# Why can ammonia act as a base?

Mar 26, 2017

Because the nitrogen can be quaternized.

#### Explanation:

We represent the acid-base behaviour of ammonia in water by the following rxn:

${H}_{2} O \left(l\right) + : N {H}_{3} \left(a q\right) \rightarrow {\text{^+NH_4 + }}^{-} O H$

In $\text{ammonium}$ one of the $N - H$ bonds is donative, or coordinate covalent. Of course, by symmetry, all of the $N - H$ bonds are equivalent. The coordinate covalency of the 4th $N - H$ is a convenient little fiction.

The nitrogen in ammonium is now coordinatively saturated, and can no longer expand its valence shell.