When ns2 electrons of outermost shell do not participate in bonding then these ns2 1 electrons are called inert pair and the effect is called inert pair effect. Due to this inert pair effect Ge, Sn and Pb of group IV have a tendency to form both +4 and +2 ions. Now the inert pair effect increases down the group, hence the stability of M2+ ions increases and M4+ + ions decreases down the group. For this reason,Pb2+ is more stable than Pb4+ and Sn4+ is more stable than Sn2+