The 'Inert Pair' Effect
This applies to groups 3, 4 & 5.
Group 4: Outer-electronic configuration of ns2np2. The normal group oxidation state is 4+ with all 4 outer electrons being used in bonding. E.g. CCl4 or SiCl4. On descending the group it becomes increasingly common for the group 4 elements to have a +2 oxidation state. E.g. SnO or PbO. With these compounds only 2 of the 4 outer electrons are used in bonding. +4 oxidation state is more sstable at the top of group 4 and its stability decreases as you descend the group. +2 oxidation state increases in stability as you go down the group.