The empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound.

 A simple example of this concept is that the empirical formula of sulphur monoxide, or SO , would simply be SO , as is the empirical formula of sulphur dioxide, ${\mathrm{S}}_2{\mathrm{O}}_2$ . 

Thus, sulfur monoxide and disulphur dioxide, both compounds of sulphur and oxygen, have the same empirical formula.

 However, their molecular formulas, which express the number of atoms in each molecule of a chemical compound, are not the same.