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 are called synthetic. 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 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 found naturally in red giant stars.
Quasi-synthetic[clarification needed] elements include
(All elements with atomic numbers 1 through 94 are naturally occurring at least in trace quantities)
