The Bronsted-Lowery concept and Arrhenius concept of acids and bases are limited to substances which contain protons. G.N. Lewis in 1923 proposed a more general and broader concept of acids and bases. Lewis concept states that:
An acid is a substance whether molecule or ion which can accept a pair of electrons while a base is a substance whether molecule or ion which can donate a pair of electrons.
In other words, acids are electron pair acceptors while bases are electron pair donors. Hence we can say that any substance which has an unshared pair of electrons can act as a Lewis base while a substance which has an empty orbital that can accommodate a pair of electrons acts as Lewis acid. Hence, coordinate bond is formed between the Lewis base and Lewis acid.
