Presently, just two rules for predicting electrides are widely accepted: (1) the metal atoms that form the coordination sphere around the electride electron must be electropositive , and (2) the compound must be electron rich . For example, in [Ca2N] + (e -) , calcium is electropositive, and the compound is electron rich because the preferred oxidation states (Ca 2+ , N ) and stoichiometry provide an extra electron, forming an electride. These rules are far too broad, however, because many compounds fulfill these rules but are not electrides.