Final answer:
The compound Mg3N2 is magnesium nitride, a binary ionic compound without Roman numerals in its formula; Roman numerals are used for metals with variable charges, not for those like magnesium with a fixed charge.
Step-by-step explanation:
The compound Mg3N2 is known as magnesium nitride, which is a binary ionic compound. When writing chemical formulas for such compounds, you write the metal ion with its charge, followed by the nonmetal ion with its charge. Magnesium has a charge of +2 and nitrogen has a charge of -3. However, no Roman numerals are included in the formula Mg3N2 because magnesium has a fixed oxidation state of +2 and does not require indicating its charge through Roman numerals, unlike transition metals with variable charges. Roman numerals are used when naming ionic compounds of cations with variable charges such as iron in iron(II) chloride or iron(III) oxide.