Light stable nuclei consist of an approximately equal number of protons and neutrons. The protons and neutrons are held close together by a very strong but short range force called the strong force. This strong attractive force overcomes the longer range electrostatic repulsive force between the like-charged protons for small nuclei. For larger, heavier nuclei, the strong force has insufficient range to encompass the whole nucleus, and the longer range electrostatic repulsive force between the protons begins to win out, unless the protons are diluted by a greater proportion of neutrons to again produce stable nuclei. Nuclei with too many or too few neutrons are unstable and radioactive. There are no stable nuclei with atomic number, Z, greater than 83 (bismuth), because these nuclei have so much positive charge and are so large that extra neutrons cannot dilute the protons sufficiently. Technetium is the lightest element (Atomic Number, Z=43) for which no stable isotopes exist.


An element may exist as a mixture of several stable isotopes. The percentage abundance shows their relative distributions as found on Earth. Some elements consist of a mixture of stable and unstable isotopes when the unstable isotopes have half-lives over 80 Million years e.g. potassium-40, vanadium-50, cadmium-113, tellurium-123, samarium-148, bismuth-209 and uranium-238. Nuclei containing an even number of both nucleons (protons and neutrons) are usually stable, whilst those containing an odd complement of both are mostly unstable.

Of the stable nuclei, there are 168 examples of even/even split, whereas there are only 9 odd/odd nuclei, and these are all light nuclei. Nuclei containing 2, 8, 20, (28), 50 or 82 nucleons are particularly stable, these numbers corresponding to a closed shell of nucleons (c.f. closed electron shells of Noble gases). Shown are carbon-12, iron-56 which is the most stable isotope. Also shown is potassium-40 (which is radioactive, and therefore not stable) which can decay by either of two routes, but it's halflife is so long (1.28×109 years that it is almost stable (hence the tri-colour colour-code for that isotope: red for the beta decay, yellow for the inverse beta decay, and blue for 'stable') )