Lewis acids and bases
The term
Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically:
An acid substance is one which can employ an electron lone pair from another molecule in completing the stable group of one of its own atoms. The modern-day definition of Lewis acid, as given by IUPAC is
a molecular entity that is an electron-pair acceptor and therefore able to react with a Lewis base to form a Lewis adduct, by sharing the electron pair furnished by the Lewis base. This definition is both more general and more specific—the electron pair need not be a lone pair, but the reaction should give an adduct. A
Lewis base, then, is any species that donates a pair of electrons to a Lewis acid to form a Lewis adduct. For example, OH− and NH3 are Lewis bases, because they can donate a lone pair of electrons. Some compounds, such as H2O, are both Lewis acids and Lewis bases, because they can either accept a pair of electrons or donate a pair of electrons, depending upon the reaction. Usually the terms
Lewis acid and
Lewis base are defined within the context of a specific chemical reaction.