Isotope - Radioactive, Primordial, and Stable Isotopes

Radioactive, Primordial, and Stable Isotopes

Some isotopes are radioactive, and are therefore described as radioisotopes or radionuclides, while others have never been observed to undergo radioactive decay and are described as stable isotopes. For example, 14C is a radioactive form of carbon while 12C and 13C are stable isotopes. There are about 339 naturally occurring nuclides on Earth, of which 288 are primordial nuclides, meaning that they have existed since the solar system's formation.

Primordial nuclides include 35 nuclides with very long half-lives (over 80 million years) and 254 which are formally considered as "stable isotopes", since they have not been observed to decay. In the cases of three elements that have one or more stable isotopes, the most abundant isotope found in nature is actually one (or two) extremely long lived radioisotope(s) of the element (these elements are tellurium, indium, and rhenium). However, in most cases for obvious reasons, if an element has stable isotopes, those isotopes predominate in the elemental abundance found on Earth and in the solar system.

Many apparently "stable" isotopes are predicted by theory to be radioactive, with extremely long half-lives (this does not count the possibility of proton decay, which would make all nuclides ultimately unstable). Of the 254 nuclides never observed to decay, only 90 of these (all from the first 40 elements) are stable in theory to all known forms of decay. Element 41 (niobium) is theoretically unstable via spontaneous fission, but this has never been detected. Many other stable nuclides are in theory energetically susceptible to other known forms of decay, such as alpha decay or double beta decay, but no decay products have yet been observed. The predicted half-lives for these nuclides often greatly exceed the estimated age of the universe, and in fact there are also 27 known radionuclides (see primordial nuclide) with half-lives longer than the age of the universe.

Adding in the radioactive nuclides that have been created artificially, there are more than 3100 currently known nuclides. These include 905 nuclides which are either stable, or have half-lives longer than 60 minutes. See list of nuclides for details.