Whether it is a combination of elements (a molecule) or a single element (atom).
“Mass” is just the measure of the matter. We use “atomic masses” as fundamental units in chemistry (even though we can break that down into elemental particle masses) because that is how we form the molecules. A “molecular mass” is the weight of a combination of elements. It is the sum of the atomic masses that make it up.
For example, Carbon and Oxygen are single atoms of elements. Carbon’s atomic mass is 12 and oxygen’s is 16. Combined into the molecule of carbon dioxide (
They are both linked by the Mole concept that says that elements (and thus, molecules) interact in discreet amounts, and the molar mass of any element or compound contains the same number of units as any other elemental or molecular mass.