CBSE Class 12-science Answered

Inert pair effect refers to the fact that valence electrons in an s orbital penetrate to the nucleus better than electrons in p orbitals, and as a result they are more tightly bound to the nucleus and less able to participate in bond formation. A pair of such electrons is called an inert pair.

or in other words, the tendency of the outermost s electrons to remain nonionized 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.

It is an effect seen especially in groups 13 and 14 of the periodic table, in which the heavier elements in the group tend to form compounds with a valency two lower than the expected group valency

The inert pair effect explains why common ions of Pb are Pb⁴⁺ and Pb²⁺, and not just Pb⁴⁺.