Most elements are metals

Alkali Metals

Alkaline Earth Metals Noble Gases

Halogens

I AII A

III A IV A VA VI A VIIA

VIII A

Transition metals

HW: 37

Non-metals

Molecules and Molecular Compounds

Some elements exist as diatomic molecules at room temp.

H2 N

2 O

2 F

2 Cl

2 Br

2 I

2

Periodic table helps predict formulas of compounds:

e.g. fluorides of metallic elements:Group 1HF, LiF, NaF, KF, RbF, CsF

Group 2. BeF2, MgF2, CaF2, SrF2, BaF2

Group 3B. ScF3,YF3, LaF3, AcF3.

Group 3A. BF3,AlF3, GaF3, InF3

Group 2B. ZnF2, CdF2, HgF2

Metallic elements

Periodic table helps predict formulas of compounds:

e.g. Hydrides (compounds with hydrogen) of

non-metallic elements: Group 7. HF, HCl, HBr, HI.Group 6. H2O,

H2S, H2Se, H2TeGroup 5. NH3, PH3, AsH3, SbH3

Group 4. CH4, SiH4, GeH4

inert

Atoms of the first four elements in the periodic table:

Hydrogen Helium Lithium Beryllium

protons: 1 2 3 4Neutrons: 0 2 4 5Electrons: 1 2 3 4

1H 2He 3Li 4Be1 4 7 9

Atoms and ions:

An atom can lose one or more electrons to become a positively charged cation, or gain electrons to become a negatively charged anion:

Ions are formed when a neutral atom

- gains electrons or- loses electrons

Li Li+

e-

Cation

BrBr

-Anion

e-

This has an effect when we count electrons in elements and Ions

This has an effect when we count electrons in elements and Ions

number of electronsnumber of protons

(Z) number of neutrons

80Br-

40Ca2+

32S 16 32 – 16 = 1616

35 + 1 = 36

20 - 2 = 18

35

20

80 – 35 = 45

40 – 20 = 20

‘-’ charge results in gain of one electron

‘2+’ charge results in loss of two electrons

one extra compared to Ztwo less compared to Z

neutral atom, same as Z

Molecules and Molecular Compounds

The periodic table helps predict how elements combine

Empirical Formulas Molecular FormulasOnly gives relative number of atoms in compound

Subscripts are smallestwhole-number ratios

Show actual number and types of atoms in a molecule

Hydrogen peroxide

Empirical Formulas Molecular Formulas

H2O

2HO

ButaneC

4H

10C2H

5

Propane

C3H

8C

3H

8

Empirical and molecular formulae of glucose

glucose

oxygen (red)

hydrogen (white)

carbon (cyan)

EmpiricalFormula: CH2O

MolecularFormula:

C6H12O6

(= empiricalformula x 6)

If you count theatoms in this modelof glucose you willsee 6 carbons, 12hydrogens, and 6oxygens

Different representations of methane (CH4)

Perspectivedrawing

Space-filling model

Structural formula

Ball-and-Stick model

Ions and Ionic Compounds

Ions are formed when a neutral atom - gains electrons or

- loses electrons

Li Li+

e-

Cation

BrBr

-Anion

e-

Metals tend to form Cations

Which Elements form Anions, which Cations?

Nonmetals tend to form Anions

I A II A

III A IV A VA VI A VIIA

VIII A

Cations and Anions can combine to form Ionic Compounds:

(Ionic Crystal – not a discrete molecule)

Sodium chloride

Sodium cation, Na+

Chloride anion, Cl-

Sodium chloride crystal

Empirical Formulas for Ionic Compounds:

(A) determine charge on ions formed

(B) add ions so that compound is neutral overall

Na, O =>

2 Na+

2+ 2-

0

Na2O

Al, O =>

O2-Al3+

O2-

6+ 6-

0

Al2O

3

Na+ Al3+O2- O2-

Mg2+MgCl

2

Al O3+ 2-

Al2O

3

O 2-

Ca2+

Ca2O

2

CaOsmallest possible ratio!

Cl-1

A simple approach: simply swap the charge onone ion to be the subscript on the other

charges

subscripts

2.8 Naming Inorganic Compounds.

• There are now more than 19 million known chemical substances. We cannot use common names for all of them, e.g. ‘rock salt’ or ‘sugar’. We need a system of naming them that indicates unambiguously what they are.

Organic and Inorganic substances:

• Organic substances are compounds of carbon and hydrogen (plus other elements such as N or S) associated with living things. Inorganic substances are substances not associated mainly with living things.

CH2CH2OH AlCl3

Ethanol aluminum chloride

Organic Inorganic

Naming Ionic Compounds:

These consist of metal cations (e.g. Na+) combined with non-metal anions (e.g. Cl-).

1. Positive cations:

a) Cations formed from the metal are given the same name as the metal: e.g. Sodium, Aluminum, Zinc.

b). If a metal can form cations of different charge, this is indicated with Roman numerals, e.g.

Fe2+ = iron (II) Fe3+ = iron(III)

Cu+ = copper(I) Cu2+ = copper(II).

An older method still widely used uses Latin names where the suffix –OUS indicates the lower charge, and –IC indicates the higher charge:

Fe(II) = ferrous Fe(III) = ferric

Cu(I) = cuprous Cu(II) = cupric

2. Negative ions (Anions).

a) The names of monoatomic anions are formed by replacing the ending of the name of the element with ide.

H- = hydride ion Cl- = chloride O2- = oxide N3- = nitride S2- = sulfide

A few simple polyatomic anions also get the suffix ide:

OH- hydroxide CN- cyanide O22-

peroxide

Oxo-anions (sulfate, sulfite, nitrate, nitrite)

b) Polyatomic anions containing oxygen end in ATE (more oxygens) or –ITE (fewer oxygens).

more oxygens

• NO3- nitrate SO4

2-

sulfate

• NO2- nitrite SO3

2- sulfite

fewer oxygens

The oxo-anions of chlorine:

Prefixes are used when there are more than two different oxo-anions of the same element:

Cl- chloride ClO- hypochlorite fewer oxygens = ite ClO2

- chlorite ClO3

- chlorate more oxygens = ate ClO4

- perchlorate

Carbonate and hydrogen carbonate:

• c) Anions derived from other anions by addition of hydrogen:

CO32- carbonate (+ H+) →

HCO3- hydrogencarbonate

O

CO--O

O

CO-HO

carbonate hydrogen carbonate

3. Ionic compounds.

These are named with the cation name first, followed by the name of the anion:

NaCl = sodium chloride

Na2CO3 = sodium carbonate

Al(NO3)3 = aluminum nitrate

Cu(ClO4)3 = copper(II) perchlorate or

cupric perchlorate

Names and Formulas of Binary Molecular Compounds.

Binary compounds are two-element compounds, e.g. carbon dioxide. Rules for naming these are as follows:

1. The name of the element further to the left in the periodic table is usually written first. Thus oxygen is always written last except when combined with fluorine. OF2 = oxygen difluoride, but Cl2O is dichlorine monoxide.

2. If elements are in same group, heavier element is written first, e.g. SO2 = sulfur dioxide.

3. the name of the second element is given an –IDE ending.

Numerical prefixes:

4. Greek prefixes are used to indicate the number of atoms of each element in the formula:

SF6 = sulfur hexafluoride

NF3 = nitrogen trifluoride

N2O4 = dinitrogen tetroxide.

P4S10 = tetraphosphorus decasulfide.

(A table of Greek prefixes is given on next slide)

Note: here the ‘a’ in‘tetra’ is omitted

Table of Greek prefixes: (bottom left p. 66) mono- = 1 di- = 2 tri- = 3 tetra- = 4 penta- = 5 hexa- = 6 hepta- = 7 octa- = 8 nona- = 9 deca- = 10 Note that if the name of the second element starts

with a vowel, e.g. oxide, then the -a is omitted for tetra-, penta-, and hepta-, so we have disulfur

tetroxide, phosphorus pentoxide, diiodine heptoxide.