molecule that in a chemical reaction seeks an atom or molecule containing an electron pair available for bonding or the negative 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, electrophiles are by definition Lewis acids. Examples include the hydronium ion (H 3 O + ), boron trifluoride (BF 3 ), and the halogen s fluorine, chlorine, bromine, and iodine in molecular form. See also acid ; nucleophile .