Polyatomic ion

A polyatomic ion is a charged species (ion) comprised of two or more atoms covalently bonded 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, but even ions of two atoms are commonly referred to as polyatomic. 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.

Hydroxide ions and ammonium ions
 * A hydroxide ion is made of one oxygen atom and one hydrogen atom: its chemical formula is −. It has a charge of −1.
 * An ammonium ion is made up of one nitrogen atom and four hydrogen atoms: its chemical formula is +. 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, 2−, 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, 2−, 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:

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.