An element subject to spontaneous degeneration of its nucleus accompanied by the emission of alpha particles, beta particles, or gamma rays. All elements with atomic numbers greater than 83 are radioactive. Naturally occurring radioactive elements include radium, thorium, and uranium. Several radioactive elements not found in nature have been produced by the bombardment of stable elements with subatomic particles in a cyclotron. Compare stable element. See also radioactivity. Subsequent to Roentgen’s discovery of X-rays, in 1896 a French scientist Henri Becquerel was experimenting with a uranium compound. While investigating the properties of fluorescent minerals, it was Becquerel who discovered that certain types of atoms disintegrate by themselves. When working on the principles of fluorescence, he utilized photographic film to record fluorescence of various minerals when exposed to sunlight. In addition, he noticed that only the film that was in the drawer with the uranium compound had an image on it. Becquerel concluded that the uranium compound gave off something invisible that could penetrate heavy paper and affect photographic film. Becquerel continued to test many samples of compounds and determined that the source of the invisible something was the element uranium. This invisible something was named radiation, and it was determined that an element that gives off radiation is a radioactive element. Today, we know uranium as one of the radioactive elements. For his discovery of radioactivity,

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