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]

An electrostatic potential map of the nitrate ion (N O−3). Areas coloured translucent red, around the outside of the red oxygen atoms themselves, signify the regions of most negative electrostatic potential.

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.^{[citation needed]}^[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 O H⁻. In contrast, an ammonium ion consists of one nitrogen atom and four hydrogen atoms, with a charge of +1; its chemical formula is N H+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, S O2−4, 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
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⁺

References

^ 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.
^ "Ionic Compounds Containing Polyatomic Ions". www.chem.purdue.edu. Retrieved 2022-04-16.
^ "IUPAC - radical (free radical) (R05066)". goldbook.iupac.org. Retrieved 25 January 2023.
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "pyro". doi:10.1351/goldbook.P04959

External links

[PetrucciA50-1] 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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Polyatomic ion

Contents

Nomenclature of polyatomic anions

Other examples of common polyatomic ions

See also

References

External links