monatomic ions are sometimes also denoted with roman numerals; for example, the example seen above is occasionally referred to as fe(ii) or fe. the roman numeral designates the "formal oxidation state" of an element, whereas the superscripted numerals denote the net charge. the two notations are, therefore, exchangeable for monatomic ions, but the roman numerals "cannot" be applied to polyatomic ions. however, it is possible to mix the notations for the individual metal centre with a polyatomic complex, as shown by the uranyl ion example.