Why is feo named using a roman numeral, iron (ii) oxide, whereas cao named without a roman numeral, calcium oxide?

Some metals have the ability to form differently charged ions. For example, iron can form 2 + or 3 + ions. If you simply gave the name iron chloride, you would not know which charge the iron ion possessed. A Roman numeral is to indicate the charge of the iron. Iron ( II ) means the iron has a 2 + charge Iron ( III ) means that the iron has a 3 + charge So, iron ( II ) oxide would have a chemical formula of FeO . (The oxide ion has a 2 â’ charge to balance the 2 + of the iron to form a neutral compound.) Iron ( III ) oxide would have a chemical formula of Fe 2 O 3 (Here you need to find the common multiple of 6, so two iron ions with a 3 + charge will balance the charge of three oxide ions with a 2 + charge.)