The radioactive element with the lowest atomic number is Hydrogen, specifically its isotope known as Tritium (³H). Tritium has one proton, making it the lightest and only radioactive isotope of hydrogen.
Tritium is naturally occurring in the environment, though it is present in very small amounts. It is produced in the upper atmosphere when cosmic rays interact with nitrogen and can also be created artificially in nuclear reactors.
While Hydrogen itself (atomic number 1) is a stable element, Tritium is the only isotope of it that is radioactive, with a half-life of about 12.3 years. It decays by beta emission into Helium-3. The study of Tritium and its properties is important not only for understanding basic nuclear physics but also for applications in nuclear fusion and radioluminescent devices.