a chemical element whose isotopes are all radioactive. The class of radioactive elements ineludes

technetium (atomic number 43), promethium (61), polonium (84), and all the elements that follow it in the Mendeleev periodic system (see Figure 1).

By 1975, 25 radioactive elements were known. Those elements that follow uranium in the periodic table are called transuranium elements. The 14 radioactive elements with atomic numbers 90–103 are very similar to one another; they make up the actinide series. Of the natural radioactive elements, only two—thorium (90) and uranium (92)—have isotopes with half-lives T_½ comparable to the age of the earth: ²³²Th (T_½ = 1.41 × 10¹⁰ yr), ²³⁵U (T_½ = 7.13 × 10⁸ yr), and ²³⁸U (T_½ = 4.51 × 10⁹ yr). Therefore, thorium and uranium have existed on our planet from the time of its formation and are the primary radioactive elements. The isotopes ²³²Th, ²³⁵U, and ²³⁸U form the basis for the natural radioactive series, whose intermediate members are secondary natural radioactive elements with atomic numbers 84–89 and 91. All the isotopes of these elements have relatively short half-lives, and if their reserves were not continuously replenished by the decay of long-lived uranium and thorium isotopes, they would have already decayed completely.

The radioactive elements with atomic numbers 43,61, and 93 and higher are called artificial, since they are produced by means of artificially conducted nuclear reactions. This division of radioactive elements into natural and artificial types is rather arbitrary. For example, astatine (85) was originally obtained artificially but was later discovered among the members of the natural radioactive series. Insignificant quantities of technetium, promethium, neptunium (93), and plutonium (94) are also known to occur in nature; they are formed during the fission of uranium nuclei, either spontaneously or by excitation (for example, under the action of neutrons in cosmic rays).

Two radioactive elements, thorium and uranium, form many different minerals. The processing of natural raw materials makes possible the production of these elements in large quantities. Radioactive elements that belong to the natural radioactive series may be isolated by radiochemical means from the waste products of thorium and uranium production, as well as from thorium-bearing and uranium-bearing preparations that have been stored over a long period of time. Neptunium, plutonium, and other light transuranium elements are produced in atomic reactors upon nuclear interaction of ²³⁸U and neutrons. Various nuclear reactions also facilitate the preparation of heavy transuranium elements. Technetium and promethium are formed in atomic reactors and may be extracted from fission products.

Many radioactive elements are of great practical value. Uranium and plutonium are used as fissile material in nuclear reactors and nuclear weapons. The neutron bombardment of thorium (its natural isotope ²³²Th) yields the isotope ²³³U, a fissile material. Promethium, polonium, plutonium, and other radioactive elements are used in the manufacture of atomic electric batteries with a continuous service life of up to several years.