Tonic compounds are represented by formula unit (formula mass) and covalent compounds are represented by their molecular mass Sodium chloride is an ionic compound and it contains sodium ions (Na⁺) and chloride ions (CI^-) present in it in a simple ratio of 1:1. Thus NaCl may also be called its formula unit in comparison, covalent compounds like Sulphur dioxide exists in form of molecules having a molecular formula SO₂ Similarly hydrogen sulphites and water have molecular formulas H₂S and H₂O respectively.