Polyatomic ion

An electrostatic potential map of the nitrate ion (NO−3). 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 bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zero. 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.

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 free radicals, which are species that have an unpaired electron and need not be charged.

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 −1; its chemical formula is OH⁻. In contrast, an ammonium ion consists of one nitrogen atom and four hydrogen atoms, with a charge of +1; its chemical formula is NH+4.

Polyatomic ions often are useful in the context of acid–base chemistry and in the formation of salts.

Often, a polyatomic ion can be considered as the conjugate acid or base of a neutral molecule. For example, the conjugate base of sulfuric acid (H₂SO₄) is the polyatomic hydrogen sulfate anion (HSO−4). The removal of another hydrogen ion produces the sulfate anion (SO2−4).

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 H⁺. For example, let us consider the carbonate(CO2−3) ion:

H⁺ + CO2−3 → HCO−3,

which is called either bicarbonate or hydrogen carbonate. The process that forms these ions is called protonation.

Most of the common polyatomic anions are oxyanions, conjugate bases of oxyacids (acids derived from the oxides of non-metallic elements). For example, the sulfate anion, SO2−4, is derived from H₂SO₄, which can be regarded as SO₃ + 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:

Oxidation state	−1	+1	+3	+5	+7
Anion name	chloride	hypochlorite	chlorite	chlorate	perchlorate
Formula	Cl−	ClO−	ClO−2	ClO−3	ClO−4
Structure

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.

bromide	hypobromite	bromite	bromate	perbromate
Br−	BrO−	BrO− 2	BrO− 3	BrO− 4
iodide	hypoiodite	iodite	iodate	periodate
I−	IO−	IO− 2	IO− 3	IO− 4 or IO5− 6
sulfide	hyposulfite	sulfite	sulfate	persulfate
S2−	S 2O2− 2	SO2− 3	SO2− 4	SO2− 5
selenide	hyposelenite	selenite	selenate
Se2−	Se 2O2− 2	SeO2− 3	SeO2− 4
telluride	hypotellurite	tellurite	tellurate
Te2−	TeO2− 2	TeO2− 3	TeO2− 4
nitride	hyponitrite	nitrite	nitrate	pernitrate
N3−	N 2O2− 2	NO− 2	NO− 3	NO− 4
phosphide	hypophosphite	phosphite	phosphate	perphosphate
P3−	H 2PO− 2	PO3− 3	PO3− 4	PO3− 5
arsenide	hypoarsenite	arsenite	arsenate
As3−	AsO3− 2	AsO3− 3	AsO3− 4

Some oxo-anions can dimerize 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. The prefix pyro is also denoted by the prefix di- . For example, dichromate ion is a dimer.

sulfite	pyrosulfite
SO2− 3	S 2O2− 5
sulfate	pyrosulfate
SO2− 4	S 2O2− 7
phosphite	pyrophosphite
PO3− 3	P 2O4− 5
phosphate	pyrophosphate
PO3− 4	P 2O4− 7
arsenate	pyroarsenate
AsO3− 4	As 2O4− 7
chromate	dichromate
CrO2− 4	Cr 2O2− 7
carbonate	dicarbonate
CO2− 3	C 2O2− 5
selenite	pyroselenite
SeO2− 3	Se 2O2− 5

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.

Anions

Tetrahydroxyborate	B(OH)−4
Acetylide	C2−2
Ethoxide or ethanolate	C2H5O−
Acetate or ethanoate	CH3COO− or C2H3O−2
Benzoate	C6H5COO− or C7H5O−2
Citrate	C6H5O3−7
Formate	HCOO−
Carbonate	CO2−3
Oxalate	C2O2−4
Cyanide	CN−
Chromate	CrO2−4
Dichromate	Cr2O2−7
Bicarbonate or hydrogencarbonate	HCO−3
Hydrogen phosphate	HPO2−4
Dihydrogen phosphate	H2PO−4
Hydrogen sulfate or bisulfate	HSO−4
Manganate	MnO2−4
Permanganate	MnO−4
Zincate	ZnO2−2
Aluminate	AlO−2
Tungstate	WO2−4
Azanide or amide	NH−2
Peroxide	O2−2
Superoxide	O−2
Hydroxide	OH−
Bisulfide	SH−
Cyanate	OCN−
Thiocyanate	SCN−
Orthosilicate	SiO4−4
Thiosulfate	S2O2−3
Azide	N−3
Tetraperoxochromate	Cr(O2)3−4

Cations

Onium ions		Carbenium ions		Others
Guanidinium	C(NH2)+3	Tropylium	C7H+7	Mercury(I)	Hg2+2
Ammonium	NH+4	Triphenylcarbenium	(C6H5)3C+	Dihydrogen	H+2
Phosphonium	PH+4	Cyclopropenium	C3H+3
Hydronium	H3O+	Trifluoromethyl	CF+3
Fluoronium	H2F+
Pyrylium	C5H5O+
Sulfonium	H3S+

See also

• Monatomic ion
• Protonation
• Onium ion

External links

• General Chemistry Online: Companion Notes: Compounds: Polyatomic ions
• List of polyatomic ions
• Tables of Common Polyatomic Ions, including PDB files