CC.7 Radioactivity and Age Dating
ESSENTIAL TO KNOW
- Atoms of an element usually have several isotopes that differ in atomic weight but are essentially identical in chemical properties.
- Some individual isotopes of many elements are radioactive. Others are stable.
- Radioisotopes called “parent isotopes” decay by losing part of the atom to become a different isotope, called a “daughter isotope.” Often the daughter is an isotope of a different element.
- Radioactive decay releases heat and radioactive particles, including alpha particles, beta particles, and gamma rays.
- Each radioisotope decays at a certain fixed rate, expressed by the isotope half-life.
- One half-life is the time it takes for exactly half of the atoms of a particular radioisotope in a sample to decay.
- The age of a rock, archaeological artifact, or skeletal or undecomposed remains of an organism can often be determined by measurement of the concentration of both the parent and daughter isotopes in the sample.
- For such age dating to be accurate, no daughter isotope must have been present when the sample was formed. In addition, no parent or daughter isotope atoms must have been gained or lost by the sample after it was formed. These conditions are often not met.
- Different pairs of parent and daughter isotopes must be used for dating samples of different ages. Parent isotopes with long half-lives are useful only for dating older samples.
- Radioisotope dating is often used to calibrate other less expensive dating techniques, such as fossil dating, that reveal only the relative dates of samples from within a group of samples under study.