Fajan´s Rule:

An ion will be formed most easily when ;

(i) the electronic structure of the ion is stable,

(ii) the charge on the ion is small, and

(iii) a small atom forms the anion and a large atom forms the cation.

Increased covalence is favored by ;

(i) high charge on the ions,

(ii) small size of the cation and large size of the anion,

(iii) cations with 18 electron structures, i.e. novel-gas configuration. Noble gas configuration of the cation produces better shielding and less polarizing power.

Examples: Development of covalent character with increase of cation charge (high charge on ions)

Examples: Decrease of covalent character with increase of cation radius

Examples: Development of covalent character with increase of anionic radius