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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.
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