http://www.ck12.org Chapter 3. Chemical Interactions
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 3.105.
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.
FIGURE 3.105
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 termradioactivity. The other scientist was