Chemistry Reference
In-Depth Information
transient, radioactive daughter isotopes are formed. Uranium-238, for
example, initially yields uranium-234, which then decays to thorium-230;
only many more steps afterward is stable lead-206 finally formed. The half-
lives of the transient daughter isotopes range from fractions of a second to
hundreds of thousands of years. The longest-lived isotopes, including
uranium-234, whose half-life is 250,000 years, and thorium-230, with a half-
life of 75,000 years, are used to date prehistorical materials (see Fig. 12).
FIGURE 12
The decay series of the uranium isotopes.
Most radioactive iso-
topes decay into stable ones. Some, however, decay into intermediate, unstable
isotopes, over and over (repeatedly) in long “decay series,” which terminates
only when a stable isotope is formed; the stable isotope formed at the end of
all the decay series is always an isotope of lead. Three decay series occur natu-
rally in the crust of the earth: one beginning with the isotope thorium-232 and
two others that begin with isotopes of uranium, namely, uranium-235 and
uranium-238. The last two, which are of use for dating archaeological materials,
are illustrated. Determining the length of burial time of objects that absorb
uranium, such as bone, teeth, and eggshell, are typical applications of the
uranium series method of dating.
