How do you know the compound is ionic in the first place? Metals readily lose electrons to nonmetals to form cations. So ionic compounds almost always contain a metal as the cation and a nonmetal as the anion (except in the case of polyatomic ions). If the compound contains a metal, it's a safe bet that the compound is ionic. You should be aware that not all ionic compounds will contain a metal as the cation. The cation could be a polyatomic ions, such as NH4+.
Now, once you have identified the compound as ionic (composed of cations and anions), you have to decide whether there is only one type of cation possible (type I) or whether there are several cations possible (type II). If the cation is a polyatomic ion, such as ammonium (NH4+), then there is only one possibility and this is a type I compound. Usually the cation is a metal, though, and so how can you tell if the metal forms only one type of cation? The general rule is this: the alkali metals (group IA on the periodic table) always form a +1 cation in an ionic compound and alkaline earth metals (group IIA) always form a +2 cation, whereas the transition metals often have several possible oxidation states (ionic charges). So magnesium always forms Mg2+ in an ionic compound, but iron may exist in one of two common oxidation states (Fe2+ and Fe3+ ). An ionic compound that contains magnesium as the cation would be named using type I rules because magnesium forms only one type of cation. On the other hand, an ionic compound that contains iron would be named using type II rules. Listed below are the rules to naming type I and type II compounds.