Polyatomic ion

A polyatomic ion, is a charged ion composed 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.

For example, 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.

Nomenclature

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, hydrogen is added to the ion's formula and its charge is increased by 1, the latter being 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.

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. 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 ions, but they do work with the most common ones (sulfate, phosphate, nitrate, chlorate).

Examples of common polyatomic ions

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)	CH3COO− or C2H3O−2
Benzoate	C6H5COO− or C7H5O−2
Bicarbonate (hydrogen carbonate)	HCO−3
Carbonate	CO2−3
Cyanide	CN−
Hydroxide	OH−
Nitrate	NO−3
Phosphate	PO3−4
Sulfate	SO2−4

Cations
Ammonium	NH+4
Hydronium	H3O+
Mercury(I)	Hg2+2