a theory, introduced independently in 1923 by the Danish chemist Johannes Nicolaus Brønsted and the English chemist Thomas Martin Lowry, stating that any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base. A proton is a nuclear particle with a unit positive electrical charge; it is represented by the symbol H⁺ because it constitutes the nucleus of a hydrogen atom.

According to the Brønsted–Lowry scheme a substance can function as an acid only in the presence of a base; similarly, a substance can function as a base only in the presence of an acid. Furthermore, when an acidic substance loses a proton, it forms a base, called the conjugate base of an acid, and when a basic substance gains a proton, it forms an acid called the conjugate acid of a base. Thus, the reaction between an acidic substance, such as hydrochloric acid, and a basic substance, such as ammonia, may be represented by the equation:

In the equation the ammonium ion (NH⁺₄ ) is the acid conjugate to the base ammonia, and the chloride ion (Cl^-) is the base conjugate to hydrochloric acid.

The Brønsted–Lowry theory enlarges the number of compounds considered to be acids and bases to include not only the neutral molecules (e.g., sulfuric, nitric, and acetic acids, and the alkali metal hydroxides) but also certain atoms and molecules with positive and negative electrical charges (cations and anions). The ammonium ion, the hydronium ion, and some hydrated metal cations are considered acids. The acetate, phosphate, carbonate, sulfide, and halogen ions are considered bases.