(ox-i-da'-tion state) (equivalent to valence) The number of electrons that must be added to or subtracted from an atom during chemical bonding to convert it to the elemental form. The oxidation states of iron (Fe) and oxygen (O) in the mineral hematite (Fe2O3) are +3 and +2, respectively. The formula of the mineral magnetite is usually written Fe3O4, but it is more informative to write Fe3+(Fe2+, Fe3+)2O4 to indicate that iron in magnetite occurs in two oxidation states. Many elements, especially the other transition metals, are similar to iron in that they have multiple oxidation states. For instance, the element vanadium (V) occurs in four oxidation states: +2, +3, +4, and +5.
the charge on an element if it is assumed that it is behaving as an ion, e.g., in calcium chloride (CaCl2), calcium has an oxidation state of +2 (Ca2+) and chlorine has an oxidation state of -1 (Cl-). As the definition suggests, this definition tends to breakdown in species with a high degree of covalent bonding.