If a material cannot be broken down into pieces constituted by different atoms or molecules then the material is a pure substance. In the case of elements all the atoms that consitute the material are equals and the material cannot be broken down into different types of atoms. In the case of compounds, all the molcules that constitue the material are equals and the material cannot be broken down into different types of molecules. Elements and compounds are the two kind of pure substances that exist. They have a uniform molecular formula.