Alpha Decay
Jean Brainard, Ph.D.
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Printed: August 25, 2016
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Alpha Decay
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Alpha Decay
• Explain how and why alpha decay occurs.
• Show how to write a balanced nuclear equation for alpha decay.
• Describe the dangers of alpha decay.
You probably associate the term decay with images like the one above. But when it comes to atoms, decay has a
different meaning. Decay in chemistry refers to changes in the nuclei of certain atoms.
Why Some Nuclei Decay
Radioactive elements and isotopes have unstable nuclei. To become more stable, the nuclei undergo radioactive
decay. In radioactive decay, the nuclei give off, or emit, radiation in the form of energy and often particles 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 Alpha Decay?
Alpha decay occurs when a nucleus is unstable because it has too many protons. The Figure 1.1 shows what happens
during alpha decay. The nucleus emits an alpha particle and energy. An alpha particle consists of two protons and
two neutrons, which is actually a helium nucleus. Losing the protons and neutrons makes the nucleus more stable.
Equations for Alpha Decay
Radioactive nuclei and particles are represented by nuclear symbols that indicate their numbers of protons and
neutrons. For example, an alpha particle (helium nucleus) is represented by the symbol 42 He, where He is the
chemical symbol for helium, the subscript 2 is the number of protons, and the superscript 4 is the mass number (2
protons + 2 neutrons).
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FIGURE 1.1
Nuclear symbols are used to write nuclear equations for radioactive decay. Let’s consider an example. Uranium-238
undergoes alpha decay to become thorium-234. (The numbers following the chemical names refer to the number
of protons plus neutrons.) In this reaction, uranium-238 loses two protons and two neutrons to become the element
thorium-234. The reaction can be represented by this nuclear equation:
238 U
92
4
→ 234
90 Th + 2 He + Energy
If you count the number of protons (subscripts) as well as the number of protons plus neutrons (superscripts), you’ll
see that the total numbers are the same on both sides of the arrow. This means that the equation is balanced. The
thorium-234 produced in this reaction is also unstable, so it will undergo radioactive decay as well. The alpha
particle (42 He) produced in the reaction can join with two free electrons to form the element helium. This is how
most of Earth’s helium formed.
Q: Fill in the missing subscript and superscript to balance the following nuclear equation for alpha decay of
Polonium-210.
210 Po
84
→ ?? Pb + 42 He + Energy
A: The subscript of Pb is 82, and the superscript is 206. This means that the new element produced in the reaction
has 82 protons. You can find the element with this number of protons in the periodic table. It is the element lead
(Pb). The new element also has 124 neutrons (206 - 82 protons = 124 neutrons).
How Dangerous Is Alpha Decay?
All types of radioactive decay pose risks to living things, but alpha decay is the least dangerous. That’s because
alpha particles are relatively heavy, so they can travel only a few centimeters through the air. They also are not very
penetrating. For example, they can’t pass through a sheet of paper or thin layer of clothing. They may burn the skin,
but they can’t penetrate to the tissues underneath the skin. However, if alpha particles are emitted inside the body,
they can do more damage. One way this can happen is by inhaling cigarette smoke. People who smoke actually
inhale the radioactive element polonium-210. It undergoes alpha decay in the lungs. Over time, exposure to alpha
particles may cause lung cancer.
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Chapter 1. Alpha Decay
Summary
• Alpha decay is one of three types of nuclear decay in which unstable nuclei emit energy with or without a
particle of matter.
• In alpha decay, energy and an alpha particle are emitted by a nucleus that is unstable because it has too many
protons. An alpha particle consists of two protons and two neutrons, so it is actually a helium nucleus.
• Alpha decay is represented by a nuclear equation. The equation is balanced if the total numbers of protons
and neutrons are the same on both sides of the arrow.
• All radioactive decay is dangerous to living things, but alpha decay is the least dangerous.
Review
1. What is alpha decay?
2. Explain why alpha decay occurs.
3. If a radioactive element with 85 protons undergoes alpha decay, how many protons will there be in the new
element that forms as the product of the reaction? What element is it?
4. Fill in the missing subscript and superscript to balance the following nuclear equation. Make sure your
equation is balanced.
222 Ra
?
→ ?86 Pb + 42 He + Energy
Resources
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/186424
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/186426
References
1. Christopher Auyeung. Diagram illustrating alpha decay . CC BY-NC 3.0
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