Lewis Definitions

General definitions of acids and bases were given by G. N. Lewis as an extension of the concept of the electron-pair covalent bond: an acid is an electron-pair acceptor, a base an electron-pair donor. All Lewis bases are Brönsted bases—an unshared electron pair is required to accept a proton. H⁺ is an acid in the Lewis sense since it can accept an electron pair from a base such as NH₃, but H₂O and NH₄⁺ are not Lewis acids. Metal ions act as Lewis acids when they accept an electron pair from Lewis bases such as NH₃ to form Complex ions . Other electron-deficient molecules such as BF₃ act as Lewis acids and form covalent bonds with bases like NH₃: