Question

Which one of the following elements does not form triiodide on reacting with iodine?

• A. $B$
• B. $Tl$
• C. $Al$
• D. $Ga$

Answer 1

Answer: (B)

Boron show consistently $+3$ oxidation state in its compounds while rest of the elements show $+1$ as well as $+3$ oxidation states. The $+1$ oxidation state becomes more and more stable as we move down the group, i.e.

$Al<Ga<In<Tl$

This is due to the presence of intervening $d-$ and $f-$orbitals which do not shield the nucleus effectively. Consequently effective nuclear charge