This is explained on the basis of inert pair effect which is significant only in elements with large values of atomic numbers. The general electronic configuration of the elements is `ns^(2)p^(2)`. Becaue of inert pair effect, in case of lst three elements. the valence p-electorns readily become availbel for bonding while the valence x-electrons remain attached to the nucleus. Thus, these elemetns show divalency while the first two elements in the absnece of inert pair effect are tetravalent.
