Radioactivity is the property exhibited by certain types of matter of emitting energy and subatomic particles spontaneously. It is, in essence, an attribute of individual atomic nuclei.

Radioactivity was first reported in 1896 by the French physicist Henri Becquerel for a double salt of uranium and potassium. Soon thereafter it was found that all uranium compounds and the metal itself were similarly radioactive. Intensity of activity was proportional to the amount of uranium present, chemical combination having no effect. In 1898 the noted French physicists Pierre and Marie Curie discovered two other strongly radioactive elements, radium and polonium, that occur in nature.

The early study of the radioactivity of the heavy elements led to revolutionary changes in ideas of the structure of matter. At the beginning of the 20th century the theory that matter consists of atoms was generally accepted by scientists; notions of the inner structure of atoms, however, were entirely speculative. By 1903 research on radioactive processes and radiations led to the realization that atoms are not of necessity permanently stable. The conclusion by 1911 was that nearly all of the mass of the atom is concentrated in a nucleus occupying only a minute portion of the total volume. Next came the important concept of isotopes (1913); and transmutation, the modification of an atomic nucleus, was achieved in a laboratory experiment six years later. Finally, in 1934, it was discovered that radioactivity could be induced in ordinary matter by transmutation in an artificially contrived arrangement. In these first experiments radioactive varieties of nitrogen, aluminum, and phosphorus were identified. Within a few months it had been shown that neutrons (uncharged nuclear particles) could effect transmutation, and the list of newly discovered radioactive isotopes covered the whole range of known elements from hydrogen to uranium. At this time there were indications that radioactive isotopes of transuranium elements (i.e., those of atomic number greater than that of uranium) might be obtained through transmutation, but it was not until 1940 that the first clear identification of such an element--neptunium--was made.

Of the various processes resulting in the production of radioactive species, neutron-induced nuclear fission, achieved in 1939, has been the most fruitful. In 1941 it was learned that fission may also occur spontaneously. In this case, certain unstable nuclei of heavier elements split into nearly equal fragments without the introduction of outside energy. With such discoveries, modern theories of nuclear structure became possible, and the large-scale release of nuclear energy was achieved in 1942.

Radioactive substances emit energy in the form of ionizing radiations. Such radiations dissipate their energy in passing through matter by producing ionization and other effects. The radiated energy is either kinetic energy of particles or quantum energy of photons; these are eventually degraded into heat. If the radioactive source is a compact portion of matter, some of the energy of radiations is dissipated in the source itself. The source then tends to maintain a temperature higher than that of its surroundings. The emission is spontaneous, and its rate is uninfluenced by changes of pressure and temperature available to laboratory study. It is, however, not inexhaustible. For each source the rate of emission of energy continually decreases, as measured by its half-life. (Half-life is defined as the period in which the rate of radioactive emission by a pure sample falls by a factor of two.) Among known radioactive isotopes, half-lives range from about 10-7 second to 1016 years.

Excerpt from the Encyclopedia Britannica without permission.