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

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s, 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, or in special case of zwitterion wear spatially separated charges where the net charge may be variable depending on acidity conditions. The term

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

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 radicals, which are

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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 Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...

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

. For example, the conjugate base of

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

(H₂SO₄) is the polyatomic hydrogen sulfate anion (). 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 particl ...

produces the sulfate anion ().

Nomenclature of polyatomic anions

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. Most of the common polyatomic anions are oxyanions, conjugate bases of oxyacids (acids derived from the

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

s 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 Chlorine is a chemical element; it has Symbol (chemistry), 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 ...

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, 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. Some oxo-anions can dimerize with loss of an oxygen atom. The prefix ''pyro'' is used, as the reaction that forms these types of chemicals often involves heating to form these types of structures. The prefix ''pyro'' is also denoted by the prefix ''di-'' . For example, dichromate ion is a dimer.

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.

Zwitterion and polycharged polyatomic ions

Many polyatomic molecules can carry spatially separated charges, forming zwitterions or, in general, polycharged polyatomic ions. A typical example are

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s, which carry both charged amino and carboxyl groups. These charges can influence the chemical and physical properties of substances.

Applications

Polyatomic ion structure may influence thin film growth. Analyses of polyatomic ion composition is key point in mass-spectrometry.

References

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External links