{{about|the ammonium ion|its neutral conjugate base|ammonia}} {{About|the molecular ion|the ancient city|Siwa Oasis}} {{Chembox | ImageFile1 = Ammonium-2D.svg | ImageClass1 = skin-invert | ImageSize1 = 170px | ImageAlt1 = 2-D skeletal version of the ammonium ion | ImageFileL1 = Ammonium-3D-balls.png | ImageClassL1 = bg-transparent | ImageAltL1 = Ball-and-stick model of the ammonium cation | ImageFileR1 = Ammonium-3D-vdW.png | ImageClassR1 = bg-transparent | ImageAltR1 = Space-filling model of the ammonium cation | SystematicName = Azanium<ref>{{RedBookRef|pages=71,105,314}}</ref> | IUPACName = Ammonium ion | OtherNames = | Section1 = {{Chembox Identifiers | CASNo = 14798-03-9 | CASNo_Ref = {{cascite|correct|CAS}} | ChEBI = 28938 | ChemSpiderID = 218 | Gmelin = 84 | KEGG = C01342 | PubChem = 16741146 | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 54S68520I4 | InChI = 1/H3N/h1H3/p+1 | InChIKey = QGZKDVFQNNGYKY-IKLDFBCSAZ | StdInChI = 1S/H3N/h1H3/p+1 | StdInChIKey = QGZKDVFQNNGYKY-UHFFFAOYSA-O | MeSHName = D000644 | SMILES = [NH4+] }} | Section2 = {{Chembox Properties | Formula = {{chem2|[NH4]+}} | N=1|H=4 | pKa = 9.25 | ConjugateBase = Ammonia }} | Section3 = {{Chembox Structure | MolShape = Tetrahedral }} | Section9 = {{Chembox Related | OtherCompounds = Ammonium radical •NH4 | OtherCations = {{ubl|Phosphonium {{chem2|[PH4]+}}|Arsonium {{chem2|[AsH4]+}}|Hydronium {{chem2|[H3O]+}}|Sulfonium {{chem2|[H3S]+}}|Fluoronium {{chem2|[H2F]+}}|Chloronium {{chem2|[H2Cl]+}}|Bromonium {{chem2|[H2Br]+}}|Iodonium {{chem2|[H2I]+}}|Carbonium {{chem2|[CH5]+}}|Trihydrogen cation {{chem2|[H3]+}}}} }} }}
'''Ammonium''' is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) molecular ion with the chemical formula {{chem2|NH4+}} or {{chem2|[NH4]+}}. It is formed by the addition of a proton (a hydrogen nucleus) to ammonia ({{chem2|NH3}}). Ammonium is also a general name for positively charged (protonated) substituted amines and quaternary ammonium cations ({{chem2|[NR4]+}}), where one or more hydrogen atoms are replaced by organic or other groups (indicated by R). Not only is ammonium a source of nitrogen and a key metabolite for many living organisms, but it is an integral part of the global nitrogen cycle.<ref name=":02">{{Cite book |last1=Schlesinger |first1=William H. |chapter=Chapter 12 - The Global Cycles of Nitrogen, Phosphorus and Potassium |date=2020-01-01 |title=Biogeochemistry |edition=Fourth |pages=483–508 |editor-last=Schlesinger |editor-first=William H. |chapter-url=https://www.sciencedirect.com/science/article/pii/B9780128146088000128 |access-date=2024-03-08 |publisher=Academic Press |doi=10.1016/b978-0-12-814608-8.00012-8 |isbn=978-0-12-814608-8 |last2=Bernhardt |first2=Emily S. |editor2-last=Bernhardt |editor2-first=Emily S.|chapter-url-access=subscription }}</ref> As such, human impact in recent years could have an effect on the biological communities that depend on it.
== Acid–base properties == [[File:Hydrochloric acid ammonia.jpg|thumb|Fumes from hydrochloric acid and ammonia forming a white cloud of ammonium chloride]]
The ammonium ion is generated when ammonia, a weak base, reacts with Brønsted acids (proton donors): : {{chem2|H+ + NH3 → [NH4]+}} The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule: : {{chem2|[NH4]+ + B- → HB + NH3}} Thus, the treatment of concentrated solutions of ammonium salts with a strong base gives ammonia. When ammonia is dissolved in water, a tiny amount of it converts to ammonium ions: : {{chem2|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 and hydroxide ions is high), 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.
== Salts and characteristic reactions == thumb|class=skin-invert-image|Formation of ammonium Ammonium cation is found in a variety of salts such as ammonium carbonate, ammonium chloride, and ammonium nitrate. Most simple ammonium salts are very soluble in water. An exception is ammonium hexachloroplatinate, the formation of which was once used as a test for ammonium. The ammonium salts of nitrate and especially perchlorate are highly explosive, in these cases, ammonium is the reducing agent.
In an unusual process, ammonium radicals ({{chem2|NH4}}) form an amalgam. Such species are prepared by the addition of sodium amalgam to a solution of ammonium chloride.<ref>{{Cite web |url=http://www.hull.ac.uk/chemistry/intro_inorganic/Chap9.htm |title=Pseudo-binary compounds |access-date=2007-10-12 |archive-date=2008-03-09 |archive-url=https://web.archive.org/web/20080309033407/http://www.hull.ac.uk/chemistry/intro_inorganic/Chap9.htm |url-status=dead }}</ref> This amalgam eventually decomposes to release ammonia and hydrogen.<ref>{{cite encyclopedia|encyclopedia = VIAS Encyclopedia|title = Ammonium Salts |url = http://www.vias.org/encyclopedia/chem_ammonia_salts.html}}</ref>
To find whether the ammonium ion is present in the salt, first, the salt is heated in presence of alkali hydroxide releasing a gas with a characteristic smell, which is ammonia. : {{chem2|[NH4]+ + OH- -> NH3 + H2O}}
To further confirm ammonia, it is passed through a glass rod dipped in an {{Chem2|HCl}} solution (hydrochloric acid), creating white dense fumes of ammonium chloride. : {{chem2|NH3 + HCl → [NH4]Cl}}
Ammonia or ammonium ion when added to Nessler's reagent gives a brown color precipitate known as the iodide of Million's base in basic medium.
Ammonium ion when added to chloroplatinic acid gives a yellow precipitate of ammonium hexachloroplatinate(IV). : {{chem2|H2[PtCl6] + [NH4]+ → [NH4]2[PtCl6](s) + 2 H+}}
Ammonium ion when added to sodium cobaltinitrite gives a yellow precipitate of ammonium cobaltinitrite. : {{chem2|Na3[Co(NO2)6] + 3 [NH4]+ → [NH4]3[Co(NO2)6](s) + 3 Na+}}
Ammonium ion gives a white precipitate of ammonium bitartrate when added to potassium bitartrate. : {{chem2|KC4H5O6 + [NH4]+ �� [NH4]C4H5O6(s) + K+}}
== Structure and bonding == The lone electron pair on the nitrogen atom (N) in ammonia, represented as a line above the N, forms a coordinate bond with a proton ({{chem2|H+}}). After that, all four {{chem2|N\sH}} bonds are equivalent, being polar covalent bonds. The ion has a tetrahedral structure and is isoelectronic with methane and the borohydride anion. In terms of size, the ammonium cation (''r''<sub>ionic</sub> = 148 pm), it is intermediate in size between potassium (144 pm) and rubidium ions (152 pm) when in an octahedral environment.<ref>{{cite journal |last1=Shannon |first1=R. D. |title=Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides |journal=Acta Crystallographica Section A |date=1976 |volume=32 |issue=5 |pages=751–767 |doi=10.1107/S0567739476001551 |bibcode=1976AcCrA..32..751S }}</ref>
The vibrational spectrum consists of two main sets of absorptions, ν<sub>N-H</sub> and δ<sub>HNH</sub>. These bands are found near 3300 and 1400 cm<sup>−1</sup>.<ref>{{cite journal |last1=Meeron |first1=Emmanuel |title=Mayer's Treatment of Ionic Solutions |journal=The Journal of Chemical Physics |date=1957 |volume=26 |issue=4 |pages=804–806 |doi=10.1063/1.1743411 |bibcode=1957JChPh..26..804M }}</ref>
== Organic ions == {{See also|Amine}} The hydrogen atoms in the ammonium ion can be substituted with an alkyl group or some other organic group to form a '''substituted ammonium ion''' (IUPAC nomenclature: '''aminium ion'''). Depending on the number of organic groups, the ammonium cation is called a primary, secondary, tertiary, or quaternary. Except the quaternary ammonium cations, the organic ammonium cations are weak acids.
An example of a reaction forming an ammonium ion is that between dimethylamine, {{chem2|(CH3)2NH}}, and an acid to give the dimethylammonium cation, {{chem2|[(CH3)2NH2]+}}: : class=skin-invert-image|400px
Quaternary ammonium cations have four organic groups attached to the nitrogen atom, they lack a hydrogen atom bonded to the nitrogen atom. These cations, such as the tetra-''n''-butylammonium cation, are sometimes used to replace sodium or potassium ions to increase the solubility of the associated anion in organic solvents. Primary, secondary, and tertiary ammonium salts serve the same function but are less lipophilic. They are also used as phase-transfer catalysts and surfactants.
An unusual class of organic ammonium salts is derivatives of amine radical cations, {{chem2|[•NR3]+}} such as tris(4-bromophenyl)ammoniumyl hexachloroantimonate.
== Biology == [[File:Nitrogen Cycle 2.svg|thumb|293x293px|Ammonium exists as a result of ammonification and decomposers. Ammonium is eventually nitrified, where it contributes to the flow of nitrogen through the ecosystem. Human impacts are not shown here, but can impact the global nitrogen cycle.]] Ammonium is utilized by living organism for biosynthesis and as an electron donor.<ref name=":02"/>
In part related to the use of fertilizers, there is much interest in ammonium in soils.<ref>{{Cite journal |last1=Barsdate |first1=Robert J. |last2=Alexander |first2=Vera |date=January 1975 |title=The Nitrogen Balance of Arctic Tundra: Pathways, Rates, and Environmental Implications |url=https://acsess.onlinelibrary.wiley.com/doi/10.2134/jeq1975.00472425000400010025x |journal=Journal of Environmental Quality |language=en |volume=4 |issue=1 |pages=111–117 |doi=10.2134/jeq1975.00472425000400010025x |bibcode=1975JEnvQ...4..111B |issn=0047-2425|url-access=subscription }}</ref><ref>{{Cite journal |last1=Nadelhoffer |first1=Knute J. |last2=Aber |first2=John D. |last3=Melillo |first3=Jerry M. |date=1984-10-01 |title=Seasonal patterns of ammonium and nitrate uptake in nine temperate forest ecosystems |url=https://doi.org/10.1007/BF02140039 |journal=Plant and Soil |language=en |volume=80 |issue=3 |pages=321–335 |doi=10.1007/BF02140039 |bibcode=1984PlSoi..80..321N |issn=1573-5036|url-access=subscription }}</ref>
===Ammonia oxidation=== oxidation of ammonium occurs nitrification, which produces nitrate and nitrite.<ref name=":1">{{Cite journal |last=Rosswall |first=T. |date=1982 |title=Microbiological regulation of the biogeochemical nitrogen cycle / Regulación microbiana del ciclo bíogeoquímico del nitrógeno |url=https://www.jstor.org/stable/42934020 |journal=Plant and Soil |volume=67 |issue=1/3 |pages=15–34 |doi=10.1007/BF02182752 |jstor=42934020 |issn=0032-079X}}</ref> This process is a form of autotrophy that is common amongst ''Nitrosomonas'', ''Nitrobacter'', ''Nitrosolobus'', and ''Nitrosospira'', amongst others.<ref name=":1" /> Conversely, nitrate and nitrite can be reduced to ammonium in a process known as assimilatory nitrate reduction.<ref name=":2">{{Cite journal |last1=Tiedje |first1=J. M. |last2=Sørensen |first2=J. |last3=Chang |first3=Y.-Y. L. |date=1981 |title=Assimilatory and Dissimilatory Nitrate Reduction: Perspectives and Methodology for Simultaneous Measurement of Several Nitrogen Cycle Processes |url=https://www.jstor.org/stable/45128674 |journal=Ecological Bulletins |issue=33 |pages=331–342 |jstor=45128674 |issn=0346-6868}}</ref>
When nitrification is slow, which is common in hypoxic soils, ammonium can accumulate in soils.<ref>{{Cite journal |last1=Wang |first1=Lixin |last2=Macko |first2=Stephen A. |date=March 2011 |title=Constrained preferences in nitrogen uptake across plant species and environments |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1365-3040.2010.02260.x |journal=Plant, Cell & Environment |language=en |volume=34 |issue=3 |pages=525–534 |doi=10.1111/j.1365-3040.2010.02260.x |pmid=21118424 |bibcode=2011PCEnv..34..525W |issn=0140-7791}}</ref>
===Excretion=== Ammonium is a waste product from some animals, although it is converted into urea in mammals, sharks, and amphibians, and into uric acid in birds, reptiles, and terrestrial snails.<ref>{{Cite book |last1=Campbell |first1=Neil A. |url=http://archive.org/details/biologyc00camp |title=Biology |last2=Reece |first2=Jane B. |date=2002 |location=San Francisco |publisher=Benjamin Cummings |via=Internet Archive |isbn=978-0-8053-6624-2}}</ref> If transport or excretion of ammonium is interrupted, accumulation of ammonium can lead to hyperammonemia.<ref>{{Citation |last1=Nimmana |first1=Bala K. |title=Hyperammonemia |date=2026 |work=StatPearls |url=http://www.ncbi.nlm.nih.gov/books/NBK557504/ |access-date=2026-04-09 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=32491436 |last2=Rout |first2=Preeti}}</ref> Toxic effects arise from high concentrations of glutamate, which can interfere with the TCA cycle.<ref>{{Cite journal |last=Visek |first=Willard J. |date=March 1984 |title=Ammonia: Its Effects on Biological Systems, Metabolic Hormones, and Reproduction |url=https://linkinghub.elsevier.com/retrieve/pii/S0022030284813314 |journal=Journal of Dairy Science |volume=67 |issue=3 |pages=481–498 |doi=10.3168/jds.S0022-0302(84)81331-4 |pmid=6371080 |issn=0022-0302}}</ref>
===Biosynthesis with ammonium=== Once assimilated, it can be incorporated into proteins and DNA.<ref>{{Cite journal |last1=Llácer |first1=José L |last2=Fita |first2=Ignacio |last3=Rubio |first3=Vicente |date=2008-12-01 |title=Arginine and nitrogen storage |url=https://www.sciencedirect.com/science/article/pii/S0959440X08001541 |journal=Current Opinion in Structural Biology |series=Catalysis and regulation / Proteins |volume=18 |issue=6 |pages=673–681 |doi=10.1016/j.sbi.2008.11.002 |pmid=19013524 |issn=0959-440X|hdl=10261/111022 |hdl-access=free }}</ref> <!--Ammonium mobilization is one of the key factors for the symbiotic association between plants and fungi, called mycorrhizae.<ref>{{Cite journal |last1=Hodge |first1=Angela |last2=Storer |first2=Kate |date=2015-01-01 |title=Arbuscular mycorrhiza and nitrogen: implications for individual plants through to ecosystems |url=https://doi.org/10.1007/s11104-014-2162-1 |journal=Plant and Soil |language=en |volume=386 |issue=1 |pages=1–19 |doi=10.1007/s11104-014-2162-1 |bibcode=2015PlSoi.386....1H |issn=1573-5036|url-access=subscription }}</ref>
Plants that use ammonium as a nitrogen source often must invest into more extensive root systems due to ammonium's limited mobility in soils compared to other nitrogen sources.<ref>{{Cite journal |last1=Raven |first1=John A. |last2=Linda |first2=Bernd Wollenweber |last3=Handley |first3=L. |date=May 1992 |title=Ammonia and ammonium fluxes between photolithotrophs and the environment in relation to the global nitrogen cycle |url=https://nph.onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1992.tb01087.x |journal=New Phytologist |language=en |volume=121 |issue=1 |pages=5–18 |doi=10.1111/j.1469-8137.1992.tb01087.x |bibcode=1992NewPh.121....5R |issn=0028-646X}}</ref><ref>{{Cite journal |last1=Bloom |first1=A. J. |last2=Jackson |first2=L. E. |last3=Smart |first3=D. R. |date=March 1993 |title=Root growth as a function of ammonium and nitrate in the root zone |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1365-3040.1993.tb00861.x |journal=Plant, Cell & Environment |language=en |volume=16 |issue=2 |pages=199–206 |doi=10.1111/j.1365-3040.1993.tb00861.x |bibcode=1993PCEnv..16..199B |issn=0140-7791|url-access=subscription }}</ref>
Ammonium cannot diffuse across the lipid bilayer.<ref>{{Cite journal |last1=Williamson |first1=Gordon |last2=Bizior |first2=Adriana |last3=Harris |first3=Thomas |last4=Pritchard |first4=Leighton |last5=Hoskisson |first5=Paul A. |last6=Javelle |first6=Arnaud |date=2024-01-31 |title=Biological ammonium transporters from the Amt/Mep/Rh superfamily: mechanism, energetics, and technical limitations |url=https://portlandpress.com/bioscirep/article/44/1/BSR20211209/233932/Biological-ammonium-transporters-from-the-Amt-Mep |journal=Bioscience Reports |language=en |volume=44 |issue=1 |article-number=BSR20211209 |doi=10.1042/BSR20211209 |pmid=38131184 |issn=0144-8463}}</ref>-->
===Ammonium transporters=== transportors or carriers occur widely.<ref name=":0">{{Citation |last1=Mohiuddin |first1=Shamim S. |title=Biochemistry, Ammonia |date=2025 |work=StatPearls |url=http://www.ncbi.nlm.nih.gov/books/NBK541039/ |access-date=2026-02-22 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31082083 |last2=Khattar |first2=Divya}}</ref> Common human transport proteins for ammonium include the Amt/Mep/Rh family.<ref>{{Cite journal |last1=Williamson |first1=Gordon |last2=Bizior |first2=Adriana |last3=Harris |first3=Thomas |last4=Pritchard |first4=Leighton |last5=Hoskisson |first5=Paul A. |last6=Javelle |first6=Arnaud |date=2024-01-31 |title=Biological ammonium transporters from the Amt/Mep/Rh superfamily: mechanism, energetics, and technical limitations |url=https://portlandpress.com/bioscirep/article/44/1/BSR20211209/233932/Biological-ammonium-transporters-from-the-Amt-Mep |journal=Bioscience Reports |language=en |volume=44 |issue=1 |article-number=BSR20211209 |doi=10.1042/BSR20211209 |pmid=38131184 |pmc=10794816 |issn=0144-8463}}</ref> <ref>{{Cite journal |last1=Ariz |first1=Idoia |last2=Boeckstaens |first2=Mélanie |last3=Gouveia |first3=Catarina |last4=Martins |first4=Ana Paula |last5=Sanz-Luque |first5=Emanuel |last6=Fernández |first6=Emilio |last7=Soveral |first7=Graça |last8=von Wirén |first8=Nicolaus |last9=Marini |first9=Anna M. |last10=Aparicio-Tejo |first10=Pedro M. |last11=Cruz |first11=Cristina |date=2018-09-07 |title=Nitrogen isotope signature evidences ammonium deprotonation as a common transport mechanism for the AMT-Mep-Rh protein superfamily |journal=Science Advances |language=en |volume=4 |issue=9 |article-number=eaar3599 |doi=10.1126/sciadv.aar3599 |issn=2375-2548 |pmc=6135547 |pmid=30214933 |bibcode=2018SciA....4.3599A }}</ref><ref>{{Cite journal |last1=Weiner |first1=I. David |last2=Hamm |first2=L. Lee |date=2007-03-01 |title=Molecular Mechanisms of Renal Ammonia Transport |journal=Annual Review of Physiology |language=en |volume=69 |issue=1 |pages=317–340 |doi=10.1146/annurev.physiol.69.040705.142215 |issn=0066-4278 |pmc=4313553 |pmid=17002591}}</ref> Glutamine synthetase is one participant.<ref name=":3">{{Cite journal |last1=Adeva |first1=Maria M. |last2=Souto |first2=Gema |last3=Blanco |first3=Natalia |last4=Donapetry |first4=Cristóbal |date=November 2012 |title=Ammonium metabolism in humans |url=https://linkinghub.elsevier.com/retrieve/pii/S0026049512002703 |journal=Metabolism |language=en |volume=61 |issue=11 |pages=1495–1511 |doi=10.1016/j.metabol.2012.07.007 |pmid=22921946 |url-access=subscription }}</ref><ref>{{Cite journal |last1=Jakhar |first1=Deepika |last2=Sarin |first2=Shiv K. |last3=Kaur |first3=Savneet |date=2024-12-04 |title=Gut microbiota and dynamics of ammonia metabolism in liver disease |url=https://www.nature.com/articles/s44355-024-00011-x |journal=npj Gut and Liver |language=en |volume=1 |issue=1 |pages=11 |doi=10.1038/s44355-024-00011-x |issn=3004-9806|doi-access=free }}</ref> [[File:Urea cycle002.svg|thumb|Urea Cycle showing the formation of carbamoyl phosphate in the mitochondria and the rest of the Urea Cycle occurring in the cytoplasm of the liver cell. ]] Other mechanisms include the Cahill cycle, glutamate, and glutamine to transport ammonium into the mitochondria.<ref>{{Cite web |last=PubChem |title=Glucose-Alanine Cycle |url=https://pubchem.ncbi.nlm.nih.gov/pathway/PathBank:SMP0087221 |access-date=2026-04-09 |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref> This ammonium combines with bicarbonate to form carbamoyl phosphate which then enters the urea cycle to be excreted as urea.<ref name=":3" />
== Human impact == Ammonium deposition from the atmosphere has increased in recent years due to volatilization from livestock waste and increased fertilizer use.<ref>{{Cite journal |last1=Ackerman |first1=Daniel |last2=Millet |first2=Dylan B. |last3=Chen |first3=Xin |date=January 2019 |title=Global Estimates of Inorganic Nitrogen Deposition Across Four Decades |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GB005990 |journal=Global Biogeochemical Cycles |language=en |volume=33 |issue=1 |pages=100–107 |doi=10.1029/2018GB005990 |bibcode=2019GBioC..33..100A |issn=0886-6236}}</ref> Because net primary production is often limited by nitrogen, increased ammonium levels could impact the biological communities that rely on it. For example, increasing nitrogen content has been shown to increase plant growth, but aggravate soil phosphorus levels, which can impact microbial communities.<ref>{{Cite journal |last1=Dong |first1=Junfu |last2=Cui |first2=Xiaoyong |last3=Niu |first3=Haishan |last4=Zhang |first4=Jing |last5=Zhu |first5=Chuanlu |last6=Li |first6=Linfeng |last7=Pang |first7=Zhe |last8=Wang |first8=Shiping |date=2022-06-20 |title=Effects of Nitrogen Addition on Plant Properties and Microbiomes Under High Phosphorus Addition Level in the Alpine Steppe |journal=Frontiers in Plant Science |volume=13 |article-number=894365 |doi=10.3389/fpls.2022.894365 |doi-access=free |issn=1664-462X |pmc=9251499 |pmid=35795351}}</ref>
== Metal == The ammonium cation has very similar properties to the heavier alkali metal cations and is often considered a close equivalent.<ref name = "Holleman&Wiberg">{{Holleman&Wiberg}}</ref><ref name="Stevenson" /><ref name="Bernal&Massey" /> Neutral ammonium is expected to behave as a metal ({{chem2|[NH4]+}} ions in a sea of electrons) at very high pressures, such as inside the giant planets Uranus and Neptune.<ref name="Stevenson">{{cite journal |last1=Stevenson |first1=D. J. |date=November 20, 1975 |title=Does metallic ammonium exist? |journal=Nature |volume=258 |issue= 5532|pages=222–223 |doi=10.1038/258222a0 |bibcode = 1975Natur.258..222S |s2cid=4199721 }}</ref><ref name="Bernal&Massey">{{cite journal |last1=Bernal |first1=M. J. M. |last2=Massey |first2=H. S. W. |date=February 3, 1954 |title=Metallic Ammonium |journal=Monthly Notices of the Royal Astronomical Society |volume=114 |issue= 2|pages=172–179 |bibcode=1954MNRAS.114..172B |doi=10.1093/mnras/114.2.172|doi-access=free }}</ref>
Under normal conditions, ammonium does not exist as a pure metal but does as an amalgam ({{chem2|NH4}} alloy with mercury).<ref>{{cite journal |last1=Reedy |first1=J.H.|date=October 1, 1929|title=Lecture demonstration of ammonium amalgam |journal=Journal of Chemical Education |volume=6 |issue=10 |pages=1767 |doi=10.1021/ed006p1767|bibcode=1929JChEd...6.1767R}}</ref>
== See also == * Onium compounds * Quaternary ammonium cation ({{chem2|[NR4]+}}, where R is organyl) * Tetrafluoroammonium ({{chem2|[NF4]+}}) * Ammonium transporter * f-ratio * Nitrification * Ammonium hydroxide
== References == {{Reflist|30em}}
{{Ammonium salts}} {{Molecules detected in outer space}} {{Nitrogen compounds}} {{Authority control}}
* Category:Cations