In $\mathrm{BCl}_{3}$, Boron is attached with chlorine by covalent bond but octet of boron is not complete, it requires two electrons or pair of electron to complete it octet.

Such electron deficient molecules have tendency to accept a pair of electrons to achieve stable electronic configuration and thus, behave as Lewis acids.

The tendency to behave as Lewis acid decreases with the increase in the size down the group. $\mathrm{BCl}_{3}$ easily accepts a lone pair of electrons from ammonia to form $\mathrm{BCl}_{3} \mathrm{NH}_{3}$.

$\mathrm{H}_{3} \mathrm{~N}:+\mathrm{BCl}_{3} \rightarrow \mathrm{H}_{3} \mathrm{~N} \rightarrow \mathrm{BCl}_{3}$

$\mathrm{AlCl}_{3}$ achieves stability by forming a dimer. In trivalent state, most of the compounds being covalent are hydrolyzed in water. For example, the trichlorides on hydrolysis in water form tetrahedral $\mathrm{M}(\mathrm{OH})_{4}^{-}$. Element $\mathrm{M}$ is $s p^{3}$ hybridized and has tetrahedral shape.