Any of several theories that give rise to alternative definitions of acid s and base s.
The original theory was based on dissociation of water into hydrogen and hydroxide ion s. To explain the behaviour of a chemical, particularly if water is not present, two other theories were developed. The most widely accepted and useful is the Brønsted-Lowry definition (1923): An acid is a chemical that tends to lose a proton (H + ), and a base is a chemical that tends to gain a proton. Another is the Lewis definition (also 1923): An acid (see electrophile ) is a chemical that can accept an electron pair from a base (see nucleophile ), which they share to form a covalent bond . The three theories have superficial similarities but subtle and important differences for certain applications.