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A polyatomic ion, also known as a molecular ion, is a
covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of attraction bet ...
set of two or more atoms, or of a
metal complex A coordination complex consists of a central atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday object ...
, 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'' (David Ford album) * Charge (Machel Montano album), ''Charge'' (Mac ...
that is not zero. Unlike a
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
, which has a net charge of zero, this
chemical species A chemical species is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touc ...
is an
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
. (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 biological classification, 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 individu ...
that have an
unpaired electron Periodic table with elements that have unpaired electrons coloured In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their comp ...
and need not be charged.) A simple example of a polyatomic ion is the
hydroxide Hydroxide is a diatomic anion with chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other sym ...
ion, which consists of one oxygen atom and one hydrogen atom, jointly carrying a net charge of
−1 In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
; its chemical formula is . In contrast, an
ammonium The ammonium cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects t ...

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 Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
. Often, a polyatomic ion can be considered as the conjugate acid or base of a neutral
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
. For example, the
conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction An acid–base reaction is a chemical reaction A chemical reactio ...
of
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
(H2SO4) is the polyatomic
hydrogen sulfate Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the sam ...

hydrogen sulfate
anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
(). The removal of another
hydrogen ion A hydrogen ion is created when a hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance cons ...
produces the
sulfate The sulfate or sulphate ion is a polyatomic anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having ...

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 hydrogen carbonate. Most of the common polyatomic anions are
oxyanionAn oxyanion, or oxoanion, is an ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyd ...
s, conjugate bases of
oxyacid An oxyacid, oxoacid, or ternary acid is an acid An acid is a molecule or ion capable of donating a proton (hydrogen ion H+) (a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid), or, alternatively, capable of forming a covalent bo ...
s (acids derived from the
oxide An oxide () is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by che ...
s of non-metallic elements). For example, the
sulfate The sulfate or sulphate ion is a polyatomic anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having ...

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 Chlorine is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemica ...

chlorine
oxyanionAn oxyanion, or oxoanion, is an ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyd ...
family: 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.


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. {, class="wikitable" , +
Cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are u ...
s , - ! colspan="2" ,
Onium ion An “-onium” (plural: onia) is a bound state In quantum physics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the ...
s ! colspan="2" ,
Carbenium ion A carbenium ion is a cation, positive ion with the structure RR′R″C+, that is, a chemical species with a Valence (chemistry), trivalent carbon that bears a +1 formal charge. In older literature the name carbonium ion was used for this class, bu ...
s ! colspan="2" , Others , - ,
Guanidinium Guanidine is the compound with the formula HNC(NH2)2. It is a colourless solid that dissolves in polar solvents. It is a strong base that is used in the production of plastics and explosives. It is found in urine Urine is a liquid by-product ...
, ,
Tropylium In organic chemistry, the tropylium ion or cycloheptatrienyl cation is an aromatic forms of benzene (top) combine to produce an average structure (bottom) In chemistry Chemistry is the scientific discipline involved with Chemical element ...
, , Mercury(I) , , - ,
Ammonium The ammonium cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects t ...

Ammonium
, ,
Triphenylcarbenium
Triphenylcarbenium
, , , , - ,
Phosphonium The phosphonium (more obscurely: phosphinium) cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having ...
, , cyclopropenium , , , , - ,
Hydronium In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...

Hydronium
, , , , , , - ,
Fluoronium The fluoronium ion is an inorganic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, propertie ...
, , , , , , - ,
Pyrylium Pyrylium is a cation An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and opposite to charged p ...
, {{chem2, C5H5O+ , , , ,


See also

*
Monatomic ion A monatomic ion is an ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objec ...


External links


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