The Ammonium cation or NH₄⁺ is a positively charged polyatomic ion resulting from protonation (bonding with H⁺) of ammonia as shown in this chemical reaction:

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.

Regarding chemical structures, any one or more of the hydrogens in an ammonium ion can be substituted with an alkyl group to form an alkyl-substituted ammonium ion.* For example, two of the hydrogens in an ammonium ion can be replaced by two methyl groups to give a dimethyl ammonium ion ( CH₃-N⁺H₂-CH₃ ), which has a single positive charge on the nitrogen. Analogously to the preceding chemical reaction in the image, an amine (alkyl-substituted ammonia) can be protonated to give a corresponding alkyl-substituted ammonium ion. See "Amine" for details.

* Actual real life formation requires going through ammonia or a corresponding amine as an intermediate. See "Amine" for details.

See also

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

da:Ammoniumion de:Ammonium fr:Ammonium nl:Ammonium pl:Jon amonowy