Named after the American chemist Gilbert Newton Lewis (1875–1946), the Lewis theory defines an acid as a species that can accept an electron pair from another atom, and a base as a species that can donate an electron pair to complete the valence shell of another atom. Hydrogen ion (proton) is the simplest substance that will do this, but Lewis acids include many compounds—such as boron trifluoride (BF₃) and aluminum chloride (AlCl₃)—that can react with ammonia, for example, to form an addition compound or Lewis salt.