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出典:『Wikipedia』 (2011/06/03 23:23 UTC 版)

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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.