The stable bivalency of Pb and trivalency of Bi is

In the last element of the group. after the removal of electrons from p - orbitals, the remaining ns2 electrons behave like stable noble gas and do not take part in compound formation. This reluctance of the s - electron pair to take part in chemical combination is called inert pair effect. Thus, for the elements at the bottom of a group. lower oxidation state is more stable.