Incomplete Electron transfer in ionic compoundsIntermediate Type of Bonding

Nature of pure ionic compounds Ways to get the value of lattice enthalpy Comparison of Theoretical and

Experimental value of lattice enthalpy Polarization of ion

Nature of pure ionic compounds

formed by complete transfer of electrons from metallic atoms to non-metallic atoms.

electrons should be solely confined and not shared with its neighbouring ions

In reality, there are some compounds that the transfer of electrons is incomplete.

Lattice Enthalpy of Ionic Compounds

Lattice enthalpy: Enthalpy change when one mole of ionic crystal

is formed from its constituent ions at gaseous state under standard conditions

How can we get the value of lattice enthalpy? By Energetics (Experimentally derived) By using the Simple Ionic Model (Theoretical)

By Energetics(Experimentally derived)

We need to construct Born-Haber cycle and apply the Hess’s Law

Let’s use NaCl as an example and calculate the lattice enthalpy

By using the Simple Ionic Model

(Theoretical) Assumptions:

Ions are perfect spheres with uniform charge distribution

The cations and anions are just in contact with each other

Lattice enthalpy is calculated in terms of electrostatic interactions within the lattice

Simple Ionic Model

The potential energy (P) required to bring two ions with charge q1 and q2 from an infinite distance r is

+ -r

q1 q2

Comparison of the Theoretical and Experimental Values of Lattice

Enthalpy Reveals the nature of the bond in the

compound

Table comparing the theoretical and experimentally derived lattice enthaplies of

some ionic compounds

Compound Lattice enthalpy (kJ mol-1)

Theoretical Experimental Difference

NaCl -766 -771 5

NaBr -732 -733 1

NaI -686 -684 -2

KCl -690 -701 11

KBr -665 -670 5

KI -632 -629 -3

AgCl -770 -905 135

AgBr -758 -890 132

AgI -736 -876 140

ZnS -3430 -3739 309

Answer for Discussion

For NaCl to KI, the differences between theoretical and experimentally derived lattice enthapies are small The Simple Ionic Model gives a good

representation of the actual lattice structure Ions are spherical Charge distribution is uniform

Bond type in these compounds is nearly purely ionic

Answer for Discussion

For AgCl, AgBr, AgI and ZnS, there is a large difference between the theoretical and experimentally derived lattice enthapies Assumptions of the Simple Ionic Model are

unsatisfactory Ions are not perfect spheres Charge distribution is not uniform

Bond type in these compounds is not purely ionic and has some degree of covalent character

Polarization of Ion Polarization:

The distortion of the electron cloud of an anion under the influence of a nearby cation

the electron cloud is attracted towards the cation

some sharing of electrons between the ions

covalent character in ionic bond

Polarizing Power of Cation

Polarizing Power: Ability of a cation to distort the electron

distribution in a neighbouring atom, molecule or ion

Depends on charge density

The charge density of a cation is high if: Higher the charge; and/or Smaller the ionic radius

The higher the charge density of a cation, the higher is its polarizing power

Think about:

1. Among Na+, Mg2+ and Al3+, which one has the highest polarizing power? Why?

2. Among Li+, Na+ and K+, which one has the highest polarizing power? Why?

Answer to Q1

Al3+ has the highest polarizing power It has the highest charge and smallest size

among Na+, Mg2+ and Al3+

The charge density of Al3+ is the greatest

Answer to Q2

Li+ has the highest polarizing power Among Li+, Na+ and K+, Li+ has the smallest size The charge density of Li+ is the highest Li+ has the highest polarizing power

Polarizability of Anion

Polarizability: A measure of the ease of distortion of its

electron cloud by neighbouring cations Higher polarizability of an anion if:

Higher the charge; and/or The larger the ionic radius of anion

Think about:

1. Among F-, Cl- and Br- and I-, which one has the highest polarizability? Why?

2. Among O2- and F-, which one has a higher polarizability? Why?

Answer to Q1

F-, Cl-, Br- and I- are all belonged to the same group

I- has the greatest number of electron shell The size of I- is the largest I- has the highest polarizability (more easily to

be polarized)

Answer to Q2

O2- and F- have the same number of electrons The charge of O2- is greater than F-

O2- is easier to be polarized than F-