In chemistry, a synthetic chemical element is too unstable to occur naturally on Earth. Synthetic elements possess half-lives so short, relative to the age of the Earth, that any atoms of these elements that may have existed when the Earth formed have long since decayed. Because of this, atoms of synthetic elements only occur on Earth as the product of experiments that involve nuclear reactors or particle accelerators, via nuclear fusion or neutron absorption. Uranium and thorium have no stable isotopes but occur naturally in the Earth's crust and atmosphere, so neither are synthetic. Unstable elements such as polonium, radium, and radon—which form through the decay of uranium and thorium—are also found in nature, despite their short half-lives.
The first element discovered through synthesis was technetium. This discovery filled a gap in the periodic table, and the fact that no stable isotopes of technetium exist explains its natural absence on Earth (and the gap). With the longest-lived isotope of technetium, Tc-98, having a 4.2 million year half-life, no technetium remains from the formation of the Earth. Only minute traces of technetium occur naturally in the Earth's crust (as a spontaneous fission product of uranium-238 or by neutron capture in molybdenum ores), but technetium is present naturally in red giant stars.
Atomic mass for natural life is based on weighted average abundance of natural isotopes that occur in the Earth's crust and atmosphere. For synthetic elements, the isotope depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore, for synthetic elements the total nucleus (protons plus neutrons) count of the most stable isotope (i.e., the isotope with the longest half-life) is listed in brackets as the atomic mass.
(All elements with atomic numbers 1 through 94 are naturally occurring at least in trace quantities)
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