Hydrogen has three common isotopes. The most common isotope of hydrogen is called protium (H), which has one proton and no neutrons. A hydrogen atom that has a proton and a neutron (atomic mass of 2) is called deuterium (H). Hydrogen with one proton and two neutrons (atomic mass of 3) is called tritium (H). Protium and deuterium are stable isotopes, while tritium is a radioactive isotope.