radium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A rare, brilliant white, luminescent, highly radioactive metallic element found in very small amounts in uranium ores, having 13 isotopes with mass numbers between 213 and 230, of which radium 226 with a half-life of 1,622 years is the most common. It is used in cancer radiotherapy, as a neutron source for some research purposes, and as a constituent of luminescent paints. Atomic number 88; melting point 700°C; boiling point 1,737°C; valence 2. See Table at element.

Century Dictionary and Cyclopedia

1. n. A chemical element of very remarkable character, discovered in 1898 by Mme. Sklodowska Curie, working with her husband and M. Bémont. The element itself has not been isolated, and many writers incorrectly use the name radium when in reality a salt of the element, generally the chlorid or bromide, is meant. In following up the researches of Becquerel on the radioactivity of uranium and its compounds, Mme. Curie found that certain ores of uranium manifest this activity to a greater extent than can be accounted for by the uranium they contain. By patient and laborious examination of the constituents of pitchblende, a mineral containing a high proportion of uranium, she was able to separate a substance exhibiting radioactive properties greatly superior to those of uranium. This substance was found associated with the barium salts removed from the pitchblende and resembled barium so closely in its chemical properties that a separation of the two elements could be effected only by the repeated fractional recrystallization of the mixed chloride, the radium chlorid becoming more concentrated in the less soluble portion. On further investigation it was found that the radium, present only in minute proportion in pitchblende, was retained and somewhat concentrated in certain residues obtained from pitchblende in the commercial extraction of uranium at the works in connection with the mines at Joachimsthal in Austria. These residues had accumulated for years under the supposition that they were valueless and a considerable quantity was placed at the disposal of M. and Mme. Curie for the continuation of their researches. The residues consist chiefly of the sulphates of lead and calcium together with the oxids of silicon, aluminium, and iron. They also contain greater or less quantities of nearly all the metals (copper, bismuth, zinc, cobalt, manganese, nickel, vanadium, antimony, thallium, the rare earths, niobium, tantalum, arsenic, barium, etc.). They were subjected to suitable chemical treatment by which the barium and radium were removed and obtained in the form of a mixed chlorid. The radium and barium chlorids were then separated from one another by fractional crystallization repeated many hundreds of times, and a pure chlorid of radium was obtained. In this way the Curies separated about one gram of fairly pure radium chlorid from eight tons of residues, representing nearly eighty tons of the original ore. Giesel has slightly modified the process, conducting the recrystallization with the bromides instead of the chlorids, which considerably facilitates the isolation of a pure radium salt. Pure radium chlorid or bromide resembles common salt in general appearance when freshly prepared, but quickly acquires a yellowish or brownish color. It gives a fine carmine-red color to flame, and affords characteristic flame and spark spectra. It is visibly luminous in a dark room. Radium belongs to the class of the alkaline-earth metals, calcium, strontium, and barium, and its compounds in general resemble theirs. Its atomic weight, as first determined by Mme. Curie, was 225 (O = 16); later she obtained the figures 226.5. The element occupies, in the periodical classification, the same position in the twelfth of Mendeléjeff's series as Ca, Sr, and Ba do in the fourth, sixth, and eighth. Its radioactivity is about one million times greater than that of uranium. It produces phosphorescent luminosity in the diamond, kunzite, and various other minerals, and on screens coated with barium platinocyanide, crystalline zinc sulphid, etc. Radium salts give off α-, β-, and γ-rays (see radioactivity). The nature of the α-particles has been closely investigated by Rutherford, who has shown that they are of atomic dimensions, have a mass equal to that of helium atoms, and are therefore in all probability the source of the helium continuously evolved b, radium salts as demonstrated in the experiments of Ramsay and Soddy. In addition to helium, a radium salt produces also a highly radioactive gaseous substance known as the radium emanation, which accumulates in the solid salt and escapes when this is heated or dissolved in water. This emanation is a gas of the argon family, autoluminescent and giving a characteristic bright-line spectrum. It has an atomic weight (as determined from the diffusion rate and assuming the molecule to be monatomic) differing but little from that of radium and is condensed at a temperature of from—150 to—155° C. It gradually loses its radioactivity and is transformed, at a rate corresponding to a change of one half the amount present in 3.8 days, into a series of successive, rapidly-changing, solid, radioactive products known as radium A, radium B, and radium C, which are deposited on the surface of any object in contact with the emanation and impart to this a temporary radioactivity. The continuous production of the emanation in radium salts is wholly independent of the chemical character of the salt and is directly proportional to the amount of radium contained in it. It has also been observed that radium salts evolve heat at the surprising rate of about 118 calories per hour for each gram of radium which they contain and are thus enabled under favorable conditions to maintain their temperature considerably above that of their surroundings. These and other considerations have led to the explanation of the behavior of radium and other radioactive elements by the so-called disintegration theory, first proposed by Rutherford and Soddy. According to this theory the radium atoms are unstable systems and a certain proportion of the total number present are constantly undergoing disintegration, being transformed into atoms of other elements having distinctive physical and chemical properties. Thus an atom of radium breaks up, expelling an α-particle constituting an atom of helium, and produces an atom of the gaseous emanation, the latter undergoing subsequent transformation into an atom of radium A, etc. The energy appearing in the course of these changes is assumed to be inherent in the original atom and to manifest itself only when this is altered or destroyed. On this assumption the behavior of radioactive substances is in no way contradictory to the doctrine of the conservation of energy. On the basis of this theory it is obvious that in any given quantity of a radium salt in the course of time the amount of radium present must gradually decrease, and it has been experimentally demonstrated that the time required for exactly one half of the radium to disappear would be about 2000 years. It can be shown that at this rate of disintegration the earth, if composed initially of pure radium, would, after the lapse of 100,000 years, contain a smaller proportion of radium than is now present in the common rocks and soil which constitute its surface. Since the age of the earth is certainly greater than 100,000 years, it is evident that the radium now in existence must have been renewed or formed in some manner. The origin of radium has therefore been the subject of careful investigation and the conclusion has been reached that radium is formed through the atomic disintegration of another radioactive element, uranium, which is much more abundant, and is widely distributed. Radium has been found present in the natural mineral substances containing uranium in a constant and unvarying proportion, three parts by weight of radium occurring associated with every one hundred million parts of uranium. It has been found that radium is not formed directly from uranium, and it has been shown by Boltwood that another radio-element, ionium, is first produced by the disintegration of the uranium atoms which after further change are converted into radium atoms. A continuation of the atomic transformation results in the successive production of a further series of radio-elements or products: radium emanation, radium A, radium B, radium C, radium D (radiolead), radium E, radium F, and radium G (polonium). As no further radioactive products have been detected in this series it is presumed that after radium G an unchanging, stable form of matter is attained. Although the evidence as to the nature of this final product is indirect and not wholly conclusive, there is reason for believing that it is ordinary lead. Radium has been found widely distributed in minute proportions in the rocks and minerals of the globe, in soils, in the waters of thermal springs, in sea-water, and in marine deposits. The chief source of the radium salts which nave been prepared has been almost entirely the pitchblende residues from the Austrian factories, and owing to the comparative rarity of this material and the laborious and costly methods of treatment necessary for its extraction, radium salts have commanded extraordinarily high prices, being frequently sold in small quantities at a rate exceeding $150,000 per gram of radium. The radiation from radium salts, especially the β- and γ-rays, reduces sensitive silver salts and produces an image upon a photographic plate screened by black paper. It also brings about other chemical changes, converting oxygen into ozone and ordinary phosphorus into red phosphorus, and causing the decomposition of water (with an excess of hydrogen over oxygen in the gaseous product as collected). It produces darkening of color in diamonds, quartz, mica, and especially glass, some specimens of glass becoming brown and others, more commonly, acquiring a violet or purple tinge. Various other substances become more or less colored. Radium salts, even at some little distance from the human skin, produce reddening in a few hours, and after days give rise to painful sores, difficult to heal. They have been used with some degree of success in the treatment of lupus and other forms of disease of the superficial tissues. They exert a marked germicidal action on various micro-organisms.

Wiktionary

1. n. a radioactive metallic chemical element (symbol Ra) with an atomic number of 88.

GNU Webster's 1913

1. n. (Chem.) An intensely radioactive metallic element found (combined) in minute quantities in pitchblende, and various other uranium minerals. Symbol, Ra; atomic weight, 226.4. Radium was discovered by M. and Mme. Curie, of Paris, who in 1902 separated compounds of it by a tedious process from pitchblende. Its compounds color flames carmine and give a characteristic spectrum. It is divalent, resembling barium chemically. The main isotope of radium found in pitchblende, radium-226, has a half-life of 1620 years, decaying first by alpha emission to radon.

WordNet 3.0

1. n. an intensely radioactive metallic element that occurs in minute amounts in uranium ores