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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 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−
structure

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).

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, O^2-, has not been observed in solution and is not considered as a component in reaction mechanisms. Similarly, orthosilicate, SiO₄^4- 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.

• Monatomic ion
• Salt
• Mass spectrometry
• Hydrogen peroxide
• Molecule
• Organic elements

• List of polyatomic ions
• A Beginner's Guide To Polyatomic Ions.
• Tables of Common Polyatomic Ions. Also a source for PDB files (PDB information)