protonation, in chemistry, addition of a proton to an atom, molecule, or ion. The proton is the nucleus of the hydrogen atom; the positive hydrogen ion, H⁺, consists of a single proton. An example of protonation is the formation of the ammonium group NH₄⁺ from ammonia, NH₃. Protonation often occurs in the reaction of an acid with a base to form a salt (see acids and bases; salts). Protonation differs from hydrogenation in that during protonation a change in charge of the protonated species occurs, whereas the charge is unaffected during hydrogenation.