Choose the correct stability order of group 13 elements in their + 1 oxidation state :

Solution

Chemistry ma’am · 4 steps

Going down group 13, the ns² pair holds on tighter, so the +1 state gets steadily more stable.

1. Inert pair effect. Group 13 atoms have the configuration ns²np¹. Losing only the np¹ electron gives the +1 state, while the ns² pair stays behind. That pair is the inert pair.
2. Why it grows. Heavier members have filled d and f subshells. These shield the outer ns² electrons poorly, so the nucleus holds them more tightly. The pair becomes harder to use in bonding as you go down the group.
3. Apply down group. Al strongly prefers +3, Ga and In show +1 more and more, and for Tl the +1 state is the most stable one. Tl⁺ is stable, while Tl³⁺ acts as an oxidising agent.
4. So. The stability of the +1 state increases down the group: Al < Ga < In < Tl, which is option (a).

The idea to keep. In the p-block, the inert pair effect grows down a group. The lower oxidation state (group number minus 2) becomes more stable as you go down: Tl(+1), Pb(+2), Bi(+3).

Answer from the NTA final answer key. Solution by PrepWiser.