(Ra), a radioactive chemical element in group II of Mendeleev’s periodic table. Atomic number, 88. Radium isotopes with mass numbers 213, 215, and 219–230 are known to exist; the most long-lived is alpha-radioactive 226Ra, with a half-life of approximately 1,600 years. The isotopes encountered in nature as members of radioactive decay series include 222Ra (special name actinium X; symbol AcX), 224Ra (thorium X; symbol ThX), 226Ra, and 228Ra (mesothorium1; symbol MsTh1).
The discovery of radium was announced in 1898 by P. Curie and his wife, M. Sktodowska-Curie, and by G. Bémont. This work followed A. Becquerel’s discovery of the phenomenon of radioactivity in uranium salts in 1896. Working in Paris in 1897, Skłodowska-Curie established that the intensity of radiation emitted by pitchblende, a variety of the mineral uraninite, is substantially greater than could be explained by the uranium content. She hypothesized that this difference was due to the presence of still unknown highly radioactive substances in the mineral. A thorough chemical analysis of pitchblende made possible the discovery of two new elements—polonium and, shortly thereafter, radium. Since it was possible during the separation of radium to trace the element’s behavior through its own radiation, radium received its name from the Latin radius, meaning “ray.” In order to isolate a pure radium compound, the Curies processed under laboratory conditions approximately 1 ton of the industrial waste that remained after the separation of uranium from pitchblende. Specifically, no fewer than 10,000 recrystallizations from aqueous solutions of BaCl2 and RaCl2 mixtures, with barium compounds serving as isomorphic carriers during the separation of radium, were carried out. In the final analysis, the Curies succeeded in obtaining 90 mg of pure RaCl2.
In the USSR, work on radium extraction from domestic sources was begun soon after the October Revolution of 1917 on direct instructions from V. I. Lenin. The first radium preparations in the USSR were obtained by V. G. Khlopin and I. Ia. Bashilov in 1921. Radium salt samples were displayed in May 1922 at the Third Mendeleev Congress.
Radium is an extremely rare element. In uranium ores, which are the main source of radium, no more than 0.34 g of Ra is obtained per ton of U. Radium is among those elements that are widely dispersed, and very small concentrations of Ra are present in the most diverse substances.
All radium compounds emit a pale blue glow upon exposure to air. Because of the self-absorption of alpha and beta particles emitted during the radioactive decay of 226Ra and its daughter products, each gram of 226Ra yields approximately 550 joules (130 calories) of heat per hour; the temperature of radium preparations is therefore always slightly higher than that of the environment.
Radium is a lustrous, silvery white metal, which tarnishes rapidly upon exposure to air. The lattice is body-centered cubic, and the calculated density is 5.5 g/cm3. According to various sources, the melting point is 700°–960°C, and the boiling point is approximately 1140°C. The outer electron shell of a Ra atom contains two electrons (configuration 7s2); radium therefore has only one oxidation state, + 2 (valence 2). Radium is closest to barium in chemical properties but is more active. At room temperature, Ra combines with oxygen to yield the oxide RaO and with nitrogen to yield the nitride Ra3N2. Radium reacts vigorously with water, evolving H2 and forming the strong base Ra(OH)2. Radium chloride, bromide, iodide, nitrate, and sulfide dissolve freely in water, whereas radium carbonate, sulfate, chromate, and oxalate are sparingly soluble.
Studies on the properties of radium have played a major role in the growth of scientific knowledge because of their bearing on many questions related to the phenomenon of radioactivity. For a long time, radium was the only element whose radioactive properties found practical application, for example, in medicine and in the preparation of phosphors. Today, however, it is more economical in most cases to use less expensive artificial radioisotopes of other elements. But in medicine radium has retained a certain value as a source of radon in radon-bath therapy. Small quantities of radium are used in the preparation of neutron sources (combined with beryllium) and in the manufacture of luminescent substances (combined with zinc sulfide).
S. S. BERDONOSOV
Radium in organisms. Of the natural radioisotopes, long-lived 226Ra has the greatest biological significance. Radium is unevenly distributed in various regions of the biosphere, and geo-chemical provinces with increased radium content are known to exist. Radium accumulations in plant organs and tissues are governed by the general patterns of absorption of mineral substances and are dependent on the type of plant as well as on the conditions of the plant’s growth. In herbaceous plants, larger quantities of radium are generally found in the roots and leaves than in the stalks and reproductive organs. The highest concentrations of radium are found in bark and xylem. The average radium content in flowering plants is 0.3–9.0 × 10–11 curie/kg, and in marine algae 0.2–3.2 × 10–11 curie/kg.
In animals and man, radium enters the body through food, where it is always present (20–26 × 10–15 g/g in wheat, 67–125 × 10–15 g/g in potatoes, and 8 × 10–15 g/g in meat), as well as through drinking water. The amount of 226Ra ingested daily through food and water by man is 2.3 × 10–12 curie, while losses through urine and feces amount to 0.8 × 10–13 and 2.2 × 10–12 curie, respectively. Approximately 80 percent of the radium that the body accumulates (Ra has properties similar to Ca) enters the bone tissues. The radium content in the human body depends on a person’s place of residence and on the nature of his diet. Large concentrations of radium have a harmful effect on animals and humans, inducing pathological changes in the form of osteoporosis, spontaneous fractures, and tumors. A radium content in soil greater than 1 × 10–7–10–8 curie/kg substantially inhibits plant growth and development.
V. A. KAL’CHENKO and V. A. SHEVCHENKO