In the same year that Brønsted and Lowry published their theory, G. N. Lewis created an alternative theory of acid–base reactions. The Lewis theory is based on electronic structure. A Lewis base is a compound that can give an electron pair to a Lewis acid, a compound that can accept an electron pair. Lewis's proposal explains the Brønsted–Lowry classification using electronic structure.
In this representation both the base, B, and the conjugate base, A, are shown carrying a lone pair of electrons and the proton, which is a Lewis acid, is transferred between them.
Lewis later wrote "To restrict the group of acids to those substances that contain hydrogen interferes as seriously with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen." In Lewis theory an acid, A, and a base, B, form an adduct, AB, where the electron pair forms a dative covalent bond between A and B. This is shown when the adduct HN−BF forms from ammonia and boron trifluoride, a reaction that cannot occur in water because boron trifluoride reacts violently with water in a hydrolysis reaction.
These reactions illustrate that BF is an acid in both Lewis and  Brønsted–Lowry classifications and show that both theories agree with each other.
Boric acid is recognised as a Lewis acid because of the reaction
In this case the acid does not split up but the base, HO, does. A solution of B(OH) is acidic because hydrogen ions are given off in this reaction.
There is strong evidence that dilute aqueous solutions of ammonia contain minute amounts of the ammonium ion
and that, when dissolved in water, ammonia functions as a Lewis base.