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Alpha Decay

Radioactive decay in which radioisotopes shed large helium-like particles.

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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 below 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.

Diagram illustrating alpha decay

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 \begin{align*}\mathrm{^4_2 He}\end{align*}, 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).

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:

\begin{align*}\mathrm{^{238}_{92} U}\end{align*}\begin{align*}\mathrm{^{234}_{90} Th}\end{align*} + \begin{align*}\mathrm{^4_2 He}\end{align*} + 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 (\begin{align*}\mathrm{^4_2 He}\end{align*}) 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.

\begin{align*}\mathrm{^{210}_{84} Po}\end{align*}\begin{align*}\mathrm{^?_? Pb}\end{align*} + \begin{align*}\mathrm{^4_2 He}\end{align*} + 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.


  • 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.


  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.
\begin{align*}\mathrm{^{222}_? Ra}\end{align*}\begin{align*}\mathrm{^?_{86} Pb}\end{align*} + \begin{align*}\mathrm{^4_2 He}\end{align*} + Energy


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alpha decay Type of radioactive decay in which an unstable atomic nucleus emits an alpha particle (two protons plus two neutrons) and energy.

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