That period of time is called the half-life for that isotope. At that point, the isotope begins its second half-life. One important example of this process is used to determine the age of objects made from once-living matter. The carbon-14 atom then becomes part of the food cycle when it combines with an oxygen molecule to form a carbon dioxide molecule. The following table shows sample data for an organism that had ten grams of carbon-14 in its cells at the time that it died.
Similar information is available for other radioactive isotopes.
In fact, another important radioactive isotope used in dating rocks here on Earth and meteorites from space is uranium -238. Such a sequence of decays is called a decay series. The uranium-238 series is shown below. Note that a diagonal line to the right indicates a beta decay, while a diagonal line to the left indicates an alpha decay. It is actually these decay series which account for certain radioactive elements that are found in nature that would not otherwise be expected.