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(rā`dēəm) [Lat. radius=ray], radioactive metallic chemical element; symbol Ra; at. no. 88; mass number of most stable isotopeisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
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 226; m.p. 700°C;; b.p. 1,140°C;; sp. gr. about 6.0; valence +2. Radium is a lustrous white radioactive metal. It is an alkaline-earth metalalkaline-earth metals,
metals constituting Group 2 of the periodic table. Generally, they are softer than most other metals, react readily with water (especially when heated), and are powerful reducing agents, but they are exceeded in each of these properties by the
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; in its chemical properties it closely resembles barium, the element above it in Group 2 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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When radium is exposed to air, a black coating of nitride rapidly forms. It combines directly with water to form the hydroxide. It reacts with acids to form the commercially important chloride and bromide. The most important property of radium and its compounds is their radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
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; radiotherapy is used in medicine in the treatment of cancer. Mixed with a phosphor such as zinc sulfide, radium compounds are used in luminous paints. Radium is also used as a neutron source (mixed with beryllium) and as a gamma-ray source.

Sixteen isotopes of radium are known, but only radium-226 (half-life 1,599 years), the most stable of the isotopes, is used commercially. It is a product in the radioactive decay series of uranium-238; it is immediately preceded in this series by thorium-230 and followed by radonradon
, gaseous radioactive chemical element; symbol Rn; at. no. 86; mass no. of most stable isotope 222; m.p. about −71°C;; b.p. −61.8°C;; density 9.73 grams per liter at STP; valence usually 0. Radon is colorless and the most dense gas known.
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-222 (a gas formerly called radium emanation). In its radioactive decay radium emits alpha, beta, and gamma rays and also produces heat (about 1,000 calories per gram per year). The curie is a unit of radioactivity defined as that amount of any radioactive substance that has the same disintegration rate as 1 gram of radium-226, i.e., 3.7×1010 disintegrations per sec. Radium decreases in radioactivity about 1% in 25 years.

Radium is a rare metal. Its compounds are found in uranium ores; there is usually about 1 part of radium to 3 million parts of uranium in these ores. Although some radium is obtained from carnotite from Colorado, the chief sources are carnotite from Congo (Kinshasa) and pitchblende from W Canada. Radium is present in all uranium minerals and is widely distributed in small amounts. Radium is usually obtained (with barium impurities) in residues from the production of uranium. It is recovered as the bromide by an involved chemical process. The small amount of the element present in any ore and the difficulty of extraction make it expensive. Radium also is a dangerous material; prolonged exposure to even small amounts may cause cancer, anemia, or other disorders. Other radioisotopes (e.g., cobalt-60) are often used in its place when they are less expensive, more powerful, or safer to use.

Radium was discovered in 1898 by Pierre and Marie CurieCurie
, family of French scientists. Pierre Curie, 1859–1906, scientist, and his wife, Marie Sklodowska Curie, 1867–1934, chemist and physicist, b. Warsaw, are known for their work on radioactivity and on radium.
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 in pitchblende given them by Austria after the uranium salts had been removed for use in glass manufacture. They had earlier found polonium in a similar sample. Metallic radium was isolated by electrolysis in 1910 by Marie Curie and André Debierne; they first formed a mercury-radium amalgam by electrolysis and then removed the mercury by distillation.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(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).


Vdovenko, V. M., and Iu. V. Dubasov. Analiticheskaia khimiia radiia. Leningrad, 1973.
Pogodin, S. I., and E. P. Libman. Kak dobyli sovetskii radii. Moscow, 1971.


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.


Vernadskii, V. I. “O kontsentratsii radiia rastitel’nymi organizmami.” Dokl. AN SSSR: Ser. A. 1930, no. 20.
Radioekologicheskie issledovaniia v prirodnykh biogeotsenozakh. Moscow, 1972.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A radioactive member of group II, symbol Ra, atomic number 88; the most abundant naturally occurring isotope has mass number 226 and a half-life of 1620 years.
A highly toxic solid that forms water-soluble compounds; decays by emission of α, β, and γ-radiation; melts at 700°C, boils at 1140°C; turns black in air; used in medicine, in industrial radiography, and as a source of neutrons and radon.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a. a highly radioactive luminescent white element of the alkaline earth group of metals. It occurs in pitchblende, carnotite, and other uranium ores, and is used in radiotherapy and in luminous paints. Symbol: Ra; atomic no.: 88; half-life of most stable isotope, 226Ra: 1620 years; valency: 2; relative density: 5; melting pt.: 700?C; boiling pt.: 1140?C
b. (as modifier): radium needle
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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