<P> Chemical elements may also be categorized by their origin on Earth, with the first 94 considered naturally occurring, while those with atomic numbers beyond 94 have only been produced artificially as the synthetic products of man - made nuclear reactions . </P> <P> Of the 94 naturally occurring elements, 83 are considered primordial and either stable or weakly radioactive . The remaining 11 naturally occurring elements possess half lives too short for them to have been present at the beginning of the Solar System, and are therefore considered transient elements . Of these 11 transient elements, 5 (polonium, radon, radium, actinium, and protactinium) are relatively common decay products of thorium and uranium . The remaining 6 transient elements (technetium, promethium, astatine, francium, neptunium, and plutonium) occur only rarely, as products of rare decay modes or nuclear reaction processes involving uranium or other heavy elements . </P> <P> Elements with atomic numbers 1 through 40 are all stable, while those with atomic numbers 41 through 82 (except technetium and promethium) are metastable . The half - lives of these metastable "theoretical radionuclides" are so long (at least 100 million times longer than the estimated age of the universe) that their radioactive decay has yet to be detected by experiment . Elements with atomic numbers 83 through 94 are unstable to the point that their radioactive decay can be detected . Three of these elements, bismuth (element 83), thorium (element 90), and uranium (element 92) have one or more isotopes with half - lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy elements before the formation of our solar system . For example, at over 1.9 × 10 years, over a billion times longer than the current estimated age of the universe, bismuth - 209 has the longest known alpha decay half - life of any naturally occurring element . The very heaviest 24 elements (those beyond plutonium, element 94) undergo radioactive decay with short half - lives and cannot be produced as daughters of longer - lived elements, and thus they do not occur in nature at all . </P> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul>

Which groups of elements are not found as free elements (pure substances) in nature
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