Explain why the following compounds behave as Lewis acids ?
(a) `BCl_(3)`
(b) `AlCl_(3)`

Explanation

In trivalent state, the number of electrons around the central atom in a molecule of compounds `BCl_(3)` and `AlCl_(3)` will be only six
Such electron deficient molecules have tendency to accept a pair of electron to achieve stable electronic configuration and thus, act as Lewis acids. The tendency to behave as Lewis acid decreases with the increase in the size down the group.