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3.59: Radioactive Decay

Difficulty Level: At Grade Created by: CK-12
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This diagram could almost illustrate a pinball game. It actually illustrates the process of radioactive decay. This is a process that occurs all around you. It may even occur inside your body.

Q: Based on the diagram, what happens during radioactive decay?

A: The nucleus of a radioactive atom gives off radiation in the form of energy and a particle of matter.

Introducing Radioactive Decay

Radioactive decay is the process in which the nuclei of radioactive atoms emit charged particles and energy, which are called by the general term radiation. Radioactive atoms have unstable nuclei, and when the nuclei emit radiation, they become more stable. Radioactive decay is a nuclear—rather than chemical—reaction because it involves only the nuclei of atoms. In a nuclear reaction, one element may change into another. For a fun introduction to radioactive decay, watch the short video at this URL: http://www.youtube.com/watch?v=o-9yt7OAYmE

Types of Radioactive Decay

There are several types of radioactive decay, including alpha, beta, and gamma decay. In all three types, nuclei emit radiation, but the nature of the radiation differs. The Table below shows the radiation emitted in each type of decay. You can watch a video about the three types at this URL: http://www.youtube.com/watch?v=3koOwozY4oc

Type Radiation Emitted
Alpha decay alpha particle (2 protons and 2 neutrons) + energy
Beta decay beta particle (1 electron or 1 positron) + energy
Gamma decay energy (gamma ray)

It’s Elemental

Both alpha and beta decay change the number of protons in an atom’s nucleus, thereby changing the atom to a different element. In alpha decay, the nucleus loses two protons. In beta decay, the nucleus either loses a proton or gains a proton. In gamma decay, no change in proton number occurs, so the atom does not become a different element.

Q: If the radioactive element polonium (Po) undergoes alpha decay, what element does it become?

A: From the periodic table, the atomic number of polonium is 84, so it has 84 protons. If it loses two protons through alpha decay, it will have 82 protons. Atoms with 82 protons are the element lead (Pb).

Dangers of Radioactive Decay

The charged particles and energy emitted during radioactive decay can harm living things, but the three types of radioactive decay aren’t equally dangerous. That’s because they differ in how far they can travel and what they can penetrate. You can see this in the Figure below and in the animation at the following URL: http://www.cna.ca/curriculum/cna_radiation/gamma_rays-eng.asp?bc=Gamma_Rays&pid=Gamma_Rays

Alpha particles can travel only a few centimeters through air. They can burn the skin but not penetrate it. Beta particles can travel up to a meter through air. They can penetrate and damage skin. Gamma rays can travel thousands of meters through air. They can penetrate and damage cells deep inside the body.


  • Radioactive decay is the process in which unstable nuclei of radioactive atoms become stable by emitting charged particles and energy.
  • There are three types of radioactive decay: alpha decay, beta decay, and gamma decay. Alpha and beta decay change one element into another. Gamma decay does not.
  • Radioactive decay can damage living things. Alpha decay is the least damaging, and gamma decay is the most damaging.


  • radioactive decay: Process in which the unstable nucleus of a radioisotope becomes stable by emitting charged particles and energy and changing to another element.


Watch the video about the decay of uranium at the following URL, and then answer the questions below. http://www.youtube.com/watch?v=F8sAkMQmOlU

  1. In the first step of the decay of uranium-238, what are the products?
  2. How does the atomic number of uranium-238 change in the first step of the decay?
  3. What happens to the new element that results from the first step of uranium-238 decay?
  4. What type of decay is the decay of uranium-238?
  5. What is the half-life of a radioactive element? What is the half-life of uranium-238?
  6. Uranium-238 decays very slowly, so why is a lump of uranium-238 dangerous?


  1. What is radioactive decay?
  2. Compare and contrast alpha, beta, and gamma decay.

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radioactive decay Process in which the unstable nucleus of a radioisotope becomes stable by emitting charged particles and energy and changing to another element.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 14, 2012
Last Modified:
Sep 17, 2016
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