The lightest and most common element in the universe. The normal hydrogen atom has only one proton and one electron. The much rarer and radioactive isotopes deuterium has a neutron as well, and the even rarer (and radioactive) and tritium has two neutrons. Hydrogen is found in great quantities in stars, the atmosphere of gas giants, and the interstellar gaseous medium. About three quarters of all conventional matter in the universe is hydrogen.