All elements that have an atomic number higher than 83 are considered radioactive. These elements do not have stable isotopes, which is the primary thing that defines them as radioactive elements.
The nuclei may degenerate at any period, which is known as the half life. While the half life varies from element to element, it provides a good probability of the life of a radioactive element.
Radioactive element includes technetium, which has the most stable isotope and what is estimated as the longest half-life. The least stable element is ununoctium, which has a half-life of 1.8 milliseconds.Learn more about Atoms & Molecules