JEE Main · 2021 · 26 Feb 2021 · Shift 2 · p-Block Elements

In $\mathrm{Tl}{\mathrm{I}}_3$ the iodine is present as the linear triiodide ion ${\mathrm{I}}_3^{-}$, exactly as it is in $\mathrm{Cs}{\mathrm{I}}_3$, so the metal has to balance a single negative charge and thallium is in the $+1$ oxidation state. Thallium has the configuration $[\mathrm{Xe}]4f^{14}5d^{10}6s^{2}6p^1$ and therefore does carry fourteen $f$ electrons, so Reason R is a correct statement as well. The preference for $+1$ arises from the inert pair effect, the reluctance of the $6s^2$ pair to take part in bonding, and the number of $f$ electrons by itself fixes no oxidation state. Both A and R are correct but R is not the correct explanation of A.