Final answer:
Roman numerals are used in ionic compound formulas to indicate the charge of the cation, which helps in specifying the correct ionic charge when a metal can form more than one type of cation.
Step-by-step explanation:
Roman numerals are used in an ionic compound formula to indicate the charge of the cation. In the naming of ionic compounds, the cation (usually a metal) is stated first, followed by the anion (usually a nonmetal). It's essential that the positive and negative charges balance to ensure the neutrality of the compound. When a metal can form cations with different charges, Roman numerals in parentheses after the name of the metal indicate its ionic charge. This clarifies which ion is being referred to in the compound.
For instance, iron can have a charge of +2 or +3, so iron chloride could refer to FeCl₂ or FeCl₃. Using Roman numerals, the compounds are named iron(II) chloride for FeCl₂ and iron(III) chloride for FeCl₃. The proper ionic formula will show the lowest ratio of ions needed to balance the charges.