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An oxyanion or oxoanion is an ion with the generic formula A''x''O''y''''z''− (where A represents a chemical element and O represents an oxygen atom). Oxoanions are formed by a large majority of the chemical elements. The formulae of simple oxoanions are determined by the octet rule. The structures of condensed oxoanions can be rationalized in terms of AO''n'' polyhedral units with sharing of corners or edges between polyhedra. The phosphate and polyphosphate esters adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are important in biology. == Monomeric oxoanions == The formula of monomeric oxoanions, AO''n''''m''−, is dictated by the oxidation state 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 oxoanion with that coordination number. Instead, carbonate (CO32−) and nitrate (NO3−) have a trigonal planar 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 oxoanions of second-row elements in the group oxidation state are tetrahedral. Tetrahedral SiO4 units are found in olivine minerals, ()SiO4, but the anion does not have a separate existence as the oxygen atoms are surrounded tetrahedrally by cations in the solid state. Phosphate (PO43−), sulfate (SO42−), and perchlorate (ClO4−) ions can be found as such in various salts. Many oxoanions 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 ClO3−. 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 ClO2−, 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 oxoanions are not known because they would carry too high an electrical charge. Thus molybdenum(VI) does not form MoO66−, but forms the tetrahedral molybdate anion, MoO42−. MoO6 units are found in condensed molybdates. Fully protonated oxoanions with an octahedral structure are found in such species as Sn(OH)62− and Sb(OH)6−. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Oxyanion」の詳細全文を読む スポンサード リンク
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