Beta Decay
Jean Brainard, Ph.D.
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Printed: October 20, 2014
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
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Chapter 1. Beta Decay
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Beta Decay
Explain how and why beta decay occurs.
Contrast beta-minus and beta-plus decay.
Show how to write a balanced nuclear equation for beta decay.
Describe dangers of beta decay.
If you hear the word decay while your dentist is checking your teeth, it’s probably bad news. But if you’re an
unstable atomic nucleus, decay is good news. When the nucleus of an atom decays, it becomes more stable.
Which Nuclei Decay
Atoms with unstable nuclei are radioactive. To become more stable, the nuclei undergo radioactive decay. In
radioactive decay, the nuclei emit energy and usually particles of matter as well. There are several types of
radioactive decay, including alpha, beta, and gamma decay. Energy is emitted in all three types of decay, but only
alpha and beta decay also emit particles.
What Is Beta Decay?
Beta decay occurs when an unstable nucleus emits a beta particle and energy. A beta particle is either an electron
or a positron. An electron is a negatively charged particle, and a positron is a positively charged electron (or antielectron). When the beta particle is an electron, the decay is called beta-minus decay. When the beta particle is
a positron, the decay is called beta-plus decay. Beta-minus decay occurs when a nucleus has too many neutrons
relative to protons, and beta-plus decay occurs when a nucleus has too few neutrons relative to protons.
Q: Nuclei contain only protons and neutrons, so how can a nucleus emit an electron in beta-minus decay or a positron
in beta-plus decay?
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FIGURE 1.1
A: Beta decay begins with a proton or neutron. You can see how in the Figure 1.1.
Q: How does beta decay change an atom to a different element?
A: In beta-minus decay an atom gains a proton, and it beta-plus decay it loses a proton. In each case, the atom
becomes a different element because it has a different number of protons. You can see an animation showing how
this happens at the following URL. http://www.furryelephant.com/player.php?subject=physics&jumpTo=re/10Ms44
Equations for Beta Decay
Radioactive nuclei and particles are represented by nuclear symbols.. For example, a beta-minus particle (electron) is
represented by the symbol 0−1 e. The subscript -1 represents the particle’s charge, and the superscript 0 shows that the
particle has virtually no mass (no protons or neutrons). Another example is the radioactive nucleus of thorium-234.
It is represented by the symbol 234
90 Th, where the subscript 90 stands for the number of protons and the superscript
234 for the number of protons plus neutrons.
Nuclear symbols are used to write nuclear equations for radioactive decay. Let’s consider the example of the betaminus decay of thorium-234 to protactinium-234. This reaction is represented by the equation:
234 Th
90
0
→ 234
91 Pa + −1 e + energy
The equation shows that thorium-234 becomes protactinium-234 and loses a beta particle and energy. The protactinium234 produced in the reaction is also radioactive, so it will decay as well.
A nuclear equation is balanced if the total numbers of protons and neutrons are the same on both sides of the arrow.
If you compare the subscripts and superscripts on both sides of the equation above, you’ll see that they are the same.
Q: What happens to the electron produced in the reaction above?
A: Along with another electron, it can combine with an alpha particle to form a helium atom. An alpha particle,
which is emitted during alpha decay, consists of two protons and two neutrons.
Q: Try to balance the following nuclear equation for beta-minus decay by filling in the missing subscript and
superscript.
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131 I
53
Chapter 1. Beta Decay
→ ?? Xe + 0−1 e + energy
A: The subscript of Xe is 54, and the superscript is 131.
Dangers of Beta Decay
Beta particles can travel about a meter through air. They can pass through a sheet of paper or a layer of cloth but not
through a sheet of aluminum or a few centimeters of wood. They can also penetrate the skin and damage underlying
tissues. They are even more harmful if they are ingested or inhaled.
Summary
• Beta decay occurs when a nucleus is unstable because it has too many or too few neutrons relative to protons.
The nucleus emits a beta particle and energy. A beta particle is either an electron (beta-minus decay) or a
positron (beta-plus decay).
• In beta-minus decay, a neutron breaks down to a proton and an electron, and the electron is emitted from the
nucleus. In beta-plus decay, a proton breaks down to a neutron and a positron, and the positron is emitted from
the nucleus.
• Balanced nuclear equations show how the numbers of protons and neutrons change in beta decay.
• Beta radiation is harmful to living things.
Vocabulary
• beta decay: Type of radioactive decay in which an unstable atomic nucleus emits a beta particle (electron or
positron) and energy.
Explore More
Read the article about beta decay at the following URL. Be sure to watch the short video at the bottom of the page.
Then answer the questions below.
http://teachnuclear.ca/contents/cna_radiation/nuclear-decay/beta_decay/
1. Write a balanced nuclear equation for the beta-minus decay of cesium-137. Identify the new element that
results from the reaction. How many neutrons does this element have?
2. What are beta emitters? Give three examples.
Review
1. Compare and contrast beta-minus and beta-plus decay.
1. Fill in the missing subscript and superscript in this nuclear equation to balance it:
14 C
?
→ ?7 N + 0−1 e + energy
Does the equation represent beta-minus or beta-plus decay? How do you know?
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References
1. Zachary Wilson. Diagram illustrating beta decay. CC BY-NC 3.0
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