Final answer:
Iron requires a Roman numeral in its ion nomenclature because it can form multiple ions (Fe²⁺ and Fe³⁺), which necessitates a distinction in naming. Zinc only forms a single common ion (Zn²⁺), so no Roman numeral is required.
Step-by-step explanation:
The best explanation of why iron requires a Roman numeral when naming an ion of iron, but zinc does not is because iron forms multiple ions with different charges, specifically Fe²⁺ and Fe³⁺. This is in contrast to zinc, which only forms one common ion, Zn²⁺, and thus there is no need for a Roman numeral to distinguish between different ionic charges.
In the Stock system of nomenclature for ions of transition metals with variable charges, the charge is indicated by a Roman numeral in parentheses, such as iron(II) for Fe²⁺ and iron(III) for Fe³⁺. When naming compounds containing these ions, distinguishing between the different charges is crucial because they can form different compounds with unique properties.