8.1: Nuclear Radiation Nuclear reactions are very different from chemical reactions. In chemical reactions, atoms become more stable by participating in a transfer of electrons or by sharing electrons with other atoms. In nuclear reactions, it is the nucleus of the atom that gains stability by undergoing a change of some kind. Some elements have no stable isotopes, which means that any atom of that element is radioactive. For some other elements, only certain isotopes are radioactive. 8.2: Half-Life Natural radioactive processes are characterized by a half-life, the time it takes for half of the material to decay radioactively. The amount of material left over after a certain number of half-lives can be easily calculated. 8.3: Effects of Radiation 8.4: Units of Radioactivity Radioactivity can be expressed in a variety of units, including rems, rads, and curies. 8.5: Uses of Radioactive Isotopes Radioactivity has several practical applications, including tracers, medical applications, dating once-living objects, and preservation of food. 8.6: Nuclear Fission 8.7: Nuclear Fusion 8.8: Nuclear Energy Nuclear energy comes from tiny mass changes in nuclei as radioactive processes occur. In fission, large nuclei break apart and release energy; in fusion, small nuclei merge together and release energy.