Ionic compounds are the result of the chemical bonding of a metal with a non-metal. The two parts of the compounds, the metal and the non-metal, are called the cation and the anion, respectively. The formula for aluminum nitride is AlN AlN. The ions of the lithium oxide compound are: Li O2− Li, −. In this case, two lithium ions are needed to balance the charge of one oxide ion. The formula for lithium oxide is Li2O O. Another way to write a correct formula for an ionic compound is to use the crisscross method. Chemical formulas are an expression of the types and numbers of atoms in a chemical compound. Writing chemical formulas involves element symbols and subscription. The oxide always has a − charge, so with three oxide ions we have a total negative − charge. This means that the two cobalt ions must be associated with each other, which for two cobalt ions means each of them. Therefore, the proper name of this ionic compound is cobalt III oxide. cobalt III oxide.