Taking stability as the factor, which one of the following represents correct relationship?
- InI3 > InI
- AlCl > AlCl3
- TlI > TlI3
- TlCl3 > TlCl
Answer
(3) TlI > TlI3
Solution
Down group 13, the ns2 electrons become increasingly reluctant to take part in bonding (inert pair effect), so the stability of the +1 oxidation state increases: Al < Ga < In < Tl.
For Al, +3 is the stable state, so AlCl3 > AlCl; option (2) is wrong.
For Tl, +1 is the predominant state and Tl3+ is strongly oxidising, so TlCl > TlCl3 (option (4) is wrong) and TlI > TlI3.
I- is a reducing anion, Tl3+ cannot exist with it; TlI3 is actually Tl+[I3]-.
The correct relationship is TlI > TlI3.
Answer: (3)