Explain why the following compounds behave as Lewis acids?

1. BCl3

2.  AlCl3

In trivalent state, the number of electrons around the central atom in a molecule of compounds BCl3 and AlCl3 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.