Describe fajan's rule.

Explanation

The ability of a cation to polarise an anion is called its polarising ability and the tendency of an anion to get polarised is called its polarisability. The extent of polarisation in an ionic compound is given by the Fajans rules
Fajans Rules :
(i) To show greater covalent character, both the cation and anion should have high charge on them. Higher the positive charge on the cation, greater will be the attraction on the electron cloud of the anion . Similarly higher the magnitude of negative charge on the anion , greater is its polarisability . Hence, the increase in charge on cation of in anion increases the covalent character Let us consider three ionic compounds aluminum chloride, magnesium chloride and sodium chloride. Since the charge of the cation increase in the order `Na^(+) lt Mg^(2+) lt Al^(3+) ` , the covalent character also follows the same order `NaCl lt MgCl_(2) lt AlCl_(3)`.
(ii) The smaller cation and larger anion show greater covalent character due to the greater extent of polarisation .
Lithium chloride is more covalent than sodium chloride. e size of `Li^(+)` is smaller than `Na^(+)` and hence the polarising power of `Li^(+)` is more. Lithium iodide is more covalent than lithium chloride as the size of `I^(-)` is larger than the `Cl^(-)`.
Hence `I^(-)` will be more polarised than `Cl^(-)` by the cation `Li^(+)`.
(iii) Cations having `ns^(2) np^(6) nd^(10)` configuration show greater polarising power than the cations with `ns^(2) np^(6)` configuration . Hence, they show greater covalent character .
CuCl is more covalent than NaCl . Compared to `Na^(+) (1.13 Å ). (cu^(+) (0.6 Å)` is small and have `3s^(2) 3p^(6) 3d^(10)` conguration .
Electronic conguration of `Cu^(+)`
` [Ar] 3d^(10)`
Electronic Conguration of `Na^(+)`
`[He] 2s^(2), p^(6)`