oxidation number (oxidation state; positive valence)

A convention for representing a charge of an atom embedded within a compound, if the compound were purely ionic. For example, H2O is a covalent compound; if it were ionic, the hydrogens would be H+ (oxidation number +1) and the oxygen would be O2- (oxidation number -2). Oxidation number rises for at least one atom in a compound that is oxidized ; oxidation number becomes smaller if the compound is reduced.