An oxyanion or oxoanion is an ion with the generic formula A_xO_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.