(Rn), a radioactive chemical element in group VIII of Mendeleev’s periodic system. Atomic number, 86. One of the inert gases. Three α-radioactive radon isotopes occur in nature as members of the natural radioactive decay series: 2l9Rn (member of actinouranium series, half-life T½ = 3.92 sec), 220Rn (thorium series, T½ = 54.5 sec), and 222Rn (uranium-radium series, T½ = 3.823 days). The isotope 219Rn is also known as actinon (symbol An), and 220Rn is known as thoron (Tn); 222Rn is called true radon and is often designated simply by the symbol Rn.
More than 20 radon isotopes, with mass numbers 201–222, have been obtained artificially through nuclear reactions. In order to synthesize the neutron-deficient radon isotopes with mass numbers 206–212, the Joint Institute for Nuclear Research (Dubna, USSR) has developed a special gas-chromatog-raphy unit, which can produce these isotopes in a radiochemi-cally pure form within a half-hour period.
The discovery of radon was the result of early research on radioactivity. In 1899 the American physicist R. B. Owens found that the decay of Th yields a certain radioactive substance, which can be extracted from solutions containing Th by means of a stream of air. E. Rutherford named this substance emanation (Latin emano, “I flow out”). In 1899, Rutherford, then working in Canada, proved that the thorium emanation discovered by Owens is a radioactive gas. That same year, F. Dorn in Germany and A.-L. Debierne in France announced that emanation (radium emanation → radon) is also formed during radium decay. In 1903 actinium emanation, or actinon (natural radon isotopes are still often called emanations), was also discovered. Thus with radon, scientists encountered —practically for the first time—the existence of several types of atoms in a single element, atoms that were later to be called isotopes. Rutherford, W. Ramsay, and F. Soddy showed that radium emanation is a new chemical element, one belonging to the inert-gas category. The name niton (Latin nitens, “shining”) was proposed because of the element’s ability to luminesce in the condensed state.
Radon is one of the rarest elements. The quantity of radon in the earth’s crust to a depth of 1.6 km amounts to approximately 115 tons. Formed in radioactive ores and minerals, radon gradually makes its way to the surface of the earth and enters the hydrosphere and atmosphere. The average radon concentration in the atmosphere is about 6 × 10–17 percent (by weight); in sea-water, the concentration can reach 0.001 picocurie per liter.
Under ordinary conditions, radon is a colorless, odorless, and tasteless gas with a boiling point of -61.8°C, a melting point of — 71°C, and a density of approximately 9.9 grams per liter at 0°C. The solubility is approximately 0.5 volume in unit volume of H2O at 0°C, and with organic solvents the solubility is considerably higher. Since the outer electron shell of the Rn atom contains eight electrons (configuration 6s2 6p6), the element is highly inactive chemically. Like xenon, radon forms a fluoride (possible composition RnF2), which at 500°C is reduced by hydrogen to elementary radon. As determined by B. A. Nikitin, radon can form clathrates with such compounds as water, phenol, and toluene.
Radon (isotope 222Rn) is obtained by passing a stream of gas (nitrogen, argon) through an aqueous solution of a radium salt. After passage through the solution, the gas will contain approximately 10–5 percent radon. Radon’s good sorption on porous solids (activated carbon) facilitates extraction, and special chemical methods for extraction are also available. The quantities of pure radon obtained do not exceed 1 mm3.
Radon is highly toxic because of its radioactive properties. Upon its decay, nonvolatile radioactive products (isotopes of Po, Bi, and Pb) are formed, which an organism can eliminate only with considerable difficulty. It is therefore necessary to use hermetic boxes and follow safety procedures when working with radon.
Radon is used primarily in medicine. Water containing radon is used in the treatment of diseases of the nervous and cardiovascular systems, respiratory and digestive organs, bones, joints, and muscles and in the treatment of gynecological and metabolic disorders.
The determination of radon concentration in the layer of air at the earth’s surface provides the basis for emanation methods used in geological prospecting. These methods permit an estimation of the U and Th content in soils and in rocks lying near the earth’s surface. Radon is also used in scientific research. The content of U and Th in, for example, rock samples, can be determined from the radioactivity of the radon in equilibrium with these elements. Studies carried out through the emanation method on the structural changes in solids are based on a measurement of the rate of radon formation upon heating solid samples containing radioisotopes that precede radon in the radioactive series 232Th and 235U.
S. S. BERDONOSOV