Pure substances that have properties entirely different from the atoms that make it up are called compounds. All compounds are made up of molecules, but not all the molecules are compounds. Compounds are composed of more than one mixture of atoms. An example of this is Hydrogen Gas, it is a molecule but it is not a compound because it is only made of only one element. But when Hydrogen is mixed with Oxygen, it forms water, which is now a compound because of the mixture of two hydrogen and one oxygen atoms.