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Ammonium group

Ammonium
2-D skeletal version of the ammonium ion
Ball-and-stick model of the ammonium cation
Space-filling model of the ammonium cation
Names
Systematic IUPAC name
Ammonium
Identifiers
14798-03-9
3D model (Jmol) Interactive image
ChEBI CHEBI:CHEBI:28938
ChemSpider 218
MeSH D000644
PubChem 16741146
Properties
NH+
4
Molar mass 18.04 g·mol−1
Acidity (pKa) 9.25
Structure
Tetrahedral
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

The ammonium cation is a positively charged polyatomic ion with the chemical formula NH+
4
. 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 (NR+
4
), where one or more hydrogen atoms are replaced by organic groups (indicated by R).

The ammonium ion is generated when ammonia, a weak base, reacts with Brønsted acids (proton donors):

The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule:

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:

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.


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