The ammonium cation is a positively charged polyatomic ion with the chemical formula NH4+.〔In the substitutive nomenclature, NH4+ is denoted by the name "azanium".〕 It is formed by the protonation of ammonia (NH3). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ammonium cations (NR4+), where one or more hydrogen atoms are replaced by organic groups (indicated by R).
==Acid base properties==
The ammonium ion is generated when ammonia, a weak base, reacts with Brønsted acids (proton donors):
:H+ + NH3 → NH4+
The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule:
:NH4+ + B− → HB + NH3
Thus, treatment of concentrated solutions of ammonium salts with strong base gives ammonia. When ammonia is dissolved in water, a tiny amount of it converts to ammonium ions:
:H2O + NH3 OH- + NH4+
The degree to which ammonia forms the ammonium ion depends on the pH of the solution. If the pH is low, the equilibrium shifts to the right: more ammonia molecules are converted into ammonium ions. If the pH is high (the concentration of hydrogen ions is low), the equilibrium shifts to the left: the hydroxide ion abstracts a proton from the ammonium ion, generating ammonia.
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.
The conversion of ammonium back to ammonia is easily accomplished by the addition of a strong base.
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