Natural isotopes
Open-knowledge reference entry
Natural isotopes are either stable isotopes or radioactive isotopes that have a sufficiently long half-life to allow them to exist in substantial concentrations in the Earth (such as bismuth-209, with a half-life of 1.9×1019 years, potassium-40 with a half-life of 1.251(3)×109 years), daughter products of those isotopes (such as 234Th, with a half-life of 24 days) or cosmogenic elements. The heaviest stable isotope is lead-208, but the heaviest 'natural' isotope is U-238.
Many elements have both natural and artificial isotopes. For example, hydrogen has three natural isotopes and another four known artificial isotopes. A further distinction among stable natural isotopes is division into primordial (existed when the Solar System formed) and cosmogenic (created by cosmic ray bombardment or other similar processes).
What defines a natural isotope
Natural isotopes must be either stable, have a half-life exceeding about 7×107 years (there are 35 isotopes in this category, see stable isotope for more details) or are generated in large amounts cosmogenically (such as 14C, which has a half-life of only 6000 years but is made by cosmic rays colliding with 14N).
Naturally occurring radioisotopes
Some radioisotopes occur in nature with a half-life of less than 7×107 years (carbon-14: 5,730 ± 40 years, tritium: 12.32 years etc.). They are synthesised all the time by cosmic radiation. A practical use is radiocarbon dating with carbon-14.
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This entry incorporates text from “Natural isotopes” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.