Answer:
This is because some molecules contain only one chemical element, and a compound, by definition, contains 2 or more elements.
Step-by-step explanation:
Consider molecular Hydrogen (Hâ‚‚) as an example. It is a molecule, albeit very simple, because it is made up of multiple atoms (2 Hydrogen atoms, to be precise). It is not a compound, however, because it only contains one chemical element: Hydrogen.