Polyatomic ion

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An electrostatic potential map of the nitrate ion (NO3). Areas coloured red are lower in energy than areas coloured yellow.

A polyatomic ion, also known as a molecular ion, is a charged chemical species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered to be acting as a single unit. The prefix "poly-" means "many," in Greek, but even ions of two atoms are commonly referred to as polyatomic. In older literature, a polyatomic ion is also referred to as a radical, and less commonly, as a radical group. In contemporary usage, the term radical refers to free radicals that are (not necessarily charged) species with an unpaired electron.

An example of a polyatomic ion is the hydroxide ion - consisting of one oxygen atom and one hydrogen atom, hydroxide has a charge of −1. Its chemical formula is OH. An ammonium ion is made up of one nitrogen atom and four hydrogen atoms: it has a charge of +1, an its chemical formula is NH4+.

Polyatomic ions are often useful in the context of acid-base chemistry or in the formation of salts. A polyatomic ion can often be considered as the conjugate acid/base of a neutral molecule. For example, the conjugate base of sulfuric acid (H2SO4) is the polyatomic hydrogen sulfate anion (HSO4-). The removal of another hydrogen ion yields the sulfate anion (SO42-).

Nomenclature of polyatomic anions[edit]

There are two "rules" 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 alternate to the bi- prefix is to use the word hydrogen in its place: the anion derived from H+
+ CO2−
3
, HCO
3
, 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 H
2
SO
4
, which can be regarded as SO
3
+ H
2
O
.

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
ClO
2
ClO
3
ClO
4
structure The chloride ion The hypochlorite ion The chlorite ion The chlorate ion The perchlorate ion

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 one more. In all situations, the charge is not affected. 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 will not work with all polyatomic anions, but they do work with the most common ones (sulfate, phosphate, nitrate, chlorate).

Examples of common polyatomic ions[edit]

The following tables give 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
Acetate (ethanoate) CH
3
COO
or C
2
H
3
O
2
Acetylide C2−
2
Benzoate C
6
H
5
COO
or C
7
H
5
O
2
Carbonate CO2−
3
Chromate CrO2−
4
Citrate C
6
H
5
O3−
7
Cyanide CN
Hypochlorite ClO
Chlorite ClO
2
Chlorate ClO
3
Perchlorate ClO
4
Dichromate Cr
2
O2−
7
Dihydrogen phosphate H
2
PO
4
Hydrogen carbonate (bicarbonate) HCO
3
Hydrogen sulfate (bisulfate) HSO
4
Hydrogen phosphate HPO2−
4
Hydroxide OH
Nitrite NO
2
Nitrate NO
3
Peroxide O2−
2
Permanganate MnO
4
Phosphate PO3−
4
Sulfite SO2−
3
Sulfate SO2−
4
Cations
Ammonium NH+
4
Phosphonium PH+
4
Hydronium H
3
O+
Fluoronium H
2
F+
Mercury(I) Hg2+
2
Tropylium C
7
H+
7
Guanidinium C(NH
2
)+
3

See also[edit]

External links[edit]