## Physics: Principles with Applications (7th Edition)

The nucleon (mass) number drops by 16, from 222 to 206, which means that four $\alpha$ decays took place. The atomic number changed by 4, from Z = 86 to Z = 82. The four alpha decays account for a decrease of eight protons, so 4 $\beta^{-}$ decays must have taken place, converting four neutrons into protons. This decay chain may be traced out in Figure 30–11.