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radiochemistry,chemistry of radioactive substances (see 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.
..... Click the link for more information. ). Radioactive isotopesradioactive isotope
natural or artificially created isotope of a chemical element having an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached.
..... Click the link for more information. are very useful as tracerstracer,
an identifiable substance used to follow the course of a physical, chemical, or biological process. In chemistry the ideal tracer has the same chemical properties as the molecule it replaces and undergoes the same reactions but can at all times be detectible and
..... Click the link for more information. to study the mechanisms of complex organic reactions, since even minute amounts of these isotopes are easily detected by means of a Geiger counter or photographic film. For example, by feeding plants carbon dioxide that contains the radioisotope carbon-14 and by monitoring the carbon compounds through the plants' life cycle, the intermediate stages of the photosynthetic process can be determined. A method developed by W. F. LibbyLibby, Willard Frank,
1908–80, American chemist, b. Grand Valley, Colo., grad. Univ. of California (B.S., 1931; Ph.D., 1933). He taught (1933–45) at the Univ. of California and was a chemist (1941–45) in the war research division at Columbia.
..... Click the link for more information. uses carbon-14 to date archaeological discoveries and other samples containing organic matter (see datingdating,
the determination of the age of an object, of a natural phenomenon, or of a series of events. There are two basic types of dating methods, relative and absolute.
..... Click the link for more information. ). Radioactive substances have also been used to investigate the properties of artificially produced elements and to measure the rates of electron transfers.
the branch of chemistry concerned with the chemistry of radioisotopes, elements, and substances, the laws governing the physicochemical behavior of this radioactive matter, the chemistry of nuclear transformations, and the physicochemical processes that accompany these transformations. Radiochemistry, because of the topics, methods, and objects of its investigations, can be subdivided into general radio-chemistry, the chemistry of nuclear transformations, the chemistry of radioactive elements, and applied radiochemistry.
General radiochemistry studies the physicochemical regularities in the behavior of radioisotopes and elements. Radioisotopes, which differ little in their chemical properties from nonradioactive isotopes, are present, though in extremely low concentrations, in ores and other natural substances, in products obtained synthetically, and in the solutions formed after processing raw materials. The decay these isotopes undergo is accompanied by nuclear radiation. Most natural radioisotopes are daughter isotopes, that is, products of the decay of 238U,
|Table 1. Concentration of daughter isotopes (in grams)|
|238U.......||7.6 × 10–11||2.14 × 10–13||3.4 × 10–7|
|235U ......||1.3 × 10–15||1 × 10–10||5.6 × 10–5|
|232Th............||1.5 × 10–9||5 × 10–14|
235U, and 232Th. The concentrations of some of these isotopes in the equiponderant ores U and Th per 1 gram of pure parent isotope are presented in Table 1 on page 421.
Radioisotopes are also obtained artificially by irradiating various substances with nuclear particles. The yield is of the order of 10–8–10–12 percent by weight. In many cases, hundreds, tens, or even just a few atoms of radioisotopes are present in many other atoms. (Only in the production of nuclear fuel is Pu obtained in significant quantities, though even here its concentration upon irradiation of U with neutrons is low.) It is therefore possible to separate radioactive elements and isotopes only from extremely dilute systems, and their weights in most cases cannot be determined. The physicochemical behavior of extremely dilute solutions is very complex. This behavior may be described by the laws of ideal solutions, though sometimes these laws are not obeyed because of secondary processes related to, for example, adsorption or radiolysis. General radio-chemistry includes the study of isotopic exchange, processes involving the distribution of trace amounts of radioisotopes between phases, processes of coprecipitation, adsorption, and extraction, the electrochemistry of radioactive elements, and the state of radioisotopes in extremely dilute systems—the dis-persity of the elements (formation of radiocolloids) and the formation of complexes.
The chemistry of nuclear transformations includes the study of the reactions of atoms formed in nuclear transformations (hot atoms), the products of nuclear reactions, and the methods for obtaining, concentrating, and separating radioisotopes and their nuclear isomers. Also studied are the properties of radioactive substances and the transformations of these substances under the effect of their own radiation.
The chemistry of radioactive elements is the chemistry of the natural radioactive elements from Po to U (atomic numbers 84–92) and of the artificial elements Tc (atomic number 43), Pm (atomic number 61), Np (atomic number 94), and all subsequent elements up to atomic number 106. By convention, the chemistry and technology of nuclear fuel is included in this subdivision. Nuclear fuel entails the production and chemical separation of 239Pu from irradiated uranium, of 233U from thorium irradiated with neutrons, and of 235U from a natural mixture of isotopes.
Applied radiochemistry is concerned with the development of methods for the synthesis of labeled compounds, the use of radioisotopes in chemistry and the chemical industry, and the use of nuclear radiation in chemical analysis, for example, in nuclear gamma-ray fluorescence spectroscopy.
The objects of radiochemical investigations are radioactive substances containing radioisotopes, many of which are characterized by a short lifetime and nuclear (radioactive) radiation, which necessitates the use of special methods of investigation.
Radioactive emissions permit the use of special radiometric methods in radiochemistry for measuring the quantity of a radioactive substance. These emissions, however, necessitate the use of special safety techniques because emissions in doses exceeding certain levels are harmful to human health. Since the methods for measuring radioactivity are of superior sensitivity, work can be done with a minimal amount of material, an amount that would not lend itself to study under any other methods. Using instruments common in radiochemical laboratories, it is possible to detect, for example, 10–10—10–15 gram of 226Ra, 10–17 gram of 32P, or 10–17 gram of 222Rn. By using especially sensitive methods for monitoring radioactive decay, it is possible to detect the presence of single atoms of a radioisotope and to establish the fact of their decay.
The development of radiochemistry into an independent branch of chemistry began at the end of the 19th century with the work of M. Skłodowska-Curie and P. Curie, who in 1898 discovered and isolated Ra and Po. Skłodowska-Curie was the first to use the methods of coprecipitation of trace amounts of radioactive elements from solution with large amounts of analogous elements. In 1911, F. Soddy defined radiochemistry as the science concerned with the study of the properties of the products of radioactive transformations and with the separation and identification of these products. Four periods can be discerned in the development of radiochemistry, each of which is related to progress in the study of radioactivity and nuclear physics.
The first period (1898–1913) was characterized by the discovery of five natural radioactive elements—Po, Ra, Rn, Ac, and Pa—and a series of their isotopes (this became clear after the discovery by Soddy in 1913 of the existence of isotopes). As a result of the formulation by K. Fajans and Soddy of the displacement law, according to which a new element formed from a radioactive element is located in D. I. Mendeleev’s periodic system either two places to the left of the parent element (α-decay) or one place to the right (β-decay), E. Rutherford and Soddy found a genetic relationship between all the discovered isotopes and determined the positions of these isotopes in the periodic system. In this period, an intense search was conducted for radioactive substances in nature, that is, radioactive minerals and waters. In Russia, A. P. Sokolov and other scientists studied the radioactivity of mineral waters and the atmosphere, P. P. Orlov initiated a study of the radioactivity of minerals, and V. I. Vernadskii did important work on the geochemistry of radioactive elements.
The second period (1914–33) was linked with the determination of a number of regularities in the behavior of radioisotopes in extremely dilute systems—solutions and gaseous systems—and with the discovery by G. de Hevesy and F. Paneth of isotopic exchange. Paneth and Fajans formulated laws of adsorption, and O. Hahn and V. G. Khlopin carried out systematic studies of the processes of coprecipitation and adsorption. As a result, Hahn formulated laws that qualitatively characterize these processes, Khlopin established a quantitative law of coprecipitation (Khlopin’s law), and Khlopin’s student A. P. Ratner developed a thermodynamic theory to account for the processes of distribution of a substance between a solid crystalline phase and a solution. Also in this period, another Soviet scientist, L. S. Kolovrat-Chervinskii, and then Hahn furthered the study of the emanations of solids containing radium isotopes, and B. A. Nikitin subsequently carried out extensive studies of the clathrate compounds of inert gases, as in the compounds of radon. In 1917, V. I. Spitsyn conducted a series of investigations on determining the solubility of a number of compounds of thorium through the use of radioactive tracers. The basis of such a method had been developed earlier by de Hevesy and Paneth. During this period, Sktodowska-Curie and Paneth studied radioisotopes in extremely dilute solutions and the conditions necessary for the formation of radiocolloids.
The third period (1934–45) began after the discovery of artificial radioactivity by Jean Frédéric Joliot-Curie and his wife, Irène. In this period, the foundation was laid for methods of obtaining, concentrating, and separating artificial radioisotopes as a result of the work of E. Fermi on the action of neutrons on chemical elements, the discovery and study of the nuclear isomerism of artificial radioisotopes by I. V. Kurchatov and his coworkers, the discovery of the fission of uranium nuclei under the effect of neutrons by Hahn and the German scientist F. Strassmann, and the discovery of the Szilard-Chalmers reaction. The use of the cyclotron permitted E. Segrè and his coworkers to synthesize the new artificial elements Tc and At. Using radiometric methods, in conjunction with precise radiochemical methods for the separation of trace amounts of radioactive elements, M. Perey (France) separated Fr (atomic number 87) from the decay products of Ac. From the mid-1930’s, applied radiochemistry developed rapidly, and the method of radioactive (isotopic) tracers has received wide application.
The current, fourth period of the development of radiochemistry is linked with the use of powerful nuclear particle accelerators and nuclear reactors. This period has seen the synthesis and separation of artificial chemical elements—promethium by the American scientists J. Marinsky and L. Glendenin and the transuranium elements, with atomic numbers 93 to 105, by G. Seaborg, G. N. Flerov, and their respective co-workers. Methods for the production of nuclear fuel, the separation of Pu and fission products from the U irradiated in nuclear reactors, and the reprocessing of spent U from reactors have all been undergoing improvement. A number of important technological problems related to nuclear fuel have been solved. The chemistry of artificial, especially transuranium, and natural, especially U, Th, and Pa, radioactive elements has undergone extensive development, particularly in the chemistry of the complexes formed by these elements. The chemistry of new atomlike species—positronium, muonium, and mesic atoms—has also been established.
Extraction and chromatography have acquired special importance in radiochemistry. The use of radioactive tracer techniques is expanding in the study of the mechanisms and kinetics of chemical reactions, the structure of chemical compounds, and the phenomena of adsorption, coprecipitation, and catalysis. Tracer techniques are also used in the measurement of physicochemical constants and in the development of methods of radiometric analysis. Radiochemical methods are widely used in the solution of many problems in geochemistry and astro-chemistry and have been used in the search for mineral resources.
A new area in radiochemistry is the chemistry of processes that occur after a nuclear reaction, when the newly formed radioisotopes possess high energy. Studies are also under way on the products of nuclear transformations under the action of high-energy particles. Soviet scientists and scientists of a number of other countries are actively working in all these areas of radiochemistry. Radiochemistry is continuing to develop and to encompass new areas in the chemistry of radioactive substances.
REFERENCESRadioaktivnye izotopy v khimicheskikh issledovaniiakh. Edited by A. N. Murin. (With others.) Leningrad-Moscow, 1965.
Starik, I. E. Osnovy radiokhimii, 2nd ed. Leningrad, 1969.
Vdovenko, V. M. Sovremennaia radiokhimiia. Moscow, 1969.
Murin, A. N. Fizicheskie osnovy radiokhimii. Moscow, 1971.
Nesmeianov, An. N. Radiokhimiia. Moscow, 1972.
AN. N. NESMEIANOV