Hetrogeneous substances are mixtures of chemicals that do not undergo chemical reactions. Thus each separate substance retains its individual identify and could easily be separated by physical or chemical means. Elements are collection of identical atoms which thus have identical chemical properties. The chemical properties of each element are determined by the arrangement of the outer electrons which, in turn, is determined by the number of protons in the nucleus. Thus the atomic number indirectly, but completely, determines the chemical properties of each element.
Compounds are the result of chemical reactions between elements.
They are stable and precise combinations of elements. The ratios of the elements that make up each compound must be exact integer ratios. The result is an mix of compounds that is everywhere similar down to the atomic level. Check melting and boiling points. A solution will usually separate into its individual compounds as they melt or boil at different temperatures.