Anoxyanion, oroxoanion, is anion with the generic formulaA
_xO^z−
_y (where A represents achemical element and O represents anoxygen atom). Oxyanions are formed by a large majority of thechemical elements.^[1] The formulae of simple oxyanions are determined by theoctet rule. The correspondingoxyacid of an oxyanion is the compoundH
_zA
_xO
_y. 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) andadenosine triphosphate (ATP) are important in biology.

The formula ofmonomeric oxyanions,AO^m−
_n, is dictated by theoxidation state of the element A and its position in theperiodic 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 (CO²⁻
₃) andnitrate (NO⁻
₃) have atrigonal planar structure withπ 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 thegroup oxidation state aretetrahedral. TetrahedralSiO₄ units are found inolivine minerals,(Mg,Fe)₂SiO₄, but the anion does not have a separate existence as the oxygen atoms are surrounded tetrahedrally by cations in the solid state.Phosphate (PO³⁻
₄),sulfate (SO²⁻
₄), andperchlorate (ClO⁻
₄) ions can be found as such in various salts. Many oxyanions of elements in lower oxidation state obey theoctet 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 isClO⁻
₃. The structure of the ion is predicted byVSEPR theory to be pyramidal, with three bonding electron pairs and one lone pair. In a similar way,The oxyanion of chlorine(III) has the formulaClO⁻
₂, and is bent with two lone pairs and two bonding pairs.

Oxidation state	Name	Formula	Image
+1	Thehypochlorite ion	ClO−
+3	Thechlorite ion	ClO− 2
+5	Thechlorate ion	ClO− 3
+7	Theperchlorate ion	ClO− 4

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 formMoO⁶⁻
₆, but forms the tetrahedralmolybdate anion,MoO²⁻
₄. MoO₆ units are found in condensed molybdates. Fully protonated oxyanions with an octahedral structure are found in such species asSn(OH)²⁻
₆ andSb(OH)⁻
₆. In addition,orthoperiodate can be only partially deprotonated,^{[Note 1]} with

${\displaystyle {\text{H}}_3^{}{\text{IO}}_6^{2-}{}_{⟶}^{⟵}{\text{H}}_2^{}{\text{IO}}_6^{3-}+{\text{H}}^{+}}$ having pK_a=11.60.^[2][3]

The naming of monomeric oxyanions follows the following rules.

Here thehalogen group (group7A, 17) is referred to as group VII and thenoble gases group (group8A) is referred to as group VIII.

If central atom is not in Group VII or VIII

If central atom is in Group VII or VIII

In aqueous solution, oxyanions with high charge can undergo condensation reactions, such as in the formation of thedichromate ion,Cr₂O2−7:

The driving force for this reaction is the reduction of electrical charge density on the anion and the elimination of thehydronium (H⁺) ion. The amount of order in the solution is decreased, releasing a certain amount ofentropy which makes theGibbs free energy more negative and favors the forward reaction. It is an example of anacid–base reaction with the monomeric oxyanion acting as a base and the condensed oxyanion acting as itsconjugate acid. The reverse reaction is ahydrolysis reaction, as awater molecule, acting as a base, is split. Further condensation may occur, particularly with anions of higher charge, as occurs with adenosine phosphates.

AMP	ADP	ATP

The conversion of ATP to ADP is a hydrolysis reaction and is an important source of energy in biological systems.

The formation of mostsilicate minerals can be viewed as the result of a de-condensation reaction in whichsilica reacts with a basic oxide, an acid–base reaction in theLux–Flood sense.

${\displaystyle \stackrel{\text{base}}{\text{CaO}}+\stackrel{\text{acid}}{{\text{SiO}}_2^{}}⟶{\text{CaSiO}}_3^{}}$

Apolyoxyanion is apolymeric oxyanion in which multiple oxyanion monomers, usually regarded asMO_n polyhedra, are joined by sharing corners or edges.^[4] When two corners of a polyhedron are shared the resulting structure may be a chain or a ring. Short chains occur, for example, inpolyphosphates. Inosilicates, such aspyroxenes, have a long chain ofSiO₄ tetrahedra each sharing two corners. The same structure occurs in so-called meta-vanadates, such asammonium metavanadate,NH₄VO₃.

The formula of the oxyanionSiO2−3 is obtained as follows: each nominal silicon ion (Si⁴⁺) is attached to two nominal oxide ions (O²⁻) and has a half share in two others. Thus the stoichiometry and charge are given by:

${\displaystyle \text{Stoichiometry: }\text{Si}+2\text{O}+(2\times \frac{1}{2})\text{O}={\text{SiO}}_3^{}}$

${\displaystyle \text{Charge: }+4+(2\times -2)+(2\times (\frac{1}{2}\times -2))=-2}$

A ring can be viewed as a chain in which the two ends have been joined. Cyclictriphosphate,P₃O3−9 is an example.

When three corners are shared the structure extends into two dimensions. Inamphiboles, (of whichasbestos 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 formulaSi₄O6−11 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 inmica,Si₂O2−5, 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 inquartz.Aluminosilicates 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.

OctahedralMO₆ units are common in oxyanions of the larger transition metals. Some compounds, such as salts of the chain-polymeric ion,Mo₂O2−7 even contain both tetrahedral and octahedral units.^[5][6] 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 calledpolyoxometalates. Typical examples occur in theKeggin structure of thephosphomolybdate ion. Edge sharing is an effective means of reducing electrical charge density, as can be seen with the hypothetical condensation reaction involving two octahedra:

${\displaystyle 2{\text{MO}}_6^{\mathit{\text{n}}-}+4{\text{H}}^{+}⟶{\text{Mo}}_2^{}{\text{O}}_{10}^{{(\mathit{\text{n}}-4)}^{-}}+2{\text{H}}_2^{}\text{O}}$

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.

The tetrahedral molybdate ion is converted into a cluster of 7 edge-linked octahedra^[6][7] giving an average charge on each molybdenum of6⁄7. 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.

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.

Most oxyanions are weakbases and can be protonated to give acids or acid salts. For example, the phosphate ion can be successively protonated to form phosphoric acid.

The extent of protonation in aqueous solution will depend on theacid dissociation constants andpH. For example, AMP (adenosine monophosphate) has a pK_a value of 6.21,^[8] so at pH 7 it will be about 10% protonated. Charge neutralization is an important factor in these protonation reactions. By contrast, the univalent anionsperchlorate andpermanganate ions are very difficult to protonate and so the corresponding acids arestrong acids.

Although acids such as phosphoric acid are written asH₃PO₄, the protons are attached to oxygen atoms forming hydroxyl groups, so the formula can also be written asOP(OH)₃ to better reflect the structure. Sulfuric acid may be written asO₂S(OH)₂; this is the molecule observed in the gas phase.

Thephosphite ion,PO3−3, is astrong base, and so always carries at least one proton. In this case the proton is attached directly to the phosphorus atom with the structureHPO2−3. In forming this ion, the phosphite ion is behaving as aLewis base and donating a pair of electrons to the Lewis acid,H⁺.

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 thepredominance diagram for chromate, shown at the right, pCr stands for the negativelogarithm of the chromium concentration andpH stands for the negative logarithm ofH⁺ ion concentration. There are two independent equilibria.Equilibrium constants are defined as follows.^[9]

	${\displaystyle K_1=\frac{[\mathrm{H}\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{-}]}{[\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{2-}][{\mathrm{H}}^{+}]}}$	${\displaystyle \log K_1=5.89}$
	${\displaystyle K_2=\frac{[\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}]}{[\mathrm{H}\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{-}{]}^2}}$	${\displaystyle \log K_2=2.05}$

The predominance diagram is interpreted as follows.

• The chromate ion,CrO2−4, 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 < pK₁ the hydrogen chromate ion,HCrO−4 is predominant in dilute solution.
• The dichromate ion,Cr₂O2−7, is predominant in more concentrated solutions, except at high pH.

The speciesH₂CrO₄ andHCr₂O−7 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,^[10] such as invanadates,molybdates, andtungstates. 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.

• Oxycation
• Fluoroanion
• Carbanion

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• Oxyanions
• Acid–base chemistry
• Equilibrium chemistry

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