In chemistry, the adjective ferrous indicates a compound that contains iron in the +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to "ferric", which indicates presence of iron in a +3 oxidation state, such as the trivalent cation Fe3+.[1] This usage has been largely replaced by the IUPAC nomenclature, which calls for the oxidation state being indicated by Roman numerals in parentheses, such as iron(II) oxide for ferrous oxide (FeO), iron(III) oxide for ferric oxide (Fe2O3), and iron(II,III) oxide for the oxide Fe 3O 4 that contains both forms of iron.