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 usually has a net charge that is not zero, or in the special case of a zwitterion, where spatially separated charges may lead the net charge to be variable depending on acidity conditions. 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. There may be more than one atom in the structure that has non-zero charge, therefore the net charge of the structure may have a cationic (positive) or anionic (negative) nature depending on those atomic details.

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.

Naming oxyanions

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 (or otherwise increases the oxidation state), 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.

Generally, the change in prefix corresponds to a change in oxidation state. The main exception is the per- prefix, as only halogens and some transition metals can be oxidized to the +7 or greater oxidation states that would normally use per-. For other elements, it is used as shorthand for peroxy-, which has the same oxidation state as the prior -ate anion, but contains a peroxide group instead of a single oxygen. There are also cases where the oxidation state increases but the number of oxygen atoms does not, such as the oxidation of manganate (MnO2−4) to permanganate (MnO−4).

Some oxyanions form dimers, usually by losing an equivalent of oxide. These anions are given the prefix di- or pyro- (as many can be prepared by heating). These anions contain X−O−X bonds, and are structurally related to acid anhydrides of the conjugate acid. The pyro- prefix is only used for these kinds of dimers; others, such as hyponitrite, contain different bond structures despite having a formula that suggests it is "made" of two nitroxide units.

The following table shows the patterns of ion naming for some common ions and their derivatives. Exceptions to the rules are highlighted in yellow, while anions too unstable to exist are marked out with a red "none".

Element	Type of anion	Reduced anion		hypo-		-ite		-ate		per- or peroxy-
Chlorine	All	Chloride	Cl−	Hypochlorite	ClO−	Chlorite	ClO−2	Chlorate	ClO−3	Perchlorate	ClO−4
Nitrogen	Simple anion	Nitride	N3−	Template:CellCategory	Template:CellCategory	Nitrite	NO−2	Nitrate	NO−3	Peroxynitrate	NO−4
	Template:Encodefirst	Template:CellCategory	Template:CellCategory	Template:CellCategory	Template:CellCategory	None		None		None
Sulfur	Simple anion	Sulfide	S2−	Template:CellCategory	Template:CellCategory	Sulfite	SO2−3	Sulfate	SO2−4	Persulfate or peroxysulfate	SO2−5
	Protonated	Bisulfide	HS−	Hydrogen sulfoxylate	HSO−2	Bisulfite or hydrogen sulfite	HSO−3	Bisulfate or hydrogen sulfate	HSO−4	Hydrogen persulfate	HSO−5
	Dimer	Disulfide	S2−2	Template:Encodefirst		Pyrosulfite or disulfite	S2O2−5	Pyrosulfate or disulfate	S2O2−7	Peroxydisulfate	S2O2−8
Phosphorus	Simple anion	Phosphide	P3−	None		None		Phosphate or orthophosphate	PO3−4	Peroxymonophosphate	PO3−5
	Protonated once	None		None		Template:CellCategory	Template:CellCategory	Hydrogen phosphate	HPO2−4	Hydrogen peroxymonophosphate	HPO2−5
	Protonated twice	Template:CellCategory	Template:CellCategory	Template:CellCategory	Template:CellCategory	Hydrogen phosphite	H2PO−3	Dihydrogen phosphate	H2PO−4	Dihydrogen peroxymonophosphate	H2PO−5
	Dimer	No dimer; many other polyphosphides	P2−4, P3−7, P3−11, etc.	None		Diphosphite or pyrophosphite	H2P2O2−5	Diphosphate or pyrophosphate	P2O4−7	Peroxydiphosphate	P4O4−8

Other examples of common polyatomic ions

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

Anions

Inorganic carbon anions		Alkoxides		Carboxylates		Transition metal oxyanions		Other notable anions
Carbonate	CO2−3	Methoxide (methanolate)	CH3O−	Formate (methanoate)	HCOO−	Manganate	MnO2−4	Hydroxide	OH−
Bicarbonate or hydrogen carbonate	HCO−3	Ethoxide (ethanolate)	CH3CH2O− or C2H5O−	Acetate (ethanoate)	CH3COO− or C2H3O−	Permanganate	MnO−4	Peroxide	O2−2
Acetylide	C2−2	Phenolate	C6H5O−	Benzoate	C6H5COO− or C7H5O−2	Chromate	CrO2−4	Superoxide	O−2
Cyanide	CN−	tert-Butoxide	(CH3)3CO−	Oxalate	C2O2−4	Dichromate	Cr2O2−7	Azanide	NH−2
Cyanate	OCN−			Citrate	C6H5O3−7	Orthotungstate	WO2−4	Orthosilicate	SiO4−4
Thiocyanate	SCN−							Borohydride	BH−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+

Zwitterion and polycharged polyatomic ions

Many polyatomic molecules can carry spatially separated charges, forming polycharged polyatomic ions. An important case of these compounds are zwitterions, which are neutral compounds but have opposing formal charges within the same molecule. A typical example are amino acids, which carry both charged amino and carboxyl groups. These charges can influence the chemical and physical properties of substances.

Many zwitterions exhibit tautomerism with a "parent" molecule without formal charges. For example, glycine reversibly converts between the parent molecule and a zwitterionic form by transfer of a labile hydrogen atom between the protonated amino group and carboxylate group. By contrast, trimethylglycine has three non-labile methyl groups, making quaternary ammonium, so it does not interconvert with the non-zwitterionic isomer (a dimethylglycine ester). These non-tautomeric zwitterions are called betaines.

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