Give below are two statements: One is labelled as Assertion A and the other is labelled as Reason R:
Assertion A: The stability order of +1 oxidation state of Ga, In and Tl is Ga < In < Tl.
Reason R: The inert pair effect stabilizes the lower oxidation state down the group.
In the light of the above statements, choose the correct answer from the options given below:

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. Check Assertion A. Ga, In and Tl all have the outer configuration ns²np¹. They can lose only the np¹ electron (+1 state) or all three (+3 state). For Ga the +3 state dominates, for Tl the +1 state is the more stable one, and In falls in between. So Ga < In < Tl for +1 stability is true.
2. Check Reason R. The inert pair effect is the reluctance of the ns² electrons to take part in bonding. It grows down the group, so the lower oxidation state, two less than the group valency, becomes more stable. R is true.
3. Why it grows. Heavier elements have filled d and f subshells, which shield poorly. The ns² electrons feel a larger effective nuclear charge and are held more tightly. Bonds also get weaker for bigger atoms, so the energy needed to use the ns² pair is not paid back.
4. So. The rising stability of the +1 state from Ga to Tl is exactly what the inert pair effect predicts. R directly explains A, so the answer is (d).

The idea to keep. In the p-block, heavier elements prefer the oxidation state two below the group valency: Tl⁺, Pb²⁺, Bi³⁺. That trend is the inert pair effect at work.

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