Lewis acids are imporant chemicals used to catalyze or promote reactions. A Lewis acid is a chemical that can accept a share of an electron pair that is donated by a Lewis base. Although Bronstad-Lowery acids are Lewis acids, most Lewis acids are not Bronstad-Lowery acids, that is, they do not give off a proton. A more complete discussion of Lewis acids and Lewis bases can be found at Lewis acid-base theory. Lewis acids can accept a share of an electron share either because they have an incomplete valence shell, as does AlCl3 or BF3 for example, or because the metal can be increase its valence, as can happen with SnCl4.