Question

what is the inert pair effect?

Answer 1

The inertness of ns electron of outer shell atom in chemical bonding due to poor shielding effect is called inert pair effect.

The increasing stability of the divalent state down the 4^th group, especially in the case of lead compounds is attributed to the "inert pair effect". Since the sum of the first two ionisation energies also decreases down the group, it becomes increasingly favourable thermodynamically, for Sn and Pb to form compounds in oxidation state +2 which are more ionic in character.