If I am clear on this, separating two elements if often simple as they have unique physical and chemical properties. One or both can be used to separate them. But if to atoms are combined in a molecule like Na2SO4, you now have to break down their bonds to get back to the basic elements and you likely have to start with some reactions. This may add other elements complicating the things as well as produce other molecules that are not single elements. The atoms in molecules have given up or taken in electrons to become what they are and reversing that is the harder direction to go. Does this help?
