There are several patterns 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 . For example, let us consider the carbonate() ion: : + → , which is called either bicarbonate or hydrogen carbonate. The process that forms these ions is called
protonation.
Naming oxyanions 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, , is derived from , which can be regarded as + . The second rule is based on the
oxidation state of the central atom in the ion, which in practice is often (but not always) directly related to the number of oxygen atoms in the ion, following the pattern shown below. The following table shows the
chlorine oxyanion family: As the number of oxygen atoms bound to chlorine increases, the chlorine's oxidation number becomes more positive. This gives rise to the following common pattern: first, the
-ate ion is considered to be the base name; adding a
per- prefix adds an oxygen (or otherwise increases the
oxidation state), while 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, all without changing the charge. 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. Generally, the change in prefix corresponds to a change in
oxidation state. The main exception is the
per- prefix, as only
halogens and some
transition metals can be oxidized to the +7 or greater oxidation states that would normally use
per-. For other elements, it is used as shorthand for
peroxy-, which has the same oxidation state as the prior
-ate anion, but contains a
peroxide group instead of a single oxygen. There are also cases where the oxidation state increases but the number of oxygen atoms does not, such as the oxidation of
manganate () to
permanganate (). Some oxyanions form
dimers, usually by losing an equivalent of
oxide. These anions are given the prefix
di- or
pyro- (as many can be prepared by heating). These anions contain bonds, and are structurally related to
acid anhydrides of the
conjugate acid. The
pyro- prefix is only used for these kinds of dimers; others, such as
hyponitrite, contain different bond structures despite having a formula that suggests it is "made" of two
nitroxide units. The following table shows the patterns of ion naming for some common ions and their derivatives. Exceptions to the rules are highlighted in yellow, while anions too unstable to exist are marked out with a red "none". ==Other examples of common polyatomic ions==