* Element: A molecule made up of only one type of atom is considered an element. For example, oxygen gas (O₂) is a molecule, but it's also an element because it's composed of only oxygen atoms.

* Compound: A molecule made up of two or more different types of atoms is considered a compound. For example, water (H₂O) is a molecule and a compound because it's made of hydrogen and oxygen atoms.

In summary: The key difference is the number of different types of atoms that make up the molecule.