Polyatomic ion


A molecular ion is a covalently bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zero. Unlike a molecule, which has a net charge of zero, this chemical species is an ion..
In older literature, a polyatomic ion may instead be referred to as a radical.
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 or base of a neutral molecule. For example, the conjugate base of sulfuric acid is the polyatomic hydrogen sulfate anion. The removal of another hydrogen ion produces the sulfate anion.

Nomenclature of polyatomic anions

There are two "rules" 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 +,, can be called either bicarbonate or hydrogencarbonate.
Most of the common polyatomic anions are oxyanions, conjugate bases of oxyacids. For example, the sulfate anion,, is derived from, which can be regarded as +.
The second rule looks at the number of oxygens in an ion. Consider the chlorine oxyanion family:
oxidation state−1+1+3+5+7
anion namechloridehypochloritechloritechlorateperchlorate
formula
structure

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 one more. 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 do not work with all polyatomic anions, but they do work with the most common ones. Following table give examples for some of these common anion groups.
bromidehypobromitebromitebromateperbromate
IodideHypoioditeIoditeIodateperiodate
or
sulfidehyposulfitesulfitesulfatepersulfate
selenidehyposeleniteseleniteselenate
telluridehypotelluritetelluritetellurate
nitridehyponitritenitritenitrate
phosphidehypophosphitephosphitephosphateperphosphate
arsenidehypoarsenitearsenitearsenate

Other examples of common polyatomic ions

The following tables give additional examples of commonly encountered polyatomic ions. Only a few representatives are given, as the number of polyatomic ions encountered in practice is very large.
Tetrahydroxyborate
Acetylide
Ethoxide or ethanolate
Acetate or ethanoate or
Benzoate or
Citrate
Carbonate
Oxalate
Cyanide
Chromate
Dichromate
Bicarbonate or hydrogencarbonate
Hydrogen phosphate
Dihydrogen phosphate
Hydrogen sulfate or bisulfate
Manganate
Permanganate
Azanide or amide
Peroxide
Hydroxide
Bisulfide
Thiocyanate
Silicate
Thiosulfate