Polyatomic ion

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.[1] 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.[2]

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.[3]

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 carbonate(CO2−3) ion.

H+ + CO2−3 → HCO−3.

It is either called as bicarbonate or hydrogen carbonate. This process 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.[4] 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

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

References
  1. Petrucci, Ralph H.; Herring, F. Geoffrey; Madura, Jeffry D.; Bissonnette, Carey (2017). General chemistry: principles and modern applications (Eleventh ed.). Toronto: Pearson. p. A50. ISBN 978-0-13-293128-1.
  2. "Ionic Compounds Containing Polyatomic Ions". www.chem.purdue.edu. Retrieved 2022-04-16.
  3. "IUPAC - radical (free radical) (R05066)". goldbook.iupac.org. Retrieved 25 January 2023.
  4. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "pyro". doi:10.1351/goldbook.P04959
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