Any of several theories that give rise to alternative definitions of acids and bases. The original theory was based on Svante Arrhenius's electrolytic theory of solutions and involved the dissociation of water into hydrogen and hydroxide ions. 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.
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