What a molecule is, and the difference between molecules and compounds?

A molecule is made up of two or more atoms combined chemically. The atoms can be of the same element, as in oxygen gas (O2), or two or more different elements combined, as in water (H20). By contrast, a compound is made up of two or more different elements combined.

Another way to see the distinction is that molecules represent a larger category in the classification of matter. A molecule can be any combination of two or more atoms. By contrast, a compound must be composed of at least two different elements; therefore, compounds are a subset of molecules.