Radioactive Decay as a
Measure of Age
Dana Desonie, Ph.D.
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Printed: January 28, 2015
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Radioactive Decay as a Measure of Age
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Radioactive Decay as a
Measure of Age
• Radioactive decay gives a way to determine the age of some types of rocks.
When do you stop counting?
Pretend that the large green cylinder is a parent isotope. Now you can visualize the decay of the parent to the
daughter. It’s easy to see that the second cylinder is half the size of the first. The third is half the size of the second.
But when the cylinders get small, the differences are much harder to see. At some point, there is too little of the
parent left. That isotope pair is no longer useful for dating.
Radioactive Decay
Radioactive decay is the breakdown of unstable elements into stable elements. To understand this process, recall
that the atoms of all elements contain the particles protons, neutrons, and electrons.
Isotopes
An element is defined by the number of protons it contains. All atoms of a given element contain the same number of
protons. The number of neutrons in an element may vary. Atoms of an element with different numbers of neutrons
are called isotopes.
Consider carbon as an example. Two isotopes of carbon are shown below ( Figure 1.1). Compare their protons and
neutrons. Both contain six protons. But carbon-12 has six neutrons and carbon-14 has eight neutrons.
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FIGURE 1.1
Isotopes are named for their number of protons plus neutrons. If a carbon atom had seven neutrons, what would
it be named?
Almost all carbon atoms are carbon-12. This is a stable isotope of carbon. Only a tiny percentage of carbon atoms
are carbon-14. carbon-14 is unstable. It is a radioactive isotope of carbon. Pictured below is carbon dioxide (
Figure 1.2), which forms in the atmosphere from carbon-14 and oxygen. Neutrons in cosmic rays strike nitrogen
atoms in the atmosphere. The nitrogen forms carbon-14. Carbon in the atmosphere combines with oxygen to form
carbon dioxide. Plants take in carbon dioxide during photosynthesis. In this way, carbon-14 enters food chains.
FIGURE 1.2
Carbon-14 forms in the atmosphere. It combines with oxygen and forms
carbon dioxide. How does carbon-14 end up in fossils?
Decay of Unstable Isotopes
Like other unstable isotopes, carbon-14 breaks down, or decays. The original atoms are called the parent isotopes.
For carbon-14 decay, each carbon-14 atom loses an beta particle. It changes to a stable atom of nitrogen-14. The
stable atom at the end is the daughter product ( Figure 1.3).
The decay of an unstable isotope to a stable element occurs at a constant rate. This rate is different for each parentdaughter isotope pair. The decay rate is measured in a unit called the half-life. The half-life is the time it takes for
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Chapter 1. Radioactive Decay as a Measure of Age
FIGURE 1.3
Unstable isotopes, such as carbon-14, decay by losing atomic particles.
They form different, stable elements when they decay.
half of a given amount of an isotope to decay. For example, the half-life of carbon-14 is 5,730 years. Imagine that
you start out with 100 grams of carbon-14. In 5,730 years, half of it decays. This leaves 50 grams of carbon-14.
Over the next 5,730 years, half of the remaining amount will decay. Now there are 25 grams of carbon-14. How
many grams will there be in another 5,730 years? The figure below graphs the rate of decay of a substance ( Figure
1.4).
FIGURE 1.4
The rate of decay of a radioactive substance is constant over time.
Summary
• A half-life is the time it takes for half of the parent isotopes of an element to change to the daughter product.
• With alpha decay, the nucleus loses two protons and two neutrons.
• Carbon-14 has a half-life of 5,730 years.
Explore More
Use the resource below to answer the questions that follow.
• What Is Carbon Dating? at http://www.chem4kids.com/files/atom_isotopes.html
1. What is an isotope?
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2.
3.
4.
5.
Describe carbon-14.
What is average atomic mass?
What is radioactive decay?
What is carbon dating?
Review
1. What makes an isotope radioactive? Are all isotopes radioactive?
2. What is a parent isotope and a daughter product?
3. Describe half-life. Use an example.
References
1. Christopher Auyeung. Diagram of carbon isotopes 12, 13, and 14 . CC BY-NC 3.0
2. User:Jynto/Wikimedia Commons. Structure of carbon dioxide . Public Domain
3. User:Inductiveload/Wikimedia Commons, modified by CK-12 Foundation. Diagram of Carbon 14 beta decay
. Public Domain
4. Kurt Rosenkrantz. Graph of the decay of an imaginary radioactive substance with a half-life of one year . CC
BY-NC 3.0
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