Polyatomic ion

Summary

An electrostatic potential map of the nitrate ion (NO3). 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 covalently 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. Unlike a molecule, which has a net charge of zero, this chemical species is an ion. (The prefix poly- carries the meaning "many" in Greek, but even ions of two atoms are commonly described as polyatomic.)

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 (HSO4). The removal of another hydrogen ion produces the sulfate anion (SO2−4).

Nomenclature of polyatomic anions

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 alternative to the bi- prefix is to use the word hydrogen in its place: the anion derived from H+ + CO2−3, HCO3, can be called either bicarbonate or hydrogen carbonate.

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 looks at the number of oxygens in an ion. Consider the chlorine oxyanion family:

oxidation state −1 +1 +3 +5 +7
anion name chloride hypochlorite chlorite chlorate perchlorate
formula Cl ClO ClO2 ClO3 ClO4
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 do not work with all polyatomic anions, but they do work with the most common ones. Following table give examples 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
2
O2−
2
SO2−
3
SO2−
4
SO2−
5
selenide hyposelenite selenite selenate
Se2−
Se
2
O2−
2
SeO2−
3
SeO2−
4
telluride hypotellurite tellurite tellurate
Te2−
TeO2−
2
TeO2−
3
TeO2−
4
nitride hyponitrite nitrite nitrate
N3−
N
2
O2−
2
NO
2
NO
3
phosphide hypophosphite phosphite phosphate perphosphate
P3−
H
2
PO
2
PO3−
3
PO3−
4
PO3−
5
arsenide hypoarsenite arsenite arsenate
As3−
AsO3−
2
AsO3−
3
AsO3−
4

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 C2H3O2
Benzoate C6H5COO or C7H5O2
Citrate C6H5O3−7
Carbonate CO2−3
Oxalate C2O2−4
Cyanide CN
Chromate CrO2−4
Dichromate Cr2O2−7
Bicarbonate or hydrogencarbonate HCO3
Hydrogen phosphate HPO2−4
Dihydrogen phosphate H2PO4
Hydrogen sulfate or bisulfate HSO4
Manganate MnO2−4
Permanganate MnO4
Azanide or amide NH2
Peroxide O2−2
Hydroxide OH
Bisulfide SH
Thiocyanate SCN
Silicate SiO2−4
Thiosulfate S2O2−3
Cations
Onium ions Carbenium ions Others
Guanidinium C(NH2)+3 Tropylium C7H+7 Mercury(I) Hg2+2
Ammonium NH+4 Triphenylcarbenium (C6H5)3C+
Phosphonium PH+4 cyclopropenium C3H+3
Hydronium H3O+
Fluoronium H2F+
Pyrylium C5H5O+

See also

External links

  • General Chemistry Online: Companion Notes: Compounds: Polyatomic ions
  • List of polyatomic ions
  • Tables of Common Polyatomic Ions, including PDB files