List of isotopes

This list of nuclides shows the >900 observed nuclides that either are stable or, if radioactive, have half-lives longer than one hour. At least 3,000 nuclides have been experimentally characterized. A selection of those with decay half-lives less than 60 minutes are shown.

A nuclide is defined conventionally as an experimentally examined bound collection of one or more protons and zero or more neutrons, that either is stable or has an observed decay mode.

254 nuclides are considered stable. Many of these in theory could decay through spontaneous fission, alpha decay, double beta decay, etc. with a very long half-life, but no radioactive decay has yet been observed. Thus the number of stable nuclides is subject to change if some of these 254 are determined to be very long-lived radioactive nuclides in the future. If a decay has been predicted theoretically but never observed experimentally, it is given in parentheses. In theory, spontaneous fission is possible for all elements with atomic numbers >40, but has not been observed for most elements up to lead (82). The list lists nuclides unstable only to this fission mechanism, before nuclides theoretically unstable to additional mechanisms. Those that have been checked for radioactivity are indicated with "> number" and show the lower limit for the half-life based on experimental observation. Such nuclides are considered to be "stable" until a decay has been observed in some fashion. For example, tellurium-123 was reported to be radioactive, but the same experimental group later retracted this report, and it presently remains observationally stable.

The next group is the primordial radioactive nuclides. These have been measured to be radioactive, or decay products have been identified (Te-130, Ba-130). There are (currently) 32 of these, of which 28 have half-lives considerably longer than the age of the universe (13.8 billion years). With most of these 28, decay is difficult to observe and for most purposes they can be regarded as effectively stable. Bismuth-209 is notable as the only natural occurring isotope of an element long considered stable. A further 4 nuclides, thorium-232, uranium-238, potassium-40 and uranium-235 have half lives between 14 billion and 700 million years, which means they have experienced measurable decay since the formation of the solar system about 4.6 billion years ago, but still exist on earth in significant quantities. They are the primary source of radiogenic heat and radioactive decay products. Together, there are a total of 286 primordial nuclides.

...
Wikipedia