Radioactivity
Jean Brainard, Ph.D.
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Printed: September 30, 2013
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Radioactivity
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Radioactivity
• Define radioactivity and radiation.
• Give examples of radioactive elements.
• Outline the discovery of radioactivity.
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.
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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 1.1.
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.
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. To learn more about the importance of Becquerel’s research, go to this URL: http://nobelprize.org/nobel_priz
es/physics/laureates/1903/becquerel-bio.html
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. You can learn more about Marie Curie at this URL: http://nobelprize.org/no
bel_prizes/physics/laureates/1903/marie-curie-bio.html
Summary
• 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.
Vocabulary
• radiation: Energy emitted by the nucleus of a radioisotope or an accelerating particle; transfer of energy by
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Concept 1. Radioactivity
FIGURE 1.1
electromagnetic waves that can travel across space as well as through matter.
• radioactivity: Ability of an atom to emit charged particles and energy from the nucleus.
Practice
Watch the video about radioactivity at the following URL, and then answer the questions below. http://www.youtu
be.com/watch?v=q1pMDMXLSBI
MEDIA
Click image to the left for more content.
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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Review
1.
2.
3.
4.
What is radioactivity?
Define radiation.
Identify some radioactive elements.
Explain how radioactivity was discovered.
References
1. Paul Nadar. . Public Domain
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