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 (NH₃), and water (H₂O). Seealso base; electrophile.

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