The stability of lower oxidation states for heavier elements in p-block (like Tl, Pb, Bi) and also for Hg (d-block, but exhibits similar behavior due to its position) is attributed to the inert pair effect.

Why (B) is correct:

The inert pair effect is the reluctance of the outermost s-electrons to participate in chemical bonding. As we move down a group in the p-block, the valence s-electrons become increasingly reluctant to participate in bonding due to poor shielding by intervening d and f electrons. This results in an increased effective nuclear charge on the s-electrons, making them more tightly bound and less available for bonding. Consequently, the lower oxidation state (where only p-electrons are involved in bonding) becomes more stable than the higher oxidation state (where both s and p-electrons are involved).

For example, in Group 13, the +1 oxidation state becomes more stable than +3 for Tl. In Group 14, +2 becomes more stable than +4 for Pb. In Group 15, +3 becomes more stable than +5 for Bi. Similarly, Hg (Group 12) predominantly shows a +2 oxidation state, but its compounds in the +1 state (like $\mathrm{H}{\mathrm{g}}_2\mathrm{C}{\mathrm{l}}_2$) are also stable, and the tendency for lower oxidation states is related to the increasing stability of the filled d-shell and the inert pair effect's influence on subsequent p-block elements.

Option Analysis:

• A) Poor shielding effect of d-orbitals: While poor shielding by d-orbitals (and f-orbitals) contributes to the inert pair effect by increasing the effective nuclear charge on the s-electrons, it is not the direct explanation for the stability of the lower oxidation state. The inert pair effect itself is the consequence of this poor shielding.
• B) Inert pair effect due to poor participation of ns² electrons: This is the correct explanation. The ns² electrons become increasingly reluctant to participate in bonding, making the lower oxidation state more stable.
• C) High electronegativity of these elements: These elements generally have lower electronegativity compared to their lighter congeners, and high electronegativity would favor higher oxidation states by attracting electrons.
• D) Increase in metallic character down the group: While metallic character does increase down the group, this trend does not directly explain the increased stability of lower oxidation states. Metallic character relates to the ease of losing electrons, but the inert pair effect specifically refers to the reluctance of the ns² electrons to be lost or shared.

Correct Answer: (B)