INERT PAIR (INERT PAIR EFFECT)

Valence electrons in an s orbital penetrate to the nucleus better than electrons in p orbitals, and as a result they're more tightly bound to the nucleus and less able to participate in bond formation. A pair of such electrons is called an "inert pair". The inert pair effect explains why common ions of Pb are Pb 4+ and Pb 2+ , and not just Pb 4+ as we might expect from the octet rule.