Half-life and Radioactive
Dating
Jean Brainard, Ph.D.
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Printed: February 19, 2014
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Half-life and Radioactive Dating
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Half-life and Radioactive
Dating
• Define radioactive dating.
• Describe how radioactive dating works.
• Explain how carbon-14 dating can be used to date the remains of living things.
The Grand Canyon, pictured above, was carved by the rushing waters of the Colorado River over millions of years.
The exposed rocks at the bottom of the canyon are almost 2 billion years old. The youngest rocks near the top are
about 230 million years old. Therefore, from top to bottom, the rocks provide a continuous record of more than 1.5
billion years of geological history in this region.
Q: How have scientists been able to determine the ages of rocks in the Grand Canyon?
A: The ages are based on the gradual decay, or break down, of radioactive isotopes.
What Is Radioactive Dating?
Radioactive isotopes, or radioisotopes, can be used to estimate the ages of not only of rocks, but also of fossils and
artifacts made long ago by human beings. Even the age of Earth has been estimated on the basis of radioisotopes. The
general method is called radioactive dating. To understand how radioactive dating works, you need to understand
radioisotopes and radioactive decay.
Radioisotopes and Radioactive Decay
A radioisotope has atoms with unstable nuclei. Unstable nuclei naturally decay, or break down. They lose energy
and particles and become more stable. As nuclei decay, they gain or lose protons, so the atoms become different
elements. This is illustrated in the Figure 1.1. The original, unstable nucleus is called the parent nucleus. After it
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FIGURE 1.1
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 the Table 1.1. 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.
TABLE 1.1: Half-Lives of Some Radioisotopes
Parent Isotope
potassium-40
uranium-235
uranium-234
carbon-14
2Focus on Carbon-14 Dating
Daughter Isotope
argon-40
lead-207
thorium-230
nitrogen-14
Half-Life
1.3 billion years
700 million years
80,000 years
5,700 years
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Concept 1. Half-life and Radioactive Dating
FIGURE 1.2
Summary
• The age of a rock or other specimen can be estimated from the remaining amount of a radioisotope it contains
and the radioisotope’s known rate of decay, or half-life. This method of dating specimens is called radioactive
dating.
• Radioisotopes with longer half-lives are used to date older specimens, and those with shorter half-lives are
used to date younger ones.
• Carbon-14 dating is used to date specimens younger than about 60,000 years old. It is commonly used to date
fossils of living things and human artifacts.
Vocabulary
• radioactive dating: Method of determining the age of fossils or rocks that is based on the rate of decay of
radioisotopes.
Practice
Play the radioactive dating game at the following URL, and then answer the questions below. http://phet.colorado
.edu/en/simulation/radioactive-dating-game
1. What is the half-life of carbon-14? What is the half-life of uranium-238?
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2. Compare the decay rates of carbon-14 and uranium-238. How long does it take for 75 percent of a sample of
carbon-14 atoms to decay? How long does it take for 75 percent of a sample of uranium-238 to decay? Do
these rates depend on the number of atoms in the samples?
3. What percentage of carbon-14 remains in a sample after 10,000 years? How many years does it take for
uranium-238 to decay to this same percentage?
4. Why would you not use carbon-14 to measure the age of the rock?
Review
1. What is radioactive dating?
2. Which radioisotope in the table in the article (see above) could you use to date a fossil thought to be about 500
million years old? Explain your choice.
3. Why does the amount of carbon-14 in an organism remain the same throughout the organism’s life? Why does
the amount change after the organism dies?
References
1. Zachary Wilson. . CC BY-NC 3.0
2. Laura Guerin. . CC BY-NC 3.0
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