The lightest radioactive element is

The lightest radioactive element is Tritium.
Tritium is a rare and radioactive isotope of hydrogen with a half-life of about 12 years. The nucleus of tritium contains one proton and two neutrons.
Tritium is used as the energy source in radioluminescent lights for watches, gun sights, numerous instruments and tools, and even novelty items such as self-illuminating key chains. It is used in a medical and scientific settings as a radioactive tracer. Tritium is also used as a nuclear fusion fuel, along with more abundant deuterium, in tokamak reactors and hydrogen bombs.