An atom is the basic unit of an element. Elements are pure substances 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. Compounds are pure substances made of molecules containing two or more elements chemically bonded together in a fixed ratio. So in summary: - An atom is to an element as a single unit is to the whole substance - An atom is to a molecule as a single unit is to a combined group of atoms - An atom is to a compound molecule as a single unit is to a combined group of different atoms bonded together. Elements are made of single atoms while compounds and molecules involve multiple atoms of