The correct formula unit should bear correct subscripts coming from the opposite charge of the other ion after the exchange. In this case, Ca2N3 is not correct since Ca has a charge of +2 and N has a charge of -3. Hence the correct should be Ca3N2.
Step-by-step explanation:
A formula unit in chemistry is the empirical formula of an ionic or covalent material hard compound used as an objective item for stoichiometric calculations. It is the cheapest whole number ratio of ions designed in an ionic compound.