Isotopes of hydrogen. Illustration showing the isotopes of hydrogen. Isotopes are forms of an element that contain different numbers of neutrons in the atomic nucleus. Hydrogen-1,known as protium,has one proton (red) and no neutrons in its nucleus,hydrogen-2,deuterium,has one proton and one neutron (blue),and hydrogen-3,tritium,has one proton and two neutrons . All the isotopes also have one electron (black) orbiting the nucleus. Tritium is a radioactive isotope,but protium and deuterium are stable |