In the reaction BF₃ + NH₃ → F₃BNH₃, which species acts as the Lewis acid?

Core Concept

A Lewis acid is an electron pair acceptor. Boron in BF₃ carries only six valence electrons and an empty orbital, so it accepts the lone pair that nitrogen of NH₃ donates. No proton transfer occurs anywhere in the reaction, which is exactly the situation the Brønsted-Lowry model cannot describe but the Lewis model covers. The new B-N bond is a coordinate covalent bond formed from nitrogen's electron pair.