The radioactive element with the lowest atomic number is Technetium, with an atomic number of 43. Technetium is the lightest element that has no stable isotopes, and every isotope of technetium is radioactive. Promethium, with an atomic number of 61, is the second-lightest element that is exclusively radioactive. It is one of only two radioactive elements that are followed in the periodic table by elements with stable forms, the other being technetium.