03. The stability of isotopes

What can we learn from the isotope ratio?

The stability of an element's nucleus is related to its protons and neutrons and to its size. It also seems to depend on whether the number of protons or neutrons is even or odd. Only atoms of five elements have odd numbers of both neutrons and protons. Natural isotopes with similar numbers of neutrons and protons tend to be stable. Natural elements with greater numbers of neutrons in their ratios, and elements with an atomic number greater than 83, are unstable. These unstable elements will 'decay' by releasing various types of particle or energy in the form of gamma radiation, until their nuclei become stable.

Most naturally occurring elements have stable nuclei and are not radioactive. There are a small number of examples of light elements that have naturally occurring radioactive isotopes, such as potassium-40 and carbon-14, but it is much more common for radioactivity to be shown by the very heavy elements from lead (atomic number 82) to uranium (atomic number 92).

A 'diagram of nuclides' describing the number of protons (Z) against the number of neutrons (N) in nuclei. Each horizontal line of atoms therefore lists all of the element's isotopes. Plotting isotopes this way also introduces two other patterns: 'isotones' which are isotopes that have the same number of neutrons in the vertical arrangement, and 'isobars' or isotopes that have the same mass in the diagonal arrangement from the lower right to the upper left.