Final answer:
Roman numerals in ionic compound formulas indicate the oxidation states of metals that can form multiple ions, ensuring proper charge balance and representation of the compound in the lowest whole number ratio.
Step-by-step explanation:
We use Roman numerals in an ionic compound formula to indicate the oxidation states of the metals that can form more than one type of positive ion. For instance, iron can form Fe2+ and Fe3+ ions, which are named iron(II) and iron(III) respectively when part of a compound. These oxidation states must be specified because different ions will bond with negative ions (anions) in different ratios to form a neutral compound. Remember, the charge balance between the cations (positively charged ions) and anions (negatively charged ions) must be equal to zero for the ionic compound to be stable. An example is iron(III) oxide, which contains iron and oxygen ions combining in a 2:3 ratio to neutralize the compound, giving us the formula Fe2O3.
It's also important to use the correct chemical formula that represents the lowest whole number ratio of the ions in the compound. When dealing with polyatomic ions as part of an ionic compound, parentheses are used if more than one of that ion is needed to balance the charge. For example, calcium nitrate has the formula Ca(NO3)2, where the parentheses indicate that two nitrate ions combine with each calcium ion.