It is important to differentiate the stoichiometric or empirical formula of a molecular substance from its molecular formula. The former expresses only the relative numbers of atoms present, in the simplest present ratio. As, the compound of stoichiometry P2O5 occupy P4O10 molecules. Molecular formulae would be used when they are known. On the other hand, in a solid where clear molecular or other units do not exist the empirical formula is usually used. As,

NaCl is an ionic substance and the formula does not apply that molecules are present.

When solids contain identifiable groups such as complex ions or molecules the formula.