What are Inert pair effects

What are Inert pair effects ?

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.

How and why does monovalency occur in group III?

The atoms in this group have an outer electronic configuration of s²p¹. Monovalency is explained by the s-electrons in the outer shell remaining paired and not participating in bonding. This is called the ‘inert pair effect’. If the energy required to unpair them exceeds the energy evolved when they form bonds, then the s-electrons will remain paired. The strength of the bonds in MX₃ compounds decreases down the group. The mean bond energy for chlorides are 

GaCl₃ = 242, InCl₃ = 206 and TlCl₃ = 153 kJ per mol. Thus the s-electrons are most likely to be inert in thallium.

The inert pair effect is not the explanation of why monovalency occurs in group III. 

It merely describes what happens, i.e. two electrons do not participate in bonding. The reason that they do not take part in bonding is energy. The univalent ions are much larger than the trivalent ions and (I) compounds are ionic and are similar in many ways to group I elements.

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 and 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⁺).