Atoms are complete in themselves. They have a consistent form that can be relied to be the same no matter where they are found. Atoms however can join together to form molecules. Molecules are made of atoms. Molecules encompass atoms.
Molecules are more complex than atoms.
Molecules are holons because they are complete and consistent within themselves and because they can be organised into a basic living cell. Cells are holons which can form a body organ. Each stage is a holon which includes the level before it and is included in the level after it. We must move from one level to the next. We cannot skip levels.