ammonium group, in chemistry, a positively charged nitrogen atom joined by single bonds to four other atoms or groups. The simplest ammonium group, NH₄⁺, is formed by protonation of ammonia, NH₃, e.g., by its reaction with hydrogen chloride, HCl, to form ammonium chloride, NH₄Cl, an ammonium compound. Organic ammonium compounds are formed by the reaction of an alkyl halide with an amine (see amino group), for example, ethyl chloride, C₂H₅Cl, reacts with triethylamine, (C₂H₅)₃N, to form tetraethyl ammonium chloride, (C₂H₅)₄N⁺Cl^-. They are also formed by reaction of an amine with a mineral acid or by reaction of an alkyl halide with ammonia.