Hydrogen has the smallest atoms of any element. (Atoms are the smallest particles that have the characteristics of a chemical element.) A hydrogen atom contains one proton, a tiny particle with a positive electrical charge, and only one electron, an even smaller, negatively charged particle. The proton is the center, or nucleus, of the hydrogen atom, and the electron travels around the nucleus. Pure hydrogen exists as hydrogen gas, in which pairs of hydrogen atoms bond together to make larger particles called molecules.