The inertness of ns orbital to participate in bonding.
Negligence of s subshell in bonding due to poor shielding effect

Reluctance of iner s electrons to participate in bond formation is celled inert pair effect
By sheilding and screening effect the core orbital electron are not participate in reaction or bonding due to increase in effective nuclear charge ,this is called inert pair effect
In the elements of 4th, 5th and 6th period of the p-block the electrons present in the intervening d and f-orbitals do not shield the r-electrons of the valence shell effectively.As a result, {tex}{{ns}^{2}}{/tex} -electrons remain more tightly held by the nucleus and hence, do not participate in bonding. This is called 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.

The inert pair effect is the tendency of the two electrons in the outermost atomic s-orbital to remain unionized or unshared in compounds of post-transition metals. Due to this, the element down the group possess lower oxidation state of that group and becomes stable….??