Inert Pair Effect

Among the elements of p-block, in groups 13, 14 and 15, there is a general trend that the higher oxidation states become less stable in going down the group. Thus although boron and aluminium are universally trivalent, gallium, indium and thallium exhibit +1 state as well.

Similar situations are noticed in groups 14 and 15. Though carbon is universally tetravalent, it is possible to prepare divalent germanium, tin and lead compounds. The stable state of +3 in antimony and bismuth in group 15 is another example.

Outer electron configurations of group 13, 14 and 15 elements are ns²np¹, ns²np² and ns²np³, respectively. They are expected to show the higher oxidation state of +3, +4 and +5 respectively. But the preference of heavier elements of these groups to show +1, +2 and +3 states, respectively indicate that two electrons do not participate in bonding. The reluctance of s-electrons to take part in chemical bonding is known as inert pair effect.

The inert pair effect is ascribed to two factors:

1. The increase in the promotion energy from the ground state (ns²np¹) to the valence state (ns¹np²)
2. Poorer overlap of the orbitals of the large atoms and hence poorer bond energy.