generalization concerning acids and bases introduced in 1923 by the U.S. chemist Gilbert N. Lewis, in which an acid is regarded as any compound which, in a chemical reaction, is able to attach itself to an unshared pair of electrons in another molecule. The molecule with an available electron pair is called a base. The reaction between an acid and a base (neutralization) results in the formation of an addition compound, in which the electron pair that constitutes the chemical bond comes from only one reactant. Included in the Lewis definition of acids are the metal ions; the oxides of certain nonmetallic elements, such as sulfur, phosphorus, and nitrogen; substances able to donate hydrogen ions or protons; and certain solid compounds, such as aluminum chloride, boron trifluoride, silica, and alumina.

In practice, substances that are considered acids by the Lewis definition, other than those associated with hydrogen ions and protons, are specifically referred to as Lewis acids. Lewis bases include ammonia and its organic derivatives, the oxides of the alkali and alkaline earth metals, and most atoms and molecules with negative electrical charges (anions).