In chemistry, the chemical elements labeled as synthetic are too unstable to be found naturally on Earth. These 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 away. Because of this, atoms of synthetic elements are only present on Earth as the product of experiments involving nuclear reactors or particle accelerators via nuclear fusion or neutron absorption. Uranium and thorium have no stable isotopes, but are found naturally in the Earth's crust and atmosphere, so neither of these two elements is called synthetic. Also, unstable elements such as polonium, radium, and radon, which are formed through the decay of uranium and thorium can also be found in nature despite having very 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 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 found naturally in red giant stars.
Atomic mass for natural elements is based on weighted average abundance of natural isotopes occurring in the Earth's crust and atmosphere. For synthetic elements the isotope formed 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 (ie, the isotope with the longest half-life) is listed in brackets as the atomic mass.
quasi-synthetic elements include:
(All elements with atomic numbers 1-94 are naturally occurring at least in trace quantities)
Provisional names for recently observed synthetic elements: