Radioactivity is the property possessed by some elements (as uranium) or isotopes (as carbon 14) of spontaneously emitting energetic particles (as electrons or alpha particles) by the disintegration of their atomic nuclei; also : the rays emitted
1. When the Strong Nuclear force of an atom isnt strong enough to overcome the electromagnetic force of repulsion, the isotope is radioactive. 2.If the Element that you are analyzing has a number 84 and above on the periodic table, it automatically is too big to stabilize causing radioactivity. 3. If the element doesn't have the correct neutron/proton ratio, it can also be radioactive