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Polyatomic ion


A polyatomic ion is a molecule that bears ionic groups, that is, a molecule with a charge. The majority of biological compounds and inorganic species conform to this strict definition. Ordinarily however, the term refers to small collections of atoms, 3 to perhaps 50 atoms, such as many metal complexes and oxyanions such as sulfate. in Greek, the prefix poly- means "many," which to a chemist means three or more atoms.

A polyatomic ion is an ion consisting of a molecule with covalently bonded atoms or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix poly- means many in Greek. Note, a polyatomic ion is also referred to in older works as a radical. In current usage the term radical refers to free radicals which are uncharged species with an unpaired electron. Some can be used in decomposition reactions, providing spectacular results, such as H2, O2 and iodine.

Hydroxide ions and ammonium ions

  • A hydroxide ion is made of one oxygen atom and one hydrogen atom: its chemical formula is (OH). It has a charge of −1.
  • An ammonium ion is made up of one nitrogen atom and four hydrogen atoms: its chemical formula is (NH4)+. It has charge of +1.

A polyatomic ion can often be considered as the conjugate acid or conjugate base of a neutral molecule. For example the sulfate anion, SO42−, is derived from H2SO4 which can be regarded as SO3 + H2O.

There are two "rules" that can be used for the learning the nomenclature of polyatomic ions. 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. It is a consequence of the hydrogen ion carrying a +1 charge. An alternate to the bi- prefix is to use the word hydrogen in its place: the anion derived from H+ + CO32−, HCO3 can be called either bicarbonate or hydrogen carbonate.

Note that many of the common polyatomic anions are conjugate bases of acids derived from the oxides of non-metallic elements. For example the sulfate anion, SO42−, is derived from H2SO4 which can be regarded as SO3 + H2O.

The second rule looks at the number of oxygens in an ion. Consider the chlorine oxoanion family:

oxidation state −1 +1 +3 +5 +7
anion name chloride hypochlorite chlorite chlorate perchlorate
formula Cl ClO ClO2 ClO3 ClO4

First, think of the -ate ion as being the "base" name, in which case the addition of a per- prefix adds an oxygen. 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 two. In all situations, the charge is not affected.

It is important to note that these rules will not work with all polyatomic ions, but they do work with the most common ones (sulfate, phosphate, nitrate, chlorate).

List of polyatomic ions

Caution: chemists classify ions and molecules even when such species do not exist to any appreciable extent. For example, small ions with high charges are very rare, as illustrated by the fact that oxide, O2-, has not been observed in solution and is not considered as a component in reaction mechanisms. Similarly, orthosilicate, SiO44- enjoys no status as an ion in aqueous solution, except perhaps under extreme temperatures. In general, ions that have charges greater than 2- do not exist in solution unless they are protonated.

Polyatomic ions
Acetate CH3COO or C2H3O2
Aluminate AlO2, Al2O42−
Amide NH2
Ammonium NH4+
Antimonate SbO43−
Antimonite SbO33−
Arsenate AsO43−
Arsenite AsO33−
Azide N3
Benzoate C6H5COO
Bicarbonate (hydrogen carbonate) HCO3
Borate BO33−
Metaborate BO2
Tetraborate B4O72−
Bromate BrO3
Bromite BrO2
Carbide C22−
Carbonate CO32−
Chlorate ClO3
Chlorite ClO2
Chromate CrO42−
Chromite CrO2
Chromyl CrO22+
Citrate C6H5O73−
Cyanate OCN
Cyanide CN
Dichromate Cr2O72−
Dihydrogen arsenate H2AsO4
Dihydrogen phosphate H2PO4
Dihydrogen phosphite H2PO3
Dioxygenyl O2+
Disulfide S22−
Ferrate FeO42−
Ferricyanide Fe(CN)63−
Ferrocyanide Fe(CN)64−
Formate (formiate) HCO2
Fulminate CNO
Hydrazide N2H3
Hydrogen carbonate (bicarbonate) HCO3
Hydrogen arsenate HAsO42−
Hydrogen phosphate HPO42−
Hydrogen phosphite HPO32−
Hydrogen sulfate HSO4
Hydrogen sulfite HSO3
Hydrogen telluride HTe
Hydronium H3O+
Hydroxide OH
Hypobromite BrO
Hypochlorite ClO
Hypoiodite IO
Hypophosphite PO23−
Hyposulfite SO22−
Periodate IO4
Iodate IO3
Triiodide I3
Iodite IO2
Isocyanate NCO
Mercury(I) Hg22+
Manganate MnO4
Molybdate MoO42−
Nitrate NO3
Nitrite NO2
Oxalate C2O42−
Ozonide O3
Perbromate BrO4
Perchlorate ClO4
Permanganate MnO4
Peroxide O22−
Perrhenate ReO4
Peroxomonosulfate SO52−
Peroxodisulfate S2O82−
Pertechnetate TcO4
Phosphate PO43−
Phosphite PO33−
Plumbate PbO32−
Plumbite PbO22−
Pyridinium C5H6N+
Pyrophosphate P2O74−
Pyrylium C5H5O+
Quaternary ammonium NR4+
Selenate SeO42−
Selenite (ion) SeO32−
Silicate SiO32−
Disilicate Si2O52−
Metasilicate SiO32−
Orthosilicate SiO44−
Pyrosilicate Si2O76−
Stannate SnO32−
Stannite SnO22−
Sulfate SO42−
Sulfite SO32−
Sulfonium R3S+
Superoxide O2
Tartrate (CH(OH)COO)22−
Metatellurate TeO42−
Orthotellurate TeO66−
Tellurite TeO32−
Thiocyanate SCN
Thiosulfate S2O32−
Tosylate (toluenesulfonate) CH3C6H4SO3
Triflate (trifluoromethanesulfonate) CF3SO3
Tungstate WO42−
Uranyl UO2+
Vanadate VO3
Vanadyl VO2+
Pervanadyl VO2+

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Polyatomic_ion". A list of authors is available in Wikipedia.
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