Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: In $$TlI_3$$, isomorphous to $$CsI_3$$, 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:

We need to evaluate both the Assertion and the Reason.

Assertion A states that in $$TlI_3$$, which is isomorphous to $$CsI_3$$, the metal is present in the +1 oxidation state. In $$CsI_3$$, cesium is in the +1 state as $$Cs^+$$ with $$I_3^-$$ (triiodide ion). Since $$TlI_3$$ is isomorphous to $$CsI_3$$, it has the same crystal structure, meaning thallium is also in the +1 state as $$Tl^+$$ with $$I_3^-$$. This is consistent with the inert pair effect, where the 6s electrons of Tl are reluctant to participate in bonding. So Assertion A is correct.

Reason R states that Tl has fourteen f electrons in its electronic configuration. The electronic configuration of Tl (Z = 81) is $$[Xe] \, 4f^{14} \, 5d^{10} \, 6s^2 \, 6p^1$$. Indeed, Tl has 14 f electrons. So Reason R is also correct.

However, the fact that Tl has 14 f electrons is not the direct reason for Tl existing in the +1 state in $$TlI_3$$. The +1 state is due to the inert pair effect. While the poor shielding by f electrons does contribute to the enhanced inert pair effect, the Reason as stated merely notes the presence of f electrons without explaining the mechanism. Therefore, R is not the correct explanation of A.

The correct answer is Option (4): Both A and R are correct but R is NOT the correct explanation of A.