3.64 Half-life and Radioactive Dating
FIGURE 3.114
loses a particle (in this case a type of particle called an alpha particle), it forms a daughter nucleus, with a different
number of protons.
The nucleus of a given radioisotope decays at a constant rate that is unaffected by temperature, pressure, or other
conditions outside the nucleus. This rate of decay is called the half-life. The half-life is the length of time it takes
for half of the original amount of the radioisotope to decay to another element.
Q:How can the half-life of a radioisotope be used to date a rock?
A:After a rock forms, nuclei of a radioisotope inside the rock start to decay. As they decay, the amount of the
original, or parent, isotope decreases, while the amount of its stable decay product, or daughter isotope, increases.
By measuring the relative amounts of parent and daughter isotopes and knowing the rate of decay, scientists can
determine how long the parent isotope has been decaying. This provides an estimate of the rock’s age.
Different Isotopes, Different Half-Lives
Different radioisotopes decay at different rates. You can see some examples in theTable3.15. Radioisotopes
with longer half-lives are used to date older rocks or other specimens, and those with shorter half-lives are used
to date younger ones. For example, the oldest rocks at the bottom of the Grand Canyon were dated by measuring
the amounts of potassium-40 in the rocks. Carbon-14 dating, in contrast, is used to date specimens that are much
younger than the rocks in the Grand Canyon. You can read more carbon-14 dating below.
TABLE3.15:Half-Lives of Some Radioisotopes
Parent Isotope Daughter Isotope Half-Life
potassium-40 argon-40 1.3 billion years
uranium-235 lead-207 700 million years
uranium-234 thorium-230 80,000 years
carbon-14 nitrogen-14 5,700 yearsFocus on Carbon-14 Dating
One of the most familiar types of radioactive dating is carbon-14 dating. Carbon-14 forms naturally in Earth’s
atmosphere when cosmic rays strike atoms of nitrogen-14. Living things take in and use carbon-14, just as they do
carbon-12. The carbon-14 in living things gradually decays to nitrogen-14. However, as it decays, it is constantly
replaced because living things keep taking in carbon-14. As a result, there is a constant ratio of carbon-14 to carbon-
12 in organisms as long as they are alive. This is illustrated in the top part of theFigure3.115.
After organisms die, the carbon-14 they already contain continues to decay, but it is no longer replaced (see the
bottom part of theFigure3.115). Therefore, the carbon-14 in a dead organism constantly declines at a fixed rate
equal to the half-life of carbon-14. Half of the remaining carbon-14 decays every 5,700 years. If you measure how
much carbon-14 is left in a fossil, you can determine how many half-lives (and how many years) have passed since
the organism died. Carbon-14 dating is illustrated in the video at this URL: http://www.youtube.com/watch?v=u
dkQwW6aLik.
Q:Why can’t carbon-14 dating be used to date specimens older than about 60,000 years?
A:Carbon-14 has a half-life of 5700 years. After about 60,000 years, too little carbon-14 is left in a specimen to be
measured.