As discussed in previous lessons, there are many different kinds of elements. The atoms of these elements are composed of a nucleus surrounded by orbital electrons. The nucleus consists of protons and neutrons. Each element has a specific number of protons, while the number of neutrons may vary, resulting in various isotopes of the same element.
In addition, a slight difference in mass-energy exists between a composite nucleus and the sum of the free-standing nucleons; that difference being the energy that binds the nucleus together. This indicates that each nucleon, i.e., proton or neutron, give up a certain amount of its mass-energy in order for the nucleus to hold together. Because only isotopes exist in nature, this must mean that only specific combinations of neutrons and protons are possible for the existence of the unchanging stable nuclei. Variations from these specific combinations are possible, but the nuclei may not be completely stable. In other words, there may be more energy present in the nuclei of some isotopes than is required to hold them together. With that excess of energy nuclei will not be stable and will, most likely, dissipate it in some manner, in order to become stable.
Stable nuclei will remain unchanged as long as there is no external influence causing them to become unstable. Unstable nuclei, however, will spontaneously emit excess energy and transform into more stable nuclei. Such transformations are independent of external influences such as temperature or chemical reactions of the atom. The emission of excess energy by unstable nuclei in order to achieve stability is the phenomenon of radioactivity.