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- JamesTaoLv 61 decade agoFavorite Answer
Helium makes up about 0.0005% of the earth's atmosphere.
This trace amount of helium is not gravitationally bound to the earth and is constantly lost to space. The earth's atmospheric helium is replaced by the decay of radioactive elements in the earth's crust. Alpha decay, one type of radioactive decay, produces particles called alpha particles. An alpha particle can become a helium atom once it captures two electrons from its surroundings. This newly formed helium can eventually work its way to the atmosphere through cracks in the crust.
2010-02-13 21:36:04 補充：