NEET (UG) 2023 — Question 178

As we move down the group, due to poor shielding effect of intervening $d$ and $f$ orbitals,

the increased effective nuclear charge holds ns electrons tightly and therefore restricting

their participation in bonding. So, the relative stability of +1 O.S increases for heavier

elements.

$\mathrm{E}^{\circ}$ for $\mathrm{In}^{3+} \mid \mathrm{In}^{+}=-0.16 \mathrm{~V}$

$\mathrm{E}^{\circ}$ for $\mathrm{T} \ell^{3+} \mid \mathrm{T} \ell^{+}=+1.6 \mathrm{~V}$

Hence, $\mathrm{T} \ell \mathrm{I}$ is more stable than $\mathrm{T} \ell l_3$