Inert Pair Effect
In the s-block, group I elements are univalent and group II elements are divalent. In group III, we would expect the elements to be trivalent. In most of their compounds this is the case, but some of the elements show lower valency states as well. There is an increasing tendency to form univalent compounds on descending the group. Compounds with Ga(I), In(I), and Tl(I) are known. With Ga and In the (I) oxidation state is less stable than the (III) state. However, the stability of the lower oxidation state increases on descending the group. Tl(I) thallous compounds are more stable than Tl(III) thallic compounds. This is called the inert pair effect.
The inert pair effect is not restricted to group III, but also occurs among the heavier elements in other groups in the p-block. Examples from group IV are Sn2+ and Pb2+ and examples from group V are Sb3+ and Bi3+. The lower oxidation state becomes more stable on descending the group. Thus, Sn2+ is a reducing agent, but Pb2+ is stable; Sb3+ is a reducing agent, but Bi3+ is stable. When the s-electrons remain paired, the oxidation state is always two lower than the usual oxidation state for the group.
Thus, in the s-block, groups I and II show only the group valency. Groups in the p-block show variable valency, differing in steps of two. Variable valency also occurs with elements in the d-block. This arises from using different number of d-electrons for bonding, so in this case the valency can change in steps of one (e.g., Cu+ and Cu2+, Fe2+ and Fe3+).