# What is a "Lewis-acid"? Of NCl_3, NH_3, PCl_3 and AlCl_3, which is the likely Lewis-acid?

A $\text{Lewis acid}$ is an electron-pair acceptor........
$N C {l}_{3}$, $N {H}_{3}$, and $P C {l}_{3}$ are all electron donors. $A l C {l}_{3}$ is an electron pair acceptor, and may thus be classified as a Lewis acid. In fact much of the chemistry of aluminum may be rationalized on the basis of the tendency of the metal to complete its valence electron shell, i.e. to satisfy its intrinsic Lewis acidity:
$A l C {l}_{3} + C {l}^{-} \rightarrow A l C {l}_{4}^{-}$