The Stock System
The Stock system of nomenclature is used for naming compounds that contain a multivalent metal (a metal that can form more than one type of ion). In this system, the oxidation number (the charge on the ion) of the metal is shown in parentheses using Roman numerals.
For example, iron oxide would be an ambiguous name for the compound FeO, because iron and oxygen can also form Fe2O3. Calling either of these compounds iron oxide would not indicate which compound you were referring to. In the compound FeO, the oxide ion has a charge of -2, so the iron ion must have a charge of +2. Therefore, FeO can be called iron(II) oxide. FM), is called iron(III) oxide, because iron has an oxidation number of +3 in that compound.
The Stock system should not be used for metals that are monovalent (have only one oxidation number). For example, NaCI would not be called sodium(I) chloride, because sodium always has an oxidation number of +1 in a compound. NaCI is simply called sodium chloride.