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Isotopes and Nuclear Stability

Two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei.

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Nuclear Medicine

Nuclear Medicine

Credit: Myohan
Source: http://commons.wikimedia.org/wiki/File:Thyroid_scan.jpg
License: CC BY-NC 3.0

In the image above, a person's thyroid is viewed using special cameras and visualization with a radioactive isotope called iodine-123. Doctors can use these methods with a number of radioactive isotopes to diagnose otherwise undetectable illnesses.

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Credit: Dnn87
Source: http://en.wikipedia.org/wiki/File:CsCrystals.JPG
License: CC BY-NC 3.0

Cesium is a common isotope used to identify and treat diseases [Figure2]

  • Radioactive isotopes of elements such as iodine, palladium and cesium are some of the common isotopes that are used in nuclear medicine to help diagnose and treat various diseases. These radioactive isotopes are combined with well-known substances to form compounds known as radiopharmaceuticals. When a radiopharmeaceutical enters the human body, it localizes in a certain region depending on which compound is used. By using gamma ray cameras to capture the radiation given off by the radiopharmaceuticals, doctors are able to see the extent of a disease in the body without having to rely on collecting samples from the body.
  • Radioactive isotopes are elements with unstable nuclei that undergo radioactive decay via the three possible forms of radioactive emission: gamma ray emission, alpha decay, and beta decay.
    • Gamma emission: When a nucleus is in an excited state, photons can be emitted to drop the nucleus to a lower energy level. Gamma decay is usually produced after alpha and/or beta decay occur because alpha and beta decay leave the nucleus in an excited state.-
    • Alpha decay: In alpha decay, the excess energy in the nucleus is emitted via release of an alpha particle. The alpha particle released consists of 2 protons and 2 neutrons, which is the equivalent of a Helium atom. This process is best represented by the following balanced equation:


In the equation above, Z represents the atomic number and N represents the number of neutrons. Therefore, it can easily be seen that the process of alpha decay causes a transformation of the original element into a new one.

    • Beta decay: Beta decay is a result of the nucleus having too many neutrons. This is identified by the ratio of NZ, where N is the number of neutrons and Z is the number of protons. The optimal ratio is reached via emission of a beta particle. Beta particles are either electrons (β) or positrons (β+) that are emitted from an atomic nucleus. In each of the possible decay processes, a transformation of the parent element into a new element is achieved.


where v and v¯¯¯ are neutrinos and anti-neutrinos respectively.

  • Learn more about the various forms of decay: 


  • Learn about the different penetration capabilities of each form of decay: 


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Using the information provided above, answer the following questions.

  1. Which of the three forms of decay emit particles or waves that have the greatest penetration ability?
  2. If someone were to direct a stream of alpha particles at you, would your t-shirt be enough to protect you, or would you need a really thick wall of lead?
  3. What type of decay would cause uranium-238 to transform into thorium-234?

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Image Attributions

  1. [1]^ Credit: Myohan; Source: http://commons.wikimedia.org/wiki/File:Thyroid_scan.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Dnn87; Source: http://en.wikipedia.org/wiki/File:CsCrystals.JPG; License: CC BY-NC 3.0

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