Acids and Bases: Molecular Structure and Behavior

Molecular Structure of Brønsted Bases

What is it about the molecular strucure of bases that allow them to accept a proton? There are two important features:

Bases must have an unshared pair of electrons.
The unshared pair of electrons is used to form a bond with the donated proton from the acid. The Lewis structure of ammonia (NH₃, a common base), is shown below; notice the unshared electron pair on the nitrogen.

A negative charge tends to increase the strength of the base.
The negative charge can aid in attracting and holding protons (since protons are positively charged).

Let's look at the structure of water again:

Water has the molecular criteria of both an acid (hydrogen atoms) and a base (lone electron pairs). These two molecular characteristics account for water's ability to be amphiprotic (act as an acid or a base).