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((Short description|Ion containing two or more atoms))
((More references|date=November 2021))
[[File:Nitrate-ion-elpot.png|thumb|right|200px|An [[electric potential|electrostatic potential]] map of the [[nitrate]] ion (((chem2|auto=yes|NO3-))). Areas coloured translucent red, around the outside of the red oxygen atoms themselves, signify the regions of most negative electrostatic potential.]]
      
A '''polyatomic ion''' (also known as a '''molecular ion''') is a [[covalent bond]]ed set of two or more atoms, or of a [[complex (chemistry)|metal complex]], that can be considered to behave as a single unit and that has a net [[electrical charge|charge]] that is not zero.<ref name="PetrucciA50">((cite book |last1=Petrucci |first1=Ralph H. |last2=Herring |first2=F. Geoffrey |last3=Madura |first3=Jeffry D. |last4=Bissonnette |first4=Carey |title=General chemistry: principles and modern applications |date=2017 |publisher=Pearson |location=Toronto |isbn=978-0-13-293128-1 |page=A50 |edition=Eleventh))</ref> The term [[molecule]] may or may not be used to refer to a polyatomic ion, depending on the definition used. The prefix ''poly-'' carries  the meaning "many" in Greek, but even ions of two atoms are commonly described as polyatomic.<ref>((Cite web |title=Ionic Compounds Containing Polyatomic Ions |url=https://www.chem.purdue.edu/gchelp/nomenclature/poly_atom.htm |access-date=2022-04-16 |website=www.chem.purdue.edu))</ref>

In older literature, a polyatomic ion may instead be referred to as a ''radical'' (or less commonly, as a ''radical group''). In contemporary usage, the term ''radical'' refers to various [[Radical (chemistry)|free radical]]s, which are [[species (chemistry)|species]] that have an [[unpaired electron]] and need not be charged.((Citation needed|date=June 2022))<ref>((cite web |title=IUPAC - radical (free radical) (R05066) |url=https://goldbook.iupac.org/terms/view/R05066 |website=goldbook.iupac.org |access-date=25 January 2023))</ref>

A simple example of a polyatomic ion is the [[hydroxide]] ion, which consists of one oxygen atom and one hydrogen atom, jointly carrying a net charge of [[elementary charge|−1]]; its chemical formula is ((chem2|auto=yes|OH-)). In contrast, an [[ammonium]] ion consists of one nitrogen atom and four hydrogen atoms, with a charge of +1; its chemical formula is ((chem2|auto=yes|NH4+)).

Polyatomic ions often are useful in the context of [[acid–base reaction|acid–base chemistry]] and in the formation of [[salt (chemistry)|salts]].

Often, a polyatomic ion can be considered as the [[conjugate acid|conjugate acid or base]] of a neutral [[molecule]]. For example, the [[conjugate base]] of [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>) is the polyatomic [[hydrogen sulfate]] [[anion]] (((chem2|HSO4-))). The removal of another [[hydron (chemistry)|hydrogen ion]] produces the [[sulfate]] anion (((chem2|SO4(2-)))).

==Nomenclature of polyatomic anions==
There are several patterns that can be used for learning the nomenclature of polyatomic anions. First, when the prefix ''bi'' is added to a name, a hydrogen is added to the ion's formula and its charge is increased by 1, the latter being a consequence of the hydrogen ion's +1 charge. An alternative to the ''bi-'' prefix is to use the word hydrogen in its place: the anion derived from ((chem2|H+)). For example, let us consider carbonate(((chem2|link=carbonate|CO3(2-)))) ion.

((chem2|H+)) + ((chem2|link=carbonate|CO3(2-))) → ((chem2|link=bicarbonate|HCO3-)).

It is either called as bicarbonate or hydrogen carbonate.
This process is called [[protonation]].

Most of the common polyatomic anions are [[oxyanion]]s, conjugate bases of [[oxyacid]]s (acids derived from the [[oxide]]s of [[Nonmetal (chemistry)|non-metallic elements]]). For example, the [[sulfate]] anion, ((chem2|auto=yes|SO4(2-))), is derived from ((chem2|link=sulfuric acid|H2SO4)), which can be regarded as ((chem2|link=sulfur trioxide|SO3)) + ((chem2|link=water|H2O)).

The second rule is based on the [[oxidation state]] of the central atom in the ion, which in practice is often (but not always) directly related to the number of oxygen atoms in the ion, following the pattern shown below. The following table shows the [[chlorine]] [[oxyanion]] family:
{| class="wikitable"
|-
! Oxidation state
| −1
| +1
| +3
| +5
| +7
|-
! Anion name
| [[chloride]]
| [[hypochlorite]]
| [[chlorite]]
| [[chlorate]]
| [[perchlorate]]
|-
! Formula
| ((chem2|Cl-))
| ((chem2|ClO-))
| ((chem2|ClO2-))
| ((chem2|ClO3-))
| ((chem2|ClO4-))
|-
! Structure
| [[File:Chloride-ion-3D-vdW.png|50px|The chloride ion]]
| [[File:Hypochlorite-ion-3D-vdW.png|70px|The hypochlorite ion]]
| [[File:Chlorite-ion-3D-vdW.png|70px|The chlorite ion]]
| [[File:Chlorate-ion-3D-vdW.png|70px|The chlorate ion]]
| [[File:Perchlorate-ion-3D-vdW.png|70px|The perchlorate ion]]
|}

As the number of oxygen atoms bound to chlorine increases, the chlorine's oxidation number becomes more positive. This gives rise to the following common pattern: first, the ''-ate'' ion is considered to be the base name; adding a ''per-'' prefix adds an oxygen, while changing the ''-ate'' suffix to ''-ite'' will reduce the oxygens by one, and keeping the suffix ''-ite'' and adding the prefix ''hypo-'' reduces the number of oxygens by one more, all without changing the charge. The naming pattern follows within many different oxyanion series based on a standard root for that particular series. The ''-ite'' has one less oxygen than the ''-ate'', but different ''-ate'' anions might have different numbers of oxygen atoms.

These rules do not work with all polyatomic anions, but they do apply to several of the more common ones. The following table shows how these prefixes are used for some of these common anion groups.

{| class="wikitable"
|-
| [[bromide]]
| [[hypobromite]]
| [[bromite]]
| [[bromate]]
| [[perbromate]]
|-
| ((chem|Br|-))
| ((chem|BrO|-))
| ((chem|BrO|2|-))
| ((chem|BrO|3|-))
| ((chem|BrO|4|-))
|-
| [[iodide]]
| [[hypoiodite]]
| [[iodite]]
| [[iodate]]
| [[periodate]]
|-
| ((chem|I||-))
| ((chem|IO||−))
| ((chem|IO|2|−))
| ((chem|IO|3|−))
| ((chem|IO|4|−)) or ((chem|IO|6|5−))
|-
| [[sulfide]]
| [[hyposulfite]]
| [[sulfite]]
| [[sulfate]]
| [[peroxomonosulfate|persulfate]]
|-
| ((chem|S|2-))
| ((chem|S|2|O|2|2-))
| ((chem|SO|3|2-))
| ((chem|SO|4|2-))
| ((chem|SO|5|2-))
|-
| [[selenide]]
| [[hyposelenite]]
| [[Selenite (ion)|selenite]]
| [[selenate]]
|
|-
| ((chem|Se|2-))
| ((chem|Se|2|O|2|2-))
| ((chem|SeO|3|2-))
| ((chem|SeO|4|2-))
|
|-
| [[Telluride (chemistry)|telluride]]
| [[hypotellurite]]
| [[tellurite (ion)|tellurite]]
| [[tellurate]]
|
|-
| ((chem|Te|2-))
| ((chem|TeO|2|2-))
| ((chem|TeO|3|2-))
| ((chem|TeO|4|2-))
|
|-
| [[nitride]]
| [[hyponitrite]]
| [[nitrite]]
| [[nitrate]]
| [[peroxynitrate|pernitrate]]
|-
| ((chem|N|3-))
| ((chem|N|2|O|2|2-))
| ((chem|NO|2|-))
| ((chem|NO|3|-))
| ((chem|NO|4|-))
|-
| [[phosphide]]
| [[hypophosphite]]
| [[phosphite]]
| [[phosphate]]
| [[perphosphate]]
|-
| ((chem|P|3-))
| ((chem|H|2|PO|2|-))
| ((chem|PO|3|3-))
| ((chem|PO|4|3-))
| ((chem|PO|5|3-))
|-
| [[arsenide]]
| [[hypoarsenite]]
| [[arsenite]]
| [[arsenate]]
|
|-
| ((chem|As|3-))
| ((chem|AsO|2|3-))
| ((chem|AsO|3|3-))
| ((chem|AsO|4|3-))
|
|-
|}

Some oxo-anions can [[dimer (chemistry)|dimer]]ize with loss of an oxygen atom. The prefix ''pyro'' is used, as the reaction that forms these types of chemicals often involves heating to form these types of structures.<ref>((GoldBookRef|file=P04959|title=pyro))</ref> The prefix ''pyro'' is also denoted by the prefix ''di-'' . For example, dichromate ion is a dimer.

{|class=wikitable
| [[sulfite]]
| [[pyrosulfite]]
|-
| ((chem|S|O|3|2-))
| ((chem|S|2|O|5|2-))
|-
| [[sulfate]]
| [[pyrosulfate]]
|-
| ((chem|S|O|4|2-))
| ((chem|S|2|O|7|2-))
|-
| [[phosphite anion|phosphite]]
| [[pyrophosphite]]
|-
| ((chem|P|O|3|3-))
| ((chem|P|2|O|5|4-))
|-
| [[phosphate]]
| [[pyrophosphate]]
|-
| ((chem|P|O|4|3-))
| ((chem|P|2|O|7|4-))
|-
| [[arsenate]]
| [[pyroarsenate]]
|-
| ((chem|As|O|4|3-))
| ((chem|As|2|O|7|4-))
|-
| [[Chromate and dichromate|chromate]]
| [[dichromate]]
|-
| ((chem|CrO|4|2-))
| ((chem|Cr|2|O|7|2-))
|-
| [[carbonate]]
| [[dicarbonate]]
|-
| ((chem|CO|3|2-))
| ((chem|C|2|O|5|2-))
|}

==Other examples of common polyatomic ions==
The following tables give additional examples of commonly encountered polyatomic ions. Only a few representatives are given, as the number of polyatomic ions encountered in practice is very large.

{|class="wikitable"
|+ [[Anion]]s
|-
| [[Tetrahydroxyborate]]
| ((chem2|B(OH)4-))
|-
| [[Acetylide]]
| ((chem2|C2(2-)))
|-
| [[Ethoxide]] or ethanolate
| ((chem2|C2H5O-))
|-
| [[Acetate]] or ethanoate
| ((chem2|CH3COO-)) or ((chem2|C2H3O2-))
|-
| [[Benzoate]]
| ((chem2|C6H5COO-)) or ((chem2|C7H5O2-))
|-
| [[Citrate]]
| ((chem2|C6H5O7(3-)))
|-
| [[Formate]]
| ((chem2|HCOO-))
|-
| [[Carbonate]]
| ((chem2|CO3(2-)))
|-
| [[Oxalate]]
| ((chem2|C2O4(2-)))
|-
| [[Cyanide]]
| ((chem2|CN-))
|-
| [[Chromate and dichromate|Chromate]]
| ((chem2|CrO4(2-)))
|-
| [[Chromate and dichromate|Dichromate]]
| ((chem2|Cr2O7(2-)))
|-
| [[Bicarbonate]] or hydrogencarbonate
| ((chem2|HCO3(-)))
|-
| [[Phosphate#Chemical properties|Hydrogen phosphate]]
| ((chem2|HPO4(2-)))
|-
| [[Phosphate#Chemical properties|Dihydrogen phosphate]]
| ((chem2|H2PO4(-)))
|-
| [[Hydrogen sulfate]] or bisulfate
| ((chem2|HSO4(-)))
|-
| [[Manganate]]
| ((chem2|MnO4(2-)))
|-
| [[Permanganate]]
| ((chem2|MnO4(-)))
|-
| [[Zincate]]
| ((chem2|ZnO2(2-)))
|-
| [[Aluminate]]
| ((chem2|AlO2-))
|-
| [[Tungstate]]
| ((chem2|WO4(2-)))
|-
| [[Azanide]] or amide
| ((chem2|NH2(-)))
|-
| [[Peroxide]]
| ((chem2|O2(2-)))
|-
| [[Superoxide]]
| ((chem2|O2(-)))
|-
| [[Hydroxide]]
| ((chem2|OH-))
|-
| [[Bisulfide]]
| ((chem2|SH-))
|-
| [[Cyanate]]
| ((chem2|OCN-))
|-
| [[Thiocyanate]]
| ((chem2|SCN-))
|-
| [[Orthosilicate]]
| ((chem2|SiO4(4-)))
|-
| [[Thiosulfate]]
| ((chem2|S2O3(2-)))
|-
| [[Azide]]
| ((chem2|N3(-)))
|-
| [[Tetraperoxochromate]]
| ((chem2|Cr(O2)4(3-)))
|}

{|class="wikitable"
|+ [[Cation]]s
|-
! colspan="2" | [[Onium ion]]s
! colspan="2" | [[Carbenium ion]]s
! colspan="2" | Others
|-
| [[Guanidine#Guanidinium cation|Guanidinium]]
| ((chem2|C(NH2)3+))
| [[Tropylium cation|Tropylium]]
| ((chem2|C7H7+))
| [[Mercury (element)#Compounds of mercury(I)|Mercury(I)]]
| ((chem2|Hg2(2+)))
|-
| [[Ammonium]]
| ((chem2|NH4+))
| [[Triphenylcarbenium]]
| ((chem2|(C6H5)3C+))
| [[Dihydrogen cation|Dihydrogen]]
| ((chem2|H2+))
|-
| [[Phosphonium]]
| ((chem2|PH4+))
| [[Cyclopropenium ion|Cyclopropenium]]
| ((chem2|C3H3+))
|
|
|-
| [[Hydronium]]
| ((chem2|H3O+))
|[[Trifluoromethyl cation|Trifluoromethyl]] 
|((chem2|CF3+))
|
|
|-
| [[Fluoronium]]
| ((chem2|H2F+))
|
|
|
|
|-
| [[Pyrylium salt|Pyrylium]]
| ((chem2|C5H5O+))
|
|
|
|
|-
| [[Sulfonium ]]
| ((chem2|H3S+))
|
|
|
|
|}

==See also==
*[[Monatomic ion]]
*[[Protonation]]
*[[Onium ion]]

==References==
((Reflist))

==External links==
*[https://antoine.frostburg.edu/chem/senese/101/compounds/polyatomic.shtml General Chemistry Online: Companion Notes: Compounds: Polyatomic ions]
*[https://web.archive.org/web/20121227231645/http://www2.pvc.maricopa.edu/tutor/chem/chem130/nomenclature/polyatomicion.html List of polyatomic ions]
*[http://www.chemistry.wustl.edu/~edudev/LabTutorials/PeriodicProperties/Ions/ions.html Tables of Common Polyatomic Ions], including [[Protein Data Bank|PDB]] files

[[Category:Ions]]

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