Question 64: Taking stability as the factor, which one of the following represents correct relationship?
(1) \mathrm{AlCl}>\mathrm{AlCl}_{3}
(2) \mathrm{TlI}>\mathrm{TlI}_{3}
(3) \mathrm{TlCl}_{3}>\mathrm{TlCl}
(4) \mathrm{InI}_{3}>\mathrm{InI}
Answer: Option (2)
Explanation:
Elements of group 13 show a decreasing stability of the +3 oxidation state on moving down
the group due to the inert pair effect.
Because of the inert pair effect, the \mathrm{ns^2} electrons remain
less available for bonding in heavier elements like thallium.
As a result, the +1 oxidation state becomes more stable than the +3 oxidation state for thallium.
Therefore, thallium(I) iodide \mathrm{TlI} is more stable than thallium(III) iodide \mathrm{TlI_3}.
Hence, the correct stability relationship is \mathrm{TlI}>\mathrm{TlI_3},
corresponding to option (2).