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3.56: Radioactivity

Difficulty Level: At Grade Created by: CK-12
Atoms Practice
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The man in this old illustration was an alchemist. Alchemists, who lived during the Middle Ages, were people who strived to turn lead into gold. They tried all sorts of chemical reactions involving lead, but they were never able to produce gold. Today, scientists know that one element cannot be changed into another by chemical processes. However, there is a way that some elements can change into others. You’ll find out what it is when you read this article.  

Elements and Protons

For an atom of one element to change into a different element, the number of protons in its nucleus must change. That’s because each element has a unique number of protons. For example, lead atoms always have 82 protons, and gold atoms always have 79 protons.

Q: So how can one element change into another?

A: The starting element must be radioactive, and its nuclei must gain or lose protons.

Radioactive Elements

Radioactivity is the ability of an atom to emit, or give off, charged particles and energy from its nucleus. The charged particles and energy are called by the general term radiation. Only unstable nuclei emit radiation. They are unstable because they have too much energy, too many protons, or an unstable ratio of protons to neutrons. For example, all elements with more than 83 protons—such as uranium, radium, and polonium—have unstable nuclei. They are called radioactive elements. The nuclei of these elements must lose protons to become more stable. When they do, they become different elements.

How Radioactivity Was Discovered

Radioactivity was discovered in 1896 by a French physicist named Antoine Henri Becquerel, who is pictured below. Becquerel was experimenting with uranium, which was known to glow after being exposed to sunlight. Becquerel wanted to see if the glow was caused by rays of energy, like rays of light or X-rays. He placed a bit of uranium on a photographic plate after exposing the uranium to sunlight. The plate was similar to the film that is used today to take X-rays, and Becquerel expected the uranium to leave an image on the plate. The next day, there was an image on the plate, just as Becquerel expected. This meant that uranium gives off rays after being exposed to sunlight.

Antoine Henri Becquerel

Becquerel was a good scientist, so he wanted to repeat his experiment to confirm his results. He placed more uranium on another photographic plate. However, the day had turned cloudy, so he tucked the plate and uranium in a drawer to try again another day. He wasn’t expecting the uranium to leave an image on the plate without first being exposed to sunlight. To his surprise, there was an image on the plate in the drawer the next day. Becquerel had discovered that uranium gives off rays of energy on its own. He had discovered radioactivity, for which he received a Nobel prize. 

Another scientist, who worked with Becquerel, actually came up with the term radioactivity. The other scientist was the French chemist Marie Curie. She went on to discover the radioactive elements polonium and radium. She won two Nobel Prizes for her discoveries. 


  • Radioactivity is the ability of an atom to emit charged particles and energy from its nucleus. The charged particles and energy are called by the general term radiation.
  • Radioactivity was discovered in 1896 by Antoine Henri Becquerel when he found that uranium leaves an image like an X-ray on a photographic plate. Besides uranium, radioactive elements include radium and polonium, both of which were discovered by Marie Curie.


  1. What is radioactivity?
  2. Define radiation.
  3. Identify some radioactive elements.
  4. Explain how radioactivity was discovered.

Explore More

Watch this video about radioactivity and then answer the questions below.






  1. If a nucleus is unstable, it eventually breaks apart. This process is called __________.
  2. The element polonium has 84 protons. What can you infer about polonium, based on this information?
  3. One type of radioactivity is alpha emission. What is an alpha particle? What happens to an alpha particle after it is emitted from an unstable nucleus?
  4. Why is the element called uranium-238 radioactive? What products form when its nuclei break apart?



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Energy emitted by the nucleus of a radioisotope or an accelerating particle; transfer of energy by electromagnetic waves that can travel across space as well as through matter.


Ability of an atom to emit charged particles and energy from the nucleus.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 14, 2012
Last Modified:
Aug 09, 2016
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