G. N. Lewis, who proposed the electron-pair theory of covalent bonding, realized that the concept of acids and bases could be generalized to include reactions of acidic and basic oxides and many other reactions. According to this concept, a Lewis acid is a species that can form a covalent bond by accepting an electron pair from another species; a Lewis base is a species that can form a covalent bond by donating an electron pair to another species.

The Lewis acid-base concept includes many reactions that do not involve proton- transfer reactions. For example, the reaction between NH₃ and BF₃ occurs because BF₃ has a vacant orbital in its valence shell. It therefore acts as an electron-pair acceptor (a Lewis acid) towards NH₃, which donates the electron pair:

The boron atom in boron trifluoride, BF3, has only six electrons in its valance shell and needs two electrons to satisfy the octet rule. Consequently, BF3 (Lewis acid) accepts a pair of electrons from NH3 (Lewis base).

This example suggests that in a Lewis acid-base reaction, we should look for:

1. A species that has an available empty orbital to accommodate an electron pair such as the B atom in BF₃, and
2. A species that has lone-pair electrons such as NH₃.

The Lewis definition allows us to consider typical Brønsted-Lowry bases, such as OH^–, NH₃, and H₂O, as Lewis bases. They all have electron pairs available to donate for electron-deficient species.

Note that any molecule or negatively charged species having an excess of electrons can be considered as a Lewis base, and any electron-deficient molecule or positively charged species can be considered as a Lewis acid.