astatine(redirected from Astat)
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(At), a radioactive chemical element in Group VII of Mendeleev’s periodic system. Its atomic number is 85. Astatine has no stable isotope; at least 20 radioactive isotopes are known. The longest-lived of them, 210At, has a half-life T ½ of 8.3 hours. Numerous attempts by scientists of various countries to discover the 85th element in natural elements by all possible chemical and physical methods were unsuccessful. In 1940, E. Segré, D. Corson, and K. Mackenzie, working on a cyclotron at Berkeley, obtained the first isotope, 211At, by bombarding bismuth with alpha particles. The name astatine comes from the Greek astatos, meaning “unstable.” Only after this artificial derivation was it shown that four of the isotopes of astatine (215At, 216At, 2,8At, and 219At) are formed in very low-probability (5 x 10 “’-0.02 percent) branches of three natural series of the radioactive decay of uranium and thorium. Astatine is adsorbed by metals (silver, gold, platinum) and evaporates easily under normal conditions and in a vacuum. Because of these properties, astatine can be separated (up to 85 percent) from the products of irradiated bismuth by means of vacuum distillation with adsorption of astatine by silver or platinum. The chemical properties of astatine are very interesting and unique. Astatine is as close to iodine as it is to polonium— that is, it displays properties of both a nonmetal (halogen) and a metal. This combination of properties is due to astatine’s position in the periodic system: it is the heaviest (and consequently the most “metallic”) element in the halogen group. Like the halogens, astatine gives insoluble salt, AgAt. Like iodine it oxidizes to a pentavalent state (the salt AgAt03 is analogous to AgIO3). However, like typical metals, astatine precipitates with hydrogen sulfide even from a highly acidic solution, it is displaced by zinc from sulfuric acid solutions, and it precipitates at the cathode during electrolysis. The presence of astatine is determined by characteristic alpha emission.
REFERENCEGol’danskii, V. I. Novye elementy v Periodicheskoi sisteme D. I. Menedeleeva, 3rd ed. Moscow, 1964. Pages 131–41.
V. I. GOL’DANSKII