Radon is a gas that is produced by the radioactive decay of naturally occurring heavy nuclei , such as thorium and uranium . Radon gas can be breathed into the lungs , where it can decay further . The nucleus of a radon atom has a mass of 222 u, where u is an atomic mass unit Assume that the nucleus is at rest when it decays into a polonium nucleus with mass 218 u and a helium nucleus with mass 4 u (called an alpha particle ), releasing 5.59 MeV of energy . The polonium and helium nucleus are emitted in exactly opposite directions . What are the kinetic energies of the polonium nucleus and the helium nucleus ? What we know