Atom or molecule that contains an electron pair available for bonding and in chemical reaction s 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 (18751946) in 1923, nucleophiles are by definition Lewis bases. Examples include the hydroxide ion (OH - ), the ions of the halogen s chlorine, bromine, and iodine (Cl - , Br - , and I - , respectively), ammonia (NH 3 ), and water (H 2 O). See also base ; electrophile .