Atomic Mass and Radioactivity

 Atomic mass is not just the number of nuclear particles contained in the nucleus, it also receives a contribution from the internal energy of the nucleus (as well as the relative abundance of the various isotopes of that element.)  Most of the departure from whole numbers of all but the lightest nuclei can be attributed to internal energy.

        Lightest elements - above nearest whole number

Medium weight - fall below nearest whole number

Heavy elements - rise back to and above whole numbers

 The internal energy per nuclear particle is lowest for medium weight nuclei and larger for lighter and heavier nuclei.

 Common isotopes of medium weight nuclei are not radioactive (they have no surplus of internal energy to get rid of). The lightest isotopes are not radioactive because the lighter nuclei into which they might decay have even a larger surplus of internal energy. Nuclei of large atomic mass elements have more surplus energy than the lighter nuclei into which they might decay, so they can and do release this energy in radioactivity.

 "We can now come back to the question of how the energy released in radioactivity got into the nuclei in the first place. The universe is believed to have started in a "Big Bang," after which the primordial hot gas of free protons and neutrons cooled rapidly and at the end of the first three minutes combined into hydrogen and helium. Hydrogen nuclei have a much higher energy per nuclear particle than helium nuclei, and helium nuclei have a higher energy per nuclear particle than those of medium atomic mass; thus, when stars form, the hydrogen nuclei fuse into helium nuclei, and helium nuclei fuse into medium‑weight nuclei, releasing enough energy to keep stars shining for billions of years.

Eventually the material of a star evolves into those elements around iron whose nuclei have the lowest energy per nuclear particle. There is no more energy to release, and the star begins to cool. Often the star winds ups as a cinder, a black dwarf. Sometimes however, it may explode as what astronomers call a supernova. During such an explosion an intense flux of neutrons is released from the inner part of the star. The neutrons that strike nuclei of medium atomic weight atoms in the outer layers of the star rapidly build them up into heavier elements, all the way up to uranium. The exploding star sheds its outer layers, which then go out to form part of the interstellar medium, out of which a later generation of stars like the sun eventually form.

       According to this picture, the energy in naturally radioactive elements such as thorium and uranium was put into them by the neutrons released in the explosion of stars, and can be ultimately traced to the force of gravitational attraction, which provided the energy for the stellar explosion."  Steven Weinberg

 Neutrons, due to their neutral charge, can easily penetrate into even heavy nuclei and cause nuclear disintegrations (called fissions) which can produce another neutron, so that a chain reaction becomes possible.