Polyatomic Ion - Nomenclature

Nomenclature

There are two "rules" that can be used for 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, 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 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. 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).