Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : In TlI₃, isomorphous to CsI₃, the metal is present in +1 oxidation state.
Reason R : Tl metal has fourteen f electrons in its electronic configuration.
In the light of the above statements, choose the most appropriate answer from the options given below :

Solution

Chemistry ma’am · 5 steps

Read TlI₃ as Tl⁺ plus a triiodide ion, just like CsI₃, and A falls into place.

1. Check A. CsI₃ is Cs+[I₃]⁻, since caesium only shows +1. Being isomorphous, TlI₃ has the same build: Tl+[I₃]⁻. Thallium is in the +1 state, so A is correct.
2. Why not +3. Tl³⁺ is a strong oxidising agent and I⁻ is a good reducing agent, so they cannot sit together. Thallium therefore stays as Tl⁺, which is also its more stable state because of the inert pair effect.
3. Check R. Thallium (Z = 81) is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. It does have fourteen 4f electrons, so R is correct.
4. Link them. R only states a fact about the configuration. The +1 state in TlI₃ comes from the triiodide structure and the instability of Tl³⁺ next to I⁻. Counting f electrons does not explain that.
5. So. Both statements are true, but R is not the correct explanation of A. The answer is (c).

The idea to keep. When a formula gives an odd oxidation state, like +3 for Tl with iodide, look for a polyatomic anion such as I₃⁻. Also check that the stated reason actually causes the effect.

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