Oxoanion
An oxyanion or oxoanion is an ion with the generic formula A
xOz−
y (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 AOn 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.
The formula of monomeric oxoanions, AOm−
n, 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 (CO2−
3) and nitrate (NO−
3) 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.
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