Final answer:
When a transition metal serves as the cation in an ionic compound, Roman numerals are used to indicate the specific charge of that metal cation, such as in iron(II) chloride and iron(III) chloride for FeCl2 and FeCl3, respectively.
Step-by-step explanation:
When a transition metal is the cation in an ionic compound, Roman numerals must be used in the name to indicate the charge of the metal cation. This is necessary because many transition metals can form two or more cations with different charges, which can affect the properties of the compound they form. For instance, iron can have a charge of 2+ or 3+, resulting in two different compounds when combined with chlorine: FeCl2 and FeCl3. Without using Roman numerals, naming would be ambiguous, as "iron chloride" could refer to either compound. To avoid this ambiguity, the two compounds are named iron(II) chloride and iron(III) chloride to reflect the differing charges of the iron cation.
When a transition metal serves as the cation in an ionic compound, Roman numerals are used to indicate the specific charge of that metal cation, such as in iron(II) chloride and iron(III) chloride for FeCl2 and FeCl3, respectively.