Question

Explain Fajan’s rule with suitable examples.

Answer 1

Fajan’s rule :

i. Smaller the size of the cation and larger the size of the anion, greater is the covalent character of the ionic bond.

For example,

Li⁺ Cl^- is more covalent than Na⁺ Cl.

Similarly,

Li⁺I^- is more covalent than Li⁺Cl^-.

ii. Greater the charge on cation, more is covalent character of the ionic bond. 

For example, 

Covalent character of AlCl₃, MgCl₂ and NaCl decreases in the following order 

Al³⁺(Cl^-)₃ > Mg²⁺(Cl^-)² > Na⁺ Cl^-

iii. A cation with the outer electronic configuration of the s²p⁶d¹⁰ type possesses greater polarising power compared to the cation having the same size and same charge but having outer electronic configuration of s²p⁶ type.

This is because d electrons of the s²p⁶d¹⁰ shell screen nuclear charge less effectively compared to s and p electrons of s²p⁶ shell. 

Hence,

The effective nuclear charge in a cation having s²p⁶d¹⁰ configuration is greater than that of the one having s p configuration.

For example :

Cu⁺ Cl^- is more covalent than Na⁺ Cl^-.

Here,

(Cu⁺ = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰; Na⁺ = 1s² 2s² 2p⁶)