Inert Pair Effect & Stability — Tl⁺ vs Tl³⁺ and Al³⁺ Explained (JEE Main)

❓ Question

Which of the following statements are correct?

(A) Tl³⁺ is a powerful oxidising agent
(B) Al³⁺ does not get reduced easily
(C) Both Al³⁺ and Tl³⁺ are very stable in solution
(D) Tl⁺ is more stable than Tl³⁺
(E) Al³⁺ and Tl⁺ are highly stable

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🖼️ Question Image

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✍️ Short Solution

We evaluate each statement using electronic structure, standard electrode potentials and chemical trends (inert-pair effect, stability of Al³⁺).

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🔹 (A) Tl³⁺ is a powerful oxidising agent — True

• Tl in +3 state has electronic configuration [Xe]4f¹⁴5d¹⁰ (6s⁰6p¹ for neutral Tl → Tl³⁺ has lost the 6p and 6s electrons partially).

• Due to the inert-pair effect, the 6s electrons are reluctant to participate in bonding; Tl³⁺ is stabilized poorly and tends to be reduced to Tl⁺.

• Thus Tl³⁺ accepts electrons readily → it is a strong oxidizing agent (gets reduced to the more stable Tl⁺).

• Example: Tl³⁺ is often reduced in aqueous solution; TlCl₃ disproportionates, etc.

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🔹 (B) Al³⁺ does not get reduced easily — True

• Standard potential for Al³⁺/Al is strongly negative (≈ −1.66 V), meaning Al³⁺ → Al⁰ reduction is unfavorable in aqueous solution.

• Aluminium prefers the +3 state (stable, small highly charged cation, strong hydration energy).

• So Al³⁺ is difficult to reduce under normal conditions — statement B is correct.

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🔹 (C) Both Al³⁺ and Tl³⁺ are very stable in solution — False

• Al³⁺ is indeed stable in aqueous solution (strongly hydrated, common oxidation state).

• Tl³⁺ is not stable — because of the inert-pair effect Tl⁺ is much more stable; Tl³⁺ is a strong oxidant and tends to be reduced.

• Therefore the claim that both are very stable is incorrect.

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🔹 (D) Tl⁺ is more stable than Tl³⁺ — True

• Tl⁺ (with electronic configuration similar to inert-gas–like closed shells plus filled 6s²) is stabilized by the inert-pair effect.

• Chemical behavior and prevalence of Tl(I) compounds (TlCl, TlBr, Tl₂O) confirm Tl⁺ is the dominant, more stable oxidation state.

• So D is correct.

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🔹 (E) Al³⁺ and Tl⁺ are highly stable — True

• Al³⁺: highly stable due to high charge/radius ratio and strong hydration/complexation.

• Tl⁺: stable due to inert-pair stabilization; Tl(I) compounds are common and robust.

• So E is correct.

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🧮 Final Answer

Correct statements:

$$\boxed{\text{(A),\ (B),\ (D),\ (E)}}$$

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✅ Quick Recap

• Inert-pair effect makes heavier p-block elements (like Tl) favor the lower oxidation state (+1) — hence Tl⁺ is stable and Tl³⁺ is a strong oxidant.

• Aluminium strongly favors +3; Al³⁺ is hard to reduce (large negative reduction potential).

• Don’t confuse “element in high oxidation state” with “stable in solution” — hydration, ligand effects and inert-pair effect control stability.