Describe fajan's rule.

Explanation

1. Charge on the ions :
● Higher the positive charge on the cation, greater will be the polarising ability.
● Higher the negative charge on the anion, greater is its Polarisibility.
● Hence, the increase in charge on cation or in anion increases the covalent character.
● Let us consider three ionic compounds `AlCl_(3), MgCl_(2)` and NaCl. 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)`.
2. Size of the ions
● The smaller cation and larger anion show greater covalent character due to the greater extent of polarization.
● LiCl is more covalent then NaCl. The size of `Li^(+)` is smaller than `Na^(+)` and hence the polarizing power of `Li^(+)` is more.
● Lil is more covalent than LiCl, as the size of `l^(-)` is larger than the `Cl^(-)`. Hence `l^(-)` will be more polarised than `Cl^(-)` by `Li^(+)`.
3. Electronic configuration :
● 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.