Astatine

Jai Ganesh · Today 15:39:49

Astatine

Gist

Astatine (At) is the heaviest naturally occurring halogen, a rare and highly radioactive element with atomic number 85. It is unstable, with all isotopes having short half-lives, and occurs only in trace amounts in nature as a decay product of other elements. Astatine is primarily synthesized for use in research and medicine, particularly in radiotherapy and the treatment of thyroid diseases, as it tends to accumulate in the thyroid gland like iodine.

Astatine's primary use is in scientific research and targeted cancer therapy, specifically with its isotope astatine-211, which delivers alpha particles to destroy cancer cells, including for thyroid and ovarian cancers. Due to its extreme rarity and intense radioactivity, astatine is not used in everyday applications but is studied in advanced labs for understanding radioactive decay, halogen chemistry, and to develop new medical treatments for hard-to-target cancers.

Summary

Astatine is a chemical element; it has symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours. Consequently, a solid sample of the element has never been seen, because any macroscopic specimen would be immediately vaporized by the heat of its radioactivity.

The bulk properties of astatine are not known with certainty. Many of them have been estimated from its position on the periodic table as a heavier analog of fluorine, chlorine, bromine, and iodine, the four stable halogens. However, astatine also falls roughly along the dividing line between metals and nonmetals, and some metallic behavior has also been observed and predicted for it. Astatine is likely to have a dark or lustrous appearance and may be a semiconductor or possibly a metal. Chemically, several anionic species of astatine are known and most of its compounds resemble those of iodine, but it also sometimes displays metallic characteristics and shows some similarities to silver.

The first synthesis of astatine was in 1940 by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio G. Segrè at the University of California, Berkeley. They named it from the Ancient Greek ástatos 'unstable'. Four isotopes of astatine were subsequently found to be naturally occurring, although much less than one gram is present at any given time in the Earth's crust. Neither the most stable isotope, astatine-210, nor the medically useful astatine-211 occur naturally; they are usually produced by bombarding bismuth-209 with alpha particles.

Details

Astatine (At) is a radioactive chemical element and the heaviest member of the halogen elements, or Group 17 (VIIa) of the periodic table. Astatine, which has no stable isotopes, was first synthetically produced (1940) at the University of California by American physicists Dale R. Corson, Kenneth R. MacKenzie, and Emilio Segrè, who bombarded bismuth with accelerated alpha particles (helium nuclei) to yield astatine-211 and neutrons. Naturally occurring astatine isotopes have subsequently been found in minute amounts in the three natural radioactive decay series, in which they occur by minor branching (astatine-218 in the uranium series, astatine-216 in the thorium series, and astatine-215 and astatine-219 in the actinium series). Thirty-two isotopes are known; astatine-210, with a half-life of 8.1 hours, is the longest lived. Because astatine has no stable or long-lived isotopes, it was given its name from the Greek word astatos, meaning “unstable.”

Element Properties

atomic number : 85
stablest isotope : 210
oxidation states : −1, +1, +3(?), +5, +7(?)

Production and use

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.

Additional Information:

Appearance

Astatine is a dangerously radioactive element.

Uses

There are currently no uses for astatine outside of research. The half-life of the most stable isotope is only 8 hours, and only tiny amounts have ever been produced.

A mass spectrometer has been used to confirm that astatine behaves chemically like other halogens, particularly iodine.

Biological role

Astatine has no known biological role. It is toxic due to its radioactivity.

Natural abundance

Astatine can be obtained in a variety of ways, but not in weighable amounts. Astatine-211 is made in nuclear reactors by the neutron bombardment of bismuth-200.

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Astatine

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