Via a radiation
All elements with an atomic number greater than 82 (like lead) are radioactive.
It has to do with the line of stability. Isotopes are made of an extra neutron, making the atomic mass greater. If the number of protons and neutrons on the graph that has the line of stability fall anywhere that's not on the line, it is unstable.
Resources I used:
https://answers.yahoo.com/question/index?qid=20100127150824AAZlAdm by SuicideMachinex2 · 6 years ago
(http://education.jlab.org/qa/transuranic_01.html) by Keith Welch, Radialogical Controls Group