chapter  2
What is Radioactivity?
Pages 14

The atomic structure of most elements contains a nucleus that is stable. Under normal conditions, these elements remain unchanged indefinitely. They are not radioactive. Radioactive elements, in contrast, contain a nucleus that is unstable. The unstable nucleus is actually in an excited state that can not be sustained indefinitely; it must relax, or decay, to a more stable configuration. Decay occurs spontaneously and transforms the nucleus from a high energy configuration to one that is lower in energy. This can only happen if the nucleus gives off energy. The energy emitted by the relaxing nucleus is radiation. All radioactive elements have unstable nuclei; that is what makes them radioactive.