The Ammonium' cation is a positively charged polyatomic ion of the chemical formula ' and a molecular mass of 18.04, resulting from protonation of ammonia. Ammonium and aminium is also a general name for positively charged substituted amines and quaternary ammonium cations N⁺R₄ where one or more hydrogen atoms are replaced by alkyl groups .

The positively charged nitrogen atom then forms four covalent bonds, instead of three as in ammonia. This reaction is reversible. The ammonium ion can act as a very weak acid in the sense that it can protonate a stronger base using any one of its hydrogen ( H ) atoms and convert back to ammonia. This means that the ammonium ion is a conjugate acid of the base ammonia. In a solution, the degree to which ammonia forms the ammonium ion depends on the pH of the solution.

However, formation of ammonium compounds can also occur in the vapor phase; for example, when ammonia vapor comes in contact with hydrogen chloride vapor, a white cloud of ammonium chloride forms, which eventually settles out as a solid in a thin white layer on surfaces. Ammonium cations resemble alkali metal ions like Na⁺ or K⁺ and can be found in salts such as ammonium bicarbonate, ammonium chloride, and ammonium nitrate. Most simple ammonium salts are very water soluble. Ammonium ions are a toxic waste product of the metabolism in animals and are excreted unchanged in the urine by water animals. Dissolving ammonia in water produces ammonium hydroxide.

Substituted ammonium ions

Any hydrogen in the ammonium ion can be substituted with an alkyl group to form an alkyl-substituted ammonium ion, also called aminium ion; see amine for details. Depending on the number of alkyl groups it is called a primary, a secondary, a tertiary, or a quaternary ammonium cation. They exist in an equilibrium with the respective substituted amine, depending on the pH. Only quaternary ammonium cations are permanently charged.

Example: (CH₃)₂NH (dimethylamine) + H⁺ <=> (CH₃)₂N⁺H₂ (dimethylammonium or dimethylaminium ion)

Other organic radical groups (typically symbolized by R) may be substituted for the alkyl groups on the ammonium ion.

See also

• Quaternary ammonium cation (R₄N⁺)
• Hydronium (H₃O⁺)