An atom is to an element as a single unit is to a pure substance. An atom is to a molecule as a single unit is to multiple units bound together. An atom is to a compound molecule as a single unit is to multiple different units bound together. Atoms are the basic building blocks that make up all matter. An element is a pure substance made of only one type of atom. A molecule is formed when two or more atoms of one or more elements are chemically bonded together. A compound is a pure substance composed of molecules made of two or more different elements chemically bonded together.