List of elements by stability of isotopes

As a result, as the number of protons increases, an increasing ratio of neutrons to protons is needed to form a stable nucleus; if too many or too few neutrons are present with regard to the optimum ratio, the nucleus becomes unstable and subject to certain types of nuclear decay.

[1] The 83rd element, bismuth, was traditionally regarded as having the heaviest stable isotope, bismuth-209, but in 2003 researchers in Orsay, France, measured the half-life of 209Bi to be 1.9×1019 years.

Only 90 isotopes are expected to be perfectly stable, and an additional 161 are energetically unstable,[citation needed] but have never been observed to decay.

[1] Additionally, about 31 nuclides of the naturally occurring elements have unstable isotopes with a half-life larger than the age of the Solar System (~109 years or more).

[b] An additional four nuclides have half-lives longer than 100 million years, which is far less than the age of the Solar System, but long enough for some of them to have survived.

[5] Another notable example is the only naturally occurring isotope of bismuth, bismuth-209, which has been predicted to be unstable with a very long half-life, but has been observed to decay.

[1] Of these, three elements (bismuth, thorium, and uranium) are primordial because they have half-lives long enough to still be found on the Earth,[d] while all the others are produced either by radioactive decay or are synthesized in laboratories and nuclear reactors.

These comprise 251 stable isotopes, and with the addition of the 35 long-lived radioisotopes with half-lives longer than 100 million years, a total of 286 primordial nuclides, as noted above.

The nuclides found naturally comprise not only the 286 primordials, but also include about 52 more short-lived isotopes (defined by a half-life less than 100 million years, too short to have survived from the formation of the Earth) that are daughters of primordial isotopes (such as radium from uranium); or else are made by energetic natural processes, such as carbon-14 made from atmospheric nitrogen by bombardment from cosmic rays.

(Double beta decay directly from even–even to even–even, skipping over an odd-odd nuclide, is only occasionally possible, and is a process so strongly hindered that it has a half-life greater than a billion times the age of the universe.)

[citation needed] The first table is for even-atomic numbered elements, which tend to have far more primordial nuclides, due to the stability conferred by proton-proton pairing.

A second separate table is given for odd-atomic numbered elements, which tend to have far fewer stable and long-lived (primordial) unstable nuclides.