To write the formulae of compounds formed by the reaction of metals with non-metals, we use the criss-cross method. In this method, we write the symbols of the cation and anion along with their respective charges. Then, the numbers of the charges are criss-crossed to become the subscripts of the opposite ions. (i) For Magnesium (Mg²⁺) and Nitrogen (N³⁻), criss-crossing the charge numbers gives Mg₃N₂. (ii) For Lithium (Li⁺) and Nitrogen (N³⁻), criss-crossing gives Li₃N. (iii) For Sodium (Na⁺) and Sulfur (S²⁻), criss-crossing gives Na₂S. (iv) For Aluminium (Al³⁺) and Oxygen (O²⁻), criss-crossing the charges gives Al₂O₃, which is explicitly mentioned in the text as the formula of aluminium oxide.