To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.
Some atoms have unstable nuclei that emit particles and high-energy electromagnetic radiation to form new elements that are more stable. This emission of particles and electromagnetic radiation is called radioactivity. There are three main types of spontaneous radioactive emission: alpha particles, which are equivalent to helium nuclei; beta particles, which are electrons; and gamma radiation, which is high-energy electromagnetic radiation. Another type of radioactive process is spontaneous fission, in which large nuclei spontaneously break apart into smaller nuclei and, often, neutrons. In all forms of radioactivity, new elements are formed from the radioactive reactants.
Radioactive isotopes decay at different rates. The rate of an isotope’s decay is expressed as a half-life, which is the amount of time required for half of the original material to decay. The length of its half-life is a characteristic of the particular isotope and can range from less than microseconds to billions of years.
Amounts of radioactivity are measured in several different ways. A becquerel is equal to one radioactive decay per second. A curie represents 3.7 × 1010 decays per second. Other units describe the amount of energy absorbed by body tissues. One rad is equivalent to 0.01 joule of energy absorbed per gram of tissue. Different tissues react differently to different types of radioactivity. The rem unit takes into account not only the energy absorbed by the tissues, but also includes a numerical multiplication factor to account for the type of radioactivity and the type of tissue. The average annual radiation exposure of a person is less than 360 millirem, over 80% of which is from natural sources. Radioactivity can be detected using photographic film or other devices such as Geiger counters.
Radioactive isotopes have many useful applications. They can be used as tracers to follow the journey of a substance through a system, like an underground waterway or a metabolic pathway. Radioactive isotopes can be used to date objects, since the amount of parent and daughter isotopes can sometimes be measured very accurately. Radioactive emission can be used to sterilize food for a longer edible lifetime. There are also a number of diagnostic and therapeutic medical applications for radioactive isotopes.
Radioactive processes occur with simultaneous changes in energy. This nuclear energy can be used to generate power for human use. Nuclear reactors use the energy released by fission of large isotopes to generate electricity. When carefully controlled, fission can produce a chain reaction that facilitates the continuous production of energy. If not carefully controlled, a very quick production of energy can result, as in an atomic bomb. Natural uranium does not contain enough of the proper isotope of uranium to work in a nuclear reactor, so it must first be enriched in uranium-235. Forcing small nuclei together to make larger nuclei, a process called fusion, also gives off energy; however, scientists have yet to achieve a controlled fusion process.