The Roman numeral is used in the Stock system of nomenclature to indicate the oxidation number of the positive element when the element has more than one positive oxidation number. For instance, iron always requires a Roman numeral because iron can have two oxidation numbers, +2 and +3. Not all elements require a Roman numeral in the name. Sodium does not require a Roman numeral because it has only one oxidation number, +1. Use it when you have a metal that can have multiple charges(oxidation states). Usually roman numerals are only used with transition metals which are the metals in the middle of the periodic table like Iron(Fe), Copper(Cu) etc. Notable exceptions are Silver(Ag), Zinc(Zn) and Cadmium(Cd). Even though these elements are in the middle of the periodic table we never use roman numerals with them because Silver is always +1, Zn is always +2 and Cadmium is always +2, so since they cannot have more than one charge there\'s no need to use roman numerals. Roman numerals are also used with Lead(Pb), Tin(Sn) and Bismuth(Bi) even though they\'re not in the middle of the periodic table because they can have more than one charge. The roman numeral indicates the charge on the metal so Iron(III) means Iron with a +3 charge which is Fe^3+(Fe with a +3 charge). Similarly, Lead(IV) means Pb^4+(Pb with a +4 charge). Note that Mercury(I) is Hg2^2+(that is Hg2 with a +2 charge). It\'s a weird metal ion. Roman numerals are also used with some of the inner transition metals which are at the bottom of the periodic table by themselves but you don\'t encounter those elements too often in chemistry classes unless you\'re studying nuclear chemistry or some form of advanced chemistry. Solution The Roman numeral is used in the Stock system of nomenclature to indicate the oxidation number of the positive element when the element has more than one positive oxidation number. For instance, iron always requires a Roman numeral because iron can have two oxidation numbers, +2 and +3. Not all elements require a Roman numeral in the name. Sodium does not require a Roman numeral because it has only one oxidation number, +1. Use it when you have a metal that can have multiple charges(oxidation states). Usually roman numerals are only used with transition metals which are the metals in the middle of the periodic table like Iron(Fe), Copper(Cu) etc. Notable exceptions are Silver(Ag), Zinc(Zn) and Cadmium(Cd). Even though these elements are in the middle of the periodic table we never use roman numerals with them because Silver is always +1, Zn is always +2 and Cadmium is always +2, so since they cannot have more than one charge there\'s no need to use roman numerals. Roman numerals are also used with Lead(Pb), Tin(Sn) and Bismuth(Bi) even though they\'re not in the middle of the periodic table because they can have more than one charge. The roman numeral indicates the charge on the metal so Iron(III) means Iron with a +3 charge which is Fe^3+(Fe with a +3 charge). Sim.