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3.61: Beta Decay

Difficulty Level: At Grade Created by: CK-12
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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 anti-electron). 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?

A: Beta decay begins with a proton or neutron. You can see how in the Figure below.

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 \begin{align*}\mathrm{^0_{-1} e}\end{align*}01e. 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 \begin{align*}\mathrm{^{234}_{90} Th}\end{align*}23490Th, 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 beta-minus decay of thorium-234 to protactinium-234. This reaction is represented by the equation:

\begin{align*}\mathrm{^{234}_{90} Th}\end{align*}23490Th\begin{align*}\mathrm{^{234}_{91} Pa}\end{align*}23491Pa + \begin{align*}\mathrm{^0_{-1} e}\end{align*}01e + energy

The equation shows that thorium-234 becomes protactinium-234 and loses a beta particle and energy. The protactinium-234 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.

\begin{align*}\mathrm{^{131}_{53} I}\end{align*}13153I\begin{align*}\mathrm{^?_? Xe}\end{align*}??Xe + \begin{align*}\mathrm{^0_{-1} e}\end{align*}01e + 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.  


  • 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.



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.


  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.


  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: \begin{align*}\mathrm{^{14}_? C}\end{align*}14?C\begin{align*}\mathrm{^?_7 N}\end{align*}?7N + \begin{align*}\mathrm{^0_{-1} e}\end{align*}01e + energy Does the equation represent beta-minus or beta-plus decay? How do you know?

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beta decay Type of radioactive decay in which an unstable atomic nucleus emits a beta particle (electron or positron) and energy.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 14, 2012
Last Modified:
Sep 13, 2016
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