Given below are two statements — Assertion A and Reason R: Assertion A: In TlI3, isomorphous to CsI3, the metal is…

🧠 $\mathrm{TlI_3}$ is isomorphous with $\mathrm{CsI_3}$, which is $\mathrm{Cs^+[I_3]^-}$ — cesium triiodide, not cesium(III) iodide.

🗺️ Evaluate Assertion A:

$\mathrm{CsI_3}$: Cs is in +1 state; $\mathrm{[I_3]^-}$ is the triiodide ion (three iodine atoms, total −1).

Similarly, $\mathrm{TlI_3} = \mathrm{Tl^+[I_3]^-}$ — Tl is in +1 oxidation state.

This is supported by the inert pair effect: the 6s² electrons of Tl are reluctant to participate in bonding, making +1 the preferred oxidation state over +3.

A is CORRECT ✓

Evaluate Reason R:

Tl (Z = 81): $[\text{Xe}]4f^{14}5d^{10}6s^26p^1$

Tl does have 14 f-electrons (the filled 4f subshell). However, the presence of 14 f-electrons is not the reason Tl prefers +1. The actual reason is the inert pair effect (poor shielding by 4f and 5d electrons increases effective nuclear charge on 6s electrons, making them harder to ionize).

R is NOT the correct explanation of A (and is considered incorrect as a reason)

$\boxed{\text{Answer: A is correct but R is not correct}}$