The nuclei of most atoms are stable. However, many elements do have isotopes in which the nuclei are unstable-carbon-14 is one example of an unstable isotope. In this context, unstable means that the nuclei change through a random process called radioactive decay. During radioactive decay, unstable isotopes radiate energy and emit particles. The rates at which unstable isotopes decay are measurable. Therefore, certain radioactive atoms are used to determine the ages of fossils, rocks, and minerals. A discussion of radioactive decay and its applications in dating past geologic events appears in Chapter 11.