A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a

metal complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

, that can be considered to behave as a single unit and that has a net

charge Charge or charged may refer to: Arts, entertainment, and media Films * '' Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * ''Charge!!'', an album by The Aqu ...

that is not zero. The term

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

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. 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 radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...

s, which are

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

that have an

unpaired electron In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair. Each atomic orbital of an atom (specified by the three quantum numbers n, l and m) has a capacity to contain ...

and need not be charged. A simple example of a polyatomic ion is the

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...

ion, which consists of one oxygen atom and one hydrogen atom, jointly carrying a net charge of

−1 In mathematics, −1 (also known as negative one or minus one) is the additive inverse of 1, that is, the number that when added to 1 gives the additive identity element, 0. It is the negative integer greater than negative two (−2) and less t ...

; 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 In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...

. Often, a polyatomic ion can be considered as the conjugate acid or base of a neutral

. For example, the

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

of sulfuric acid (H₂SO₄) is the polyatomic hydrogen sulfate

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

(). The removal of another

hydrogen ion A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...

produces the

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

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 hydrogen carbonate. Most of the common polyatomic anions are

oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...

s, conjugate bases of

oxyacid An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce ...

s (acids derived from the oxides of non-metallic elements). For example, the

anion, , is derived from , which can be regarded as + . The second rule is based on the

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

of the "main" 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. Consider the

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

family: As the chlorine atom's oxidation state becomes more positive, the number of oxygen atoms attached increases. This gives rise to the following common pattern: first, think of the ''-ate'' ion as being the "base" name; adding a ''per-'' prefix adds an oxygen, 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. These rules do not work with all polyatomic anions, but they do apply to several of the more common ones. The following table shows how these prefixes are used for some of these common anion groups.

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.

References

{{Reflist

External links

General Chemistry Online: Companion Notes: Compounds: Polyatomic ions
including PDB files Ions

Nomenclature of polyatomic anions

Other examples of common polyatomic ions

See also

References

External links