A radioactive nucleus decays into a daughter nucleus. In many cases, the daughter nucleus is also radioactive and decays to produce its own daughter nucleus. The process continues until reaching a daughter nucleus that is stable. The sequence of isotopes, starting with the original unstable isotope and ending with the stable isotope, is called a decay series.

Decay series are especially important for very heavy nuclei. As an example, FIGURE 42.24 shows the decay series of ${}^{235}\text{U}$, an isotope of uranium with a 700-million-year half-life. This is a very long time, but it is only about 15% the age of the earth, thus most (but not all) of the ${}^{235}\text{U}$ nuclei present when the earth was formed have now decayed. There are many unstable nuclei along the way, but all ${}^{235}\text{U}$ nuclei eventually end as the ${}^{207}\text{Pb}$ isotope of lead, a stable nucleus.

FIGURE 42.24 The decay series of ${}^{235}\text{U}$.

Notice that some nuclei can decay by either alpha or beta decay. Thus there are a variety of paths that a decay can follow, but they all end at the same point.