Question

Group 13 elements react with halogens to form trihalides except

• A. BCl$_3$
• B. AlCl$_3$
• C. InCl$_3$
• D. TlI$_3$

Answer 1

Answer: (D)

TlI$_3$

Answer 2

Group 13 elements typically form trihalides (MX₃) with a +3 oxidation state. Due to the inert pair effect, the stability of the +1 oxidation state increases down the group, making Tl⁺ more stable than Tl³⁺. TlI₃ is an exception because Tl³⁺ is a strong oxidizing agent and I⁻ is a good reducing agent, leading to a redox reaction. The compound TlI₃ is actually Tl⁺[I₃]⁻, where Thallium is in the +1 oxidation state, not +3.