An atom is one of the basic units of matter. Atoms form the building blocks of the simplest substances called chemical elements. Atoms vary greatly in weight, but they are all about the same size, ranging from 0.3 to 3 angstroms. For example, an atom of plutonium, one of the heaviest elements, weighs more than 200 times as much as an atom of hydrogen, the lightest element. Yet the diameter of a plutonium atom is about 151 picometers, only about three times that of a hydrogen atom, which is about 53 picometers..