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JEE Advanced levelSingle correct optionDifficulty 3 of 3pt170

Thallium triiodide, formulated as $TlI X_{3}$ , is found to be isostructural with $CsI X_{3}$ and gives $Tl X^{+}$ and $I X_{3} X^{-}$ on analysis. The best conclusion is that

Athallium is in the $+ 3$ state because iodine is a poor oxidant
Bthallium is in the $+ 1$ state, since Tl(III) would oxidise iodide to iodine, in line with the inert pair effect
Cthallium is in the $+ 2$ state
Dthallium is present as $Tl X^{3 +}$ and the triiodide is covalent

Answer: B

The

6 s^{2}

electrons of thallium are an inert pair, so Tl(III) is a strong oxidant. It oxidises

I^{-}

to iodine, which combines with excess iodide as

I X_{3} X^{-}

, leaving Tl(I). The solid is therefore

Tl X^{+} I X_{3} X^{-}

, with oxidation states

+ 1

for Tl and

- 1/3

for each iodine.

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