The process of decay by which one species of nuclei transform into another is called radioactivity or radioactive decay. The decay occurs with certain rules or laws of radioactivity. The decaying or unstable nuclei are called radioactive elements.
Radioactivity can be either spontaneous or induced.
Spontaneous radioactive elements occur naturally like uranium, thorium, radium, etc. For example, all transuranic elements , man made elements beyond uranium are radioactive. Cathode rays 3. It has positive protons bound together. Like charges repel each other.