Ionic bonds and main

group chemistry

Learning objectives

Write Lewis dot structures of atoms and ions

Describe physical basis underlying octet rule

Predict ionic charges using periodic table

Define lattice energy

Apply Born-Haber diagrams to calculations of

lattice energy

Three types of bonding

Ionic

Metal + nonmetal

Electron transfer

Covalent

Nonmetal + nonmetal

Electron sharing

Metallic

Metal + metal

Electron “pooling”

It’s all about Coulomb’s law

Like charges attract

E inversely proportional

to r

E proportional to q x q

r

qqE

o

21

4

1

Towards the noble gas configuration

Noble gases are unreactive – they have filled

shells

Shells of reactive elements are unfilled

Achieve noble gas configuration by gaining or

losing electrons

Metals lose electrons – form positive ions

Nonmetals gain electrons – form negative ions

Lewis dot model

The nucleus and all of the core electrons are represented by the element symbol

The valence electrons are represented by dots – one for each

Number of dots in Lewis model is equal to group number (in 1 – 8 system)

The Octet Rule

All elements strive to

become a noble gas, at

least as far as the

electrons are concerned.

Filling the outer shell –

8 electrons

Achieve this by adding

electrons

Or taking them away

Predicting ion charges

s and p block elements are easy:

charge = group number for cations

charge = -(8 – group number) for anions

Group 1A Group 2A Group 3A Group 5A Group 6A Group 7A

H+

Li+ Be2+ N3- O2- F-

Na+ Mg2+ Al3+ P3- S2- Cl-

K+ Ca2+ Ga3+ As3- Se2- Br-

Rb+ Sr2+ In3+ Te2- I-

Cs+ Ba2+ Tl3+

The Octet Rule

Main-group elements undergo reactions which leave them with eight valence electrons

Group 1 [NG](ns1) – e → M+ [NG]+

Group 2 [NG](ns2) – 2e → M2+ [NG]2+

Group 6 [NG](ns2np4) + 2e → X2- [NG]2-

Group 7 [NG](ns2np5) + e → X- [NG]-

Works very well for second row (Li – F)

Many violations in heavier p-block elements

(Pb2+ not Pb4+, Tl+ not Tl3+, Sb3+ not Sb5+ or Sb3-)

Less predictable for transition

metals Occurrence of variable ionic charge

Cr2+, Cr3+, Cr4+, Cr6+ etc.

4s electrons are lost first and then the 3d

Maximum oxidation states in first half correspond to loss of all electrons (4s + 3d)

Ti4+, V5+, Cr6+, Mn7+

Doesn’t continue beyond half-filled shell – 3d electron energy decreases (more tightly held) across period

No Fe8+ etc.

Desirable configurations tend to coincide with empty, half-filled or filled 3d orbitals

Fe2+ ([Ar]3d6) is readily oxidized to Fe3+ ([Ar]3d5)

Ionization energy Energy required to remove an electron from a

neutral gaseous atom Always positive

Follows periodic trend Increases across period

Decreases down group

Removal of electrons from filled or half-filled shells is not as favourable

[He]2s2

[He]2s22p3

[He]2s22p4

[He]2s22p1

Higher ionization energies

Depend on group number

Much harder to remove electrons from a filled shell

Stepwise trend below illustrates this

Partially filled –

valence

electrons

Completely

filled – core

electrons

Electron affinity

Energy released on adding an electron to a neutral gaseous atom

Values are either negative – energy released, meaning negative ion formation is favourable

Or zero – meaning can’t be measured and negative ions are not formed

Addition of electrons to filled or half-filled shells is not favoured (e.g. He, N)

It is easier to add an electron to Na (3s1) than to Mg (3s2)

Ionic bonding

Reaction between elements that form positive and negative ions

Metals (positive ions) and nonmetals (negative ions)

Neutral Na + Cl → ionic Na+Cl-

[Ne]3s1 + [Ne]3s23p5 = [Ne]+ + [Ar]-

Stability of the ionic lattice

Forming ions does not give energy payout:

Ei(Na) = 496 kJ/mol

Ea(Cl) = -349 kJ/mol

Net energy investment (+150 kJ/mol)

Formation of lattice from gaseous ions releases energy to compensate

M+(g) + X-(g) → MX(s) +

energy

Lattice energy is energy released on bringing ions from gas phase into lattice (negative value)

Or: lattice energy is energy required to separate lattice into gas phase ions (positive value)

Cation F- Cl- Br- I- O2-

Li+ 1036 853 807 757 2925

Na+ 923 787 747 704 2695

K+ 821 715 682 649 2360

Be2+ 3505 3020 2914 2800 4443

Mg2+ 2957 2524 2440 2327 3791

Ca2+ 2630 2258 2176 2074 3401

Al3+ 5215 5492 5361 5218 15,916

Lattice energies follow simple trends

Depends on coulombic attraction between ions

-U = κz1z2/d (κ = 8.99x109 JmC-2) As ionic charge increases, U increases (U z1z2)

U(NaF) < U(CaO)

As ion size decreases, U increases (U 1/d) U(LiCl) > U(NaCl) > U(KCl)

d

Born-Haber cycle for calculating

energy

Lattice energy difficult

to measure directly

Can be estimated very

well by models

Can be obtained using

other experimentally

determined quantities

and conservation of

energy

Drawing the Born-Haber cycle