The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.(The nucleus of an atom is made up of protons and neutrons.) For example, the element carbon, which always has six protons in its nucleus, has three isotopes: one with six neutrons in the nucleus, one with seven, and one with eight.Some isotopes are stable, but some are unstable or radioactive.Note that at time 0, the time of the mineral's formation, the crystal contains only parent atoms.At time 1, 50% of the parent atoms remain; at time 2, only 25% remain, and so on.After one half-life, 50 percent of the original parents remains; after two, only 25 percent remains, and so on.
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Scientists determined the Earth's age using a technique called radiometric dating.Radiometric dating is based upon the fact that some forms of chemical elements are radioactive, which was discovered in 1896 by Henri Becquerel and his assistants, Marie and Pierre Curie.
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