10.1  Radioactivity (continued)

Nuclear Decay

Becquerel's experiment marked the discovery of radioactivity. Radioactivity is the process in which an unstable atomic nucleus emits charged particles and energy. Any atom containing an unstable nucleus is called a radioactive isotope, or radioisotope for short.

Radioisotopes of uranium—primarily uranium-238—were the source of radioactivity in Becquerel's experiment. (Recall that the name of an isotope includes its mass number.) Another common radioisotope is carbon-14, which can be found in fossils like the ones shown in Figure 2.

Figure 2  About 26,000 years ago, more than 100 mammoths died at a sinkhole in Hot Springs, South Dakota. Scientists figured out how old the remains were by measuring amounts of the radioisotope carbon-14 contained in the mammoth bones.

Unlike stable isotopes such as carbon-12 or oxygen-16, radioisotopes spontaneously change into other isotopes over time. When the composition of a radioisotope changes, the radioisotope is said to undergo nuclear decay.   During nuclear decay, atoms of one element can change into atoms of a different element altogether. For example, uranium-238 decays into thorium-234, which is also a radioisotope.