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In chemistry, the chemical elements labeled as synthetic are too unstable to be found naturally on Earth. These synthetic elements possess a half-life 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. 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.