what is inert pair effect?

The inert pair effect is the tendency of the outermost s electrons to remain unionized or unshared in compounds of post-transition metals. The term inert pair effect is often used in relation to the increasing stability of oxidation states that are 2 less than the group valency for the heavier elements of groups 13, 14, 15 and 16.

The increasing stability of N-2 oxn state in elements of lower periods due to no participation of S orbital electrns in bonding is termed as inert pair effect. The effect can be attributed to greater  penetration effect of S orbital electrons