118.

How will you explain the ionic character of alkali metal halides?

Ionic character of alkali metal halides: When a cation approaches an anion, the electron cloud of the anion is attracted towards the cation, thus it gets distorted or polarised. The capacity of the cation to polarise the anion is called polarising power, and the tendency of the anion to become polarised, is known as its polarizability. Now greater the polarisation caused, greater is the neutralisation of charge and consequently the ionic character is decreased (or covalent character is increased). The polarising power of a cation and the polarizability of an anion are determined in term of following.

Fajan’s rules:
(i) Cation’s size: Smaller is the cation, greater is its polarising power, e.g. LiCl is less ionic (or more covalent) than KCl, because the size of Li⁺ ion is much smaller than that of K⁺ ion.

(ii) Anion’s size: Larger is the anion, higher is its polarizability, since the hold on the electron-cloud by the nucleus of anion decreases. Hence, the ionic character of lithium halides is in the order : LiF > LiCl > LiBr > Lil.
Alternatively, the covalent character is in the order Lil > LiBr > LiCl > LiF. Since higher the ionic character, higher is the melting point, consequently melting point of LiF > LiCl > LiBr > Lil.