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Taking stability as the factor, which one of the following represents correct relationship?
Down Group 13, the inert pair effect makes +1 oxidation state dominant for Thallium (Tl). Thus Tl⁺ is far more stable than Tl³⁺ ⟹ TlI > TlI₃ in 5 seconds! (In fact, Tl³⁺ oxidizes I⁻ to I₂).
As we descend Group 13 from boron to thallium, the valence electron configuration is ns² np¹. In heavier members (especially thallium, Z = 81), the intervening 4f and 5d electrons provide poor screening of the nuclear charge. Consequently, the effective nuclear charge experienced by the valence 6s electrons increases sharply, holding them tightly and preventing their participation in chemical bonding. This reluctance of the s-electron pair to participate in bonding is known as the inert pair effect.
Because the inert pair effect reaches its maximum at thallium, the +1 oxidation state is far more stable than the +3 oxidation state:
Tl³⁺ has a high reduction potential (E° = +1.26 V) and acts as a powerful oxidizing agent. Iodide (I⁻) is a strong reducing agent. Tl³⁺ readily oxidizes I⁻ to I₂, rendering covalent TlI₃ thermodynamically unstable:
Therefore, TlI is much more stable than TlI₃ (TlI > TlI₃). In contrast, TlCl is more stable than TlCl₃ (making Option 2 false); InI₃ is more stable than InI (making Option 3 state InI₃ > InI, but between TlI > TlI₃ it is the quintessential example tested); AlCl₃ is far more stable than AlCl (making Option 4 false).
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Accurate: Due to the inert pair effect and redox incompatibility of Tl³⁺ with I⁻, TlI (+1) is far more stable than TlI₃.
False: TlCl (+1) is more stable than TlCl₃ (+3) due to the inert pair effect.
While In³⁺ is more stable than In⁺, in the presence of strongly reducing iodide, InI is favored or disproportionates, but NTA designated TlI > TlI₃ as the single correct key.
Completely false: For aluminum, Al³⁺ is vastly more stable than Al⁺ (AlCl₃ is the stable species).
In lighter elements (B, Al), +3 is dominant (AlCl₃ is stable, AlCl is unstable). But for the bottom-most element (Tl), the inert pair effect REVERSES this order: +1 becomes far more stable than +3!
"Owing to the inert pair effect, the stability of the +1 oxidation state increases down Group 13: Al < Ga < In < Tl. Tl⁺ is more stable than Tl³⁺."
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