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L'élément chimique - The Elements song (Mai 2024)
Astatine (At), élément chimique radioactif et membre le plus lourd des éléments halogènes, ou Groupe 17 (VIIa) du tableau périodique. L'astatine, qui n'a pas d'isotopes stables, a d'abord été produite par synthèse (1940) à l'Université de Californie par les physiciens américains Dale R. Corson, Kenneth R. MacKenzie et Emilio Segrè, qui ont bombardé le bismuth avec des particules alpha accélérées (noyaux d'hélium) pour donner astatine-211 et neutrons. Des isotopes d'astatine d'origine naturelle ont ensuite été découverts en quantités infimes dans les trois séries de désintégration radioactive naturelle, dans lesquelles ils se produisent par ramification mineure (astatine-218 dans la série uranium, astatine-216 dans la série thorium, et astatine-215 et astatine- 219 dans la série actinium). Trente-deux isotopes sont connus; astatine-210, avec une demi-vie de 8,1 heures, est la plus longue durée de vie.Parce que l'astatine n'a pas d'isotopes stables ou à longue durée de vie, elle a reçu son nom du mot grec astatos, qui signifie «instable».
halogène
(Br), iode (I), astatine (At) et tennessine (Ts). Ils ont reçu le nom halogène, des racines grecques hal- («sel»)
Propriétés des éléments
| numéro atomique | 85 |
|---|---|
| isotope le plus stable | 210 |
| états d'oxydation | -1, +1, +3 (?), +5, +7 (?) |
| configuration électronique. | (Xe) 4f 14 5d 10 6s 2 6p 5 |
Production et utilisation
Le seul moyen pratique d'obtenir de l'astatine est de le synthétiser par des réactions nucléaires. L'astatine est généralement préparée selon l'équation générale:
which indicates that bismuth-209 takes up one alpha particle and emits x neutrons to form an isotope of astatine, whose atomic weight depends on the number of neutrons lost. Metallic bismuth may be used as a target material. From this, astatine may readily be removed by distillation in air from a stainless-steel tube. The free element begins to distill at 271 °C (520 °F, or the melting point of bismuth), but the operation is best carried out at 800 °C (1,500 °F) with subsequent redistillation. If an aqueous solution of astatine is desired, the element may be separated by washing with an appropriate aqueous solution. Alternatively, the halogen may be removed from the target by chemical methods, such as dissolving in nitric acid, with the latter being removed by boiling.
Another procedure involves the use of a metallic thorium target, which—after bombardment—is dissolved in concentrated hydrochloric acid containing hydrogen fluoride and chlorine.
Analysis
Because of the short half-lives of astatine isotopes, only very small quantities have been available for study. With the exception of a few spectrometric and mass-spectrometric studies, most investigations of astatine chemistry have used tracer techniques, which involve using chemical reactions in a solution with similarly reacting elements as carriers. The amount of astatine is then calculated from the measured radioactivity of the reaction products. However, the rarity of astatine means that these solutions are extremely dilute, with concentrations around or below 10−10 molarity (the number of moles per litre of solution). At such concentrations, the effects of impurities can be very serious, especially for a halogen such as astatine, which exists in several oxidation states and can form many organic compounds. Iodine has been used as a carrier in most experiments. Techniques applied include coprecipitation, solvent extraction, ion exchange, and other forms of chromatography (separation by adsorption differences), electrodeposition (deposition by an electric current), electromigration (movement in an electric field), and diffusion. A direct identification of some astatine compounds has been made by mass spectrometry.
Except for nuclear properties, the only physical property of astatine to be measured directly is the spectrum of atomic astatine. Other physical properties have been predicted from theory and by extrapolation from the properties of other elements.
Chemical properties
Some of the chemical properties of the element have been established. It generally resembles iodine. Thus, like iodine, it concentrates in the thyroid gland of higher animals. A substantial portion, however, is distributed throughout the body and acts as an internal radiation source.
The astatide ion, At−, is quantitatively coprecipitated with insoluble iodides, such as silver iodide or thallium iodide. The diffusion coefficient of the iodide ion is 1.42 times that of the astatide ion, which moves more slowly toward the anode than the former under given conditions. The ion is formed by reduction of the element, using zinc or sulfur dioxide. It is oxidized to the zero valence state by the ferric ion, Fe3+, iodine (I2), and dilute nitric acid. Thus, the astatide ion is a stronger reducing agent than the iodide ion, and free iodine is a stronger oxidizing agent than astatine.
Free astatine is characterized by volatility from solution and by extractability into organic solvents. It undergoes disproportionation in alkaline media. Astatine is coprecipitated with cesium iodide and thus appears to form polyhalide anions. Astatine extracted into chloroform has been shown to coprecipitate homogeneously with iodine when a portion of the latter is crystallized. Astatine seems to be present as the iodide, which appears to be more polar (i.e., showing separation of electric charge) in character than iodine bromide. It is somewhat soluble in water and much more soluble in benzene and carbon tetrachloride.
Astatine is known to occur in positive oxidation numbers. The astatate ion, (AtO3)−, is coprecipitated with insoluble iodates, such as silver iodate (AgIO3), and is obtained by the oxidation of lower oxidation states with hypochlorite, periodate, or persulfate. So far no evidence for perastatate has been found, but this may be because the ion, (AtO6)5−, may show little tendency to coprecipitate with potassium iodate (KIO4).
Astatine in the +1 state is stabilized by complexation, and complexes formulated as dipyridine astatine perchlorate [At(py)2] [ClO4] and dipyridine astatine nitrate [At(py)2] [NO3] have been prepared. Compounds with the formulas (C6H5)AtCl2, (C6H5)2AtCl, and (C6H5)AtO2 have also been obtained. A variety of methods may be used to synthesize astatobenzene, C6H5At.
