Criss cross method question chemistry?

Yes criss cross method is used to write formula when the compound is mostly formed by metal + nonmetal. Let the valency of aluminium is 3 and that of oxygen is two. the formula of aluminium chloride is Al2O3. Let the valency of Magnesium is 2 and that of Nitrogen is 3 . The formula of magnesium nitride is Mg3N2.