Taking stability as the factor, which one of the following represents correct relationship?

Answer: 1. TlI > TlI\(_3\)

Key idea: In group 13 the +1 state becomes more stable down the group because of the inert pair effect; for Tl, +1 is more stable than +3.

Tl has a strongly reluctant 6s\(^2\) pair, so \(\mathrm{Tl^+}\) is favoured and TlI is more stable than \(\mathrm{TlI_3}\) (in fact \(\mathrm{TlI_3}\) exists as \(\mathrm{Tl^+I_3^-}\)). So option 1 is right.

• Option 2: \(\mathrm{TlCl_3}\) is less stable than TlCl, so this is wrong.
• Option 3: In(III) is the common state of indium, so \(\mathrm{InI_3 > InI}\) is not the relation the question is built on; the +1 stability trend is clearest for Tl.
• Option 4: Al shows +3 almost always; \(\mathrm{AlCl_3}\) is more stable than AlCl.