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

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

TS EAMCET 2015

Solution:

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