Atom or molecule that contains an electron pair available for bonding and in chemical reactions therefore seeks a positive centre, such as the nucleus of an atom or the positive end of a polar molecule (see covalent bond; electric dipole). In the Lewis electron theory (see acid-base theory), advanced by the U.S. chemist Gilbert Lewis (1875–1946) in 1923, nucleophiles are by definition Lewis bases. Examples include the hydroxide ion (OH), the ions of the halogens chlorine, bromine, and iodine (Cl, Br, and I, respectively), ammonia (NH), and water (HO). See also base; electrophile.

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