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An oxyanion, or oxoanion, is an
ion 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 conv ...
with the generic formula (where A represents a chemical element and O represents an
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well a ...
atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determined by the octet rule. The corresponding
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 t ...
of an oxyanion is the compound . The structures of condensed oxyanions can be rationalized in terms of AO''n'' polyhedral units with sharing of corners or edges between polyhedra. The oxyanions (specifically, phosphate and polyphosphate esters) adenosine monophosphate ( AMP), adenosine diphosphate ( ADP) and
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
(ATP) are important in biology.

# Monomeric oxyanions

The formula of
monomeric In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification M ...
oxyanions, , is dictated by 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. Co ...
of the element A and its position in the periodic table. Elements of the first row are limited to a maximum coordination number of 4. However, none of the first row elements has a monomeric oxyanion with that coordination number. Instead,
carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
() and nitrate () have a
trigonal planar In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands a ...
structure with π bonding between the central atom and the oxygen atoms. This π bonding is favoured by the similarity in size of the central atom and oxygen. The oxyanions of second-row elements in the
group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic ide ...
oxidation state are
tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...
. Tetrahedral units are found in
olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
minerals, , but the anion does not have a separate existence as the oxygen atoms are surrounded tetrahedrally by cations in the solid state.
Phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...
(),
sulphate 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 a ...
(), and perchlorate () ions can be found as such in various salts. Many oxyanions of elements in lower oxidation state obey the octet rule and this can be used to rationalize the formulae adopted. For example, chlorine(V) has two valence electrons so it can accommodate three electron pairs from bonds with oxide ions. The charge on the ion is +5 − 3 × 2 = −1, and so the formula is . The structure of the ion is predicted by VSEPR theory to be pyramidal, with three bonding electron pairs and one lone pair. In a similar way, the oxyanion of chlorine(III) has the formula , and is bent with two lone pairs and two bonding pairs. In the third and subsequent rows of the periodic table, 6-coordination is possible, but isolated octahedral oxyanions are not known because they would carry too high an electrical charge. Thus molybdenum(VI) does not form , but forms the tetrahedral
molybdate In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, althou ...
anion, . MoO6 units are found in condensed molybdates. Fully protonated oxyanions with an octahedral structure are found in such species as and . In addition,
orthoperiodate Periodate is an anion composed of iodine and oxygen. It is one of a number of oxyanions of iodine and is the highest in the series, with iodine existing in oxidation state +7. Unlike other perhalogenates, such as perchlorate, it can exist in two ...
can be only partially protonated, with , , , , , , + H+ having p''K''a=11.60.

## Naming

The naming of monomeric oxyanions follows the following rules. ; If central atom is not in Group VII or VIII: ; If central atom is in Group VII or VIII:

# Condensation reactions

In aqueous solution, oxyanions with high charge can undergo condensation reactions, such as in the formation of the dichromate ion, : :2 CrO4^2- + 2 H+ <=> Cr2O7^2- + H2O The driving force for this reaction is the reduction of electrical charge density on the anion and the elimination of the hydronium () ion. The amount of order in the solution is decreased, releasing a certain amount of
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
which makes the Gibbs free energy more negative and favors the forward reaction. It is an example of an acid–base reaction with the monomeric oxyanion acting as a base and the condensed oxyanion acting as its conjugate acid. The reverse reaction is a
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...
reaction, as a
water molecule Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "unive ...
, acting as a base, is split. Further condensation may occur, particularly with anions of higher charge, as occurs with adenosine phosphates. The conversion of ATP to ADP is an hydrolysis reaction and is an important source of energy in biological systems. The formation of most
silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name i ...
minerals can be viewed as the result of a de-condensation reaction in which
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
reacts with a basic oxide, an acid–base reaction in the Lux–Flood sense. :\overset + \overset -> CaSiO3

# Structures and formulae of polyoxyanions

A polyoxyanion is a
polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
ic oxyanion in which multiple oxyanion monomers, usually regarded as polyhedra, are joined by sharing corners or edges. When two corners of a polyhedron are shared the resulting structure may be a chain or a ring. Short chains occur, for example, in
polyphosphate Polyphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 (phosphate) structural units linked together by sharing oxygen atoms. Polyphosphates can adopt linear or a cyclic ring structures. In biology, the polyphosphate e ...
s. Inosilicates, such as pyroxenes, have a long chain of tetrahedra each sharing two corners. The same structure occurs in so-called meta-vanadates, such as ammonium metavanadate, . The formula of the oxyanion is obtained as follows: each nominal silicon ion () is attached to two nominal oxide ions () and has a half share in two others. Thus the stoichiometry and charge are given by: : :$\text +4 + \left(2 \times -2\right) + \left(2 \times \left(\tfrac \times -2\right)\right) = -2$ A ring can be viewed as a chain in which the two ends have been joined. Cyclic
triphosphate Polyphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 (phosphate) structural units linked together by sharing oxygen atoms. Polyphosphates can adopt linear or a cyclic ring structures. In biology, the polyphosphate e ...
, is an example. When three corners are shared the structure extends into two dimensions. In amphiboles, (of which asbestos is an example) two chains are linked together by sharing of a third corner on alternate places along the chain. This results in an ideal formula and a linear chain structure which explains the fibrous nature of these minerals. Sharing of all three corners can result in a sheet structure, as in mica, , in which each silicon has one oxygen to itself and a half-share in three others. Crystalline mica can be cleaved into very thin sheets. The sharing of all four corners of the tetrahedra results in a 3-dimensional structure, such as in quartz.
Aluminosilicate Aluminosilicate minerals ( IMA symbol: Als) are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals. Andalusite, kyanite, and sillimanite are naturally occurri ...
s are minerals in which some silicon is replaced by aluminium. However, the oxidation state of aluminium is one less than that of silicon, so the replacement must be accompanied by the addition of another cation. The number of possible combinations of such a structure is very large, which is, in part, the reason why there are so many aluminosilicates. Octahedral units are common in oxyanions of the larger transition metals. Some compounds, such as salts of the chain-polymeric ion, even contain both tetrahedral and octahedral units. p446 Edge-sharing is common in ions containing octahedral building blocks and the octahedra are usually distorted to reduce the strain at the bridging oxygen atoms. This results in 3-dimensional structures called
polyoxometalate In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are ...
s. Typical examples occur in the Keggin structure of the phosphomolybdate ion. Edge sharing is an effective means of reducing electrical charge density, as can be seen with the hypothetical condensation reaction involving two octahedra: :2 MO6^ + 4H+ -> Mo2O10^ + 2 H2O Here, the average charge on each M atom is reduced by 2. The efficacy of edge-sharing is demonstrated by the following reaction, which occurs when an alkaline aqueous solution of molybdate is acidified. :7 MoO4^2- + 8 H+ <=> Mo7O24^6- + 4 H2O The tetrahedral molybdate ion is converted into a cluster of 7 edge-linked octahedra giving an average charge on each molybdenum of . The heptamolybdate cluster is so stable that clusters with between 2 and 6 molybdate units have not been detected even though they must be formed as intermediates.

# Heuristic for acidity

The pKa of the related acids can be guessed from the number of double bonds to oxygen. Thus perchloric acid is a very strong acid while hypochlorous acid is very weak. A simple rule usually works to within about 1 pH unit.

# Acid–base properties

Most oxyanions are weak bases and can be protonated to give acids or acid salts. For example, the phosphate ion can be successively protonated to form phosphoric acid. :PO4^3- + H+ <=> HPO4^2- :HPO4^2- + H+ <=> H2PO4- :H2PO4- + H+ <=> H3PO4 The extent of protonation in aqueous solution will depend on the
acid dissociation constant In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction :HA ...
s and pH. For example, AMP (adenosine monophosphate) has a p''K''a value of 6.21, so at pH 7 it will be about 10% protonated. Charge neutralization is an important factor in these protonation reactions. By contrast, the univalent anions perchlorate and
permanganate A permanganate () is a chemical compound containing the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom is in the +7 oxidation state, the permanganate(VII) ion is a strong oxidizing agent. The ion is a ...
ions are very difficult to protonate and so the corresponding acids are
strong acids Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions ...
. Although acids such as phosphoric acid are written as , the protons are attached to oxygen atoms forming hydroxyl groups, so the formula can also be written as to better reflect the structure. Sulfuric acid may be written as ; this is the molecule observed in the gas phase. The
phosphite The general structure of a phosphite ester showing the lone pairs on the P In organic chemistry, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR)3. They can be considered as esters of ...
ion, , is a strong base, and so always carries at least one proton. In this case the proton is attached directly to the phosphorus atom with the structure . In forming this ion, the phosphite ion is behaving as a
Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any s ...
and donating a pair of electrons to the Lewis acid, . As mentioned above, a condensation reaction is also an acid–base reaction. In many systems, both protonation and condensation reactions can occur. The case of the chromate ion provides a relatively simple example. In the predominance diagram for chromate, shown at the right, pCr stands for the negative
logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number  to the base  is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 of ...
of the chromium concentration and pH stands for the negative logarithm of  ion concentration. There are two independent equilibria. Equilibrium constants are defined as follows. : The predominance diagram is interpreted as follows. *The chromate ion, , is the predominant species at high pH. As pH rises the chromate ion becomes ever more predominant, until it is the only species in solutions with pH > 6.75. *At pH < p''K''1 the hydrogen chromate ion, is predominant in dilute solution. *The dichromate ion, , is predominant in more concentrated solutions, except at high pH. The species and are not shown as they are formed only at very low pH. Predominance diagrams can become very complicated when many polymeric species can be formed, such as in vanadates,
molybdate In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, althou ...
s, and
tungstate In chemistry, a tungstate is a compound that contains an oxyanion of tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as co ...
s. Another complication is that many of the higher polymers are formed extremely slowly, such that equilibrium may not be attained even in months, leading to possible errors in the equilibrium constants and the predominance diagram.