Question

Why ammonia (NH3) acts as a lewis base and boron trifluoride acts as a lewis acid​

Answer 1

Answer:

Ammonia has a formal lone pair of electrons on the nitrogen atom. This can donate to Lewis acids (including boron trichloride) in a Lewis base/Lewis acid interaction.

On the other hand, the boron atom in boron trichloride has a formal, vacant p orbital that can accept electron density and quaternize the boron.