An oxyanion, or oxoanion, is an ion with the generic formula  (where A represents a chemical element and O represents an oxygen 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 of an oxyanion is the compound .  The structures of condensed oxyanions can be rationalized in terms of AO 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 (ATP) are important in biology.