Question

BF3 exist as monomer but Alcl3 exist as dimer .why

Answer 1

$BF_{3}$ exists as monomer whereas $AlCl_{3}$ exists as dimer

Reason:

In $AlCl_{3}$ , there are 6 electrons around Al. Therefore, Al accepts a pair of electron from the Cl of other $AlCl_{3}$ to get noble gas configuration. Therefore, $AlCl_{3}$ dimerises as $Al_{2}Cl_{6}$

In the trihalides of B, B cannot accommodate four halogens around it due to its small atomic size. Therefore, the trihalides of B are monomeric in nature.

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