Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Question:

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $\mathrm{R}$.

Assertion $\mathrm{A}: \mathrm{In} \mathrm{TII}_{3}$, isomorphous to $\mathrm{CsI}_{3}$, the metal is present in $+1$ oxidation state.

Reason R: TI metals 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:

1. Both $A$ and $R$ are correct and $R$ is the correct explanation of $A$

2. A is not correct but $R$ is correct

3. Both $A$ and $R$ are correct $R$ is NOT the correct explanation of $A$

4. A is correct but $R$ is not correct

Correct Option: , 3

Solution:

$\mathrm{T} \ell \mathrm{I}_{3}$ is $\mathrm{T} \ell^{+} \mathrm{I}_{3}^{-}$

$\mathrm{CSI}_{3}$ is $\mathrm{Cs}^{+} \mathrm{I}_{3}^{-}$

Thallium shows $\mathrm{T} \ell^{+}$state due to inert pair effect.