Steric Hindrance: Steric hindrance is a description of physical state of a molecule . It describes an overcrowded condition usually resulting in an unstable structure. Cis isomers are normally more sterically hindered than trans isomers.
Inert pair effect:
The tendency of the outermost s electrons to remain non ionized or unshared in post-transition compounds is called inert pair effect. The term inert pair effect is often used to the increasing stability of oxidation states which is 2 less than the group valency for the heavier elements of groups especially 13, 14, 15 and 16.
The inert pair effect explains why common ions of Pb are Pb4+and Pb2+, and not just Pb4+ as we might expect from the octet rule.