Radiometric dating meteorites
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.
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.
Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma.
The discovery gave scientists a tool for dating rocks that contain radioactive elements.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.
(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.
Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay.
Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.
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.
After one half-life, 50 percent of the original parents remains; after two, only 25 percent remains, and so on.
Decay curve of a radioactive element with a half-life equal to one time unit.