1923 American chemist gilbert newton lewis pointed out that there is no reason to think that acids must be confined to hydrogen-containing compounds. His understanding comes from the fact that the oxidation reaction does not necessarily involve non-oxygen. Louis is the founder of valence bond theory. He put forward the theory of acid-base electrons (Lewis acid-base theory) from the structural point of view: acid is the acceptor of electrons and alkali is the donor of electrons. Acid-base reaction is a process in which an acid receives a pair of electrons from a base, forms a coordination bond, and obtains an acid-base adduct. The acid-base reaction in this theoretical system is called acid-base addition reaction. Its general formula is:

Usually, the "acid" in the acid-base electronic theory is called Lewis acid, and the "base" is called Lewis base to show the difference. The hardness of acid-base and the strength of acid-base are different concepts, so it is necessary to consider the hardness and hardness of reactants comprehensively when dealing with acid-base reaction. When the acid-base hardness is equal, the acid-base hardness plays a leading role in the reaction direction.