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A polyatomic ion (also known as a molecular ion) is a set of two or more , or of a metal complex, that can be considered to behave as a single unit and that usually has a net charge that is not zero, (2025). 9780132931281, Pearson. ISBN 9780132931281 or in special case of zwitterion wear spatially separated charges where the net charge may be variable depending on Acidity function conditions. The term molecule may or may not be used to refer to a polyatomic ion, depending on the definition used. The prefix poly- carries the meaning "many" in Greek, but even ions of two atoms are commonly described as polyatomic. There may be more than one atom in the structure that has non-zero charge, therefore the net charge of the structure may have a (positive) or nature depending on those atomic details.
In older literature, a polyatomic ion may instead be referred to as a radical (or less commonly, as a radical group). In contemporary usage, the term radical refers to various free radicals, which are species that have an unpaired electron and need not be charged.
A simple example of a polyatomic ion is the hydroxide ion, which consists of one oxygen atom and one hydrogen atom, jointly carrying a net charge of −1; its chemical formula is . In contrast, an ammonium ion consists of one nitrogen atom and four hydrogen atoms, with a charge of +1; its chemical formula is .
Polyatomic ions often are useful in the context of acid–base chemistry and in the formation of salts.
Often, a polyatomic ion can be considered as the conjugate acid of a neutral molecule. For example, the conjugate base of sulfuric acid (H2SO4) is the polyatomic hydrogen sulfate anion (). The removal of another hydrogen ion produces the sulfate anion ().
which is called either bicarbonate or hydrogen carbonate. The process that forms these ions is called protonation.
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 and some 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 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".
| + ! colspan=2 | Inorganic carbon anions ! colspan=2 | ! colspan=2 | ! colspan=2 | Transition metal oxyanions ! colspan=2 | Other notable anions |
| or | |||||
| + | |||||
| Guanidinium | Tropylium cation | Mercury(I) | |||
| Ammonium | Triphenylcarbenium | Dihydrogen | |||
| Phosphonium | Cyclopropenium | ||||
| Hydronium | Trifluoromethyl | ||||
| Fluoronium | |||||
| Pyrylium salt | |||||
| Sulfonium |
Many zwitterions exhibit with a "parent" molecule without formal charges. For example, glycine reversibly converts between the parent molecule and a zwitterionic form by transfer of a labile hydrogen atom between the protonated amino group and carboxylate group. By contrast, trimethylglycine has three non-labile methyl groups, making quaternary ammonium, so it does not interconvert with the non-zwitterionic isomer (a dimethylglycine ester). These non-tautomeric zwitterions are called .
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