What is inert pair effect

The inert-pair effect refers to the empirical observation that the heavier elements of groups 13–17 often have oxidation states that are lower by 2 than the maximum predicted for their group.

For example, although an oxidation state of +3 is common for group 13 elements, the heaviest element in Group 13, thallium (Tl), is more likely to form compounds in which it has a +1 oxidation state.

The inert pair effect says that the ns² valence electrons of metallic elements, especially the 5s² and 6s² pairs that follow the second and third row of transition metals, are less reactive than would be expected based on periodic trends such as effective nuclear charge, atomic sizes, and ionization energies.

In, Tl, Sn, Pb, Sb, Bi, and,  Po do not always show their expected maximum oxidation states. Rather  they form compounds in which their oxidation states are 2 less than what would be expected.