Astatine

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In traces

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In traces

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Astatine [ asta ː t] (from Greek άστατος: " inconstant, unsteady " ) is a radioactive chemical element with the chemical symbol At and atomic number 85 in the periodic table it is in the seventh main group and is one of the halogens. Astatine is produced in the natural decay of uranium. Apart from the super-heavy elements astatine is the rarest element on earth, so it is artificially generated, if necessary.

History

When Dmitri Mendeleev in 1869 his periodic table stipulated, he predicted the existence of some at this time of undiscovered elements, including one that would occupy the space under iodine. As a result, some scientists tried to find this item that was referred to as " eka- iodine ".

In 1931, Fred Allison, he and his staff claimed at Alabama Polytechnic Institute (now Auburn University) had discovered the missing element and gave him the name Alabamine (AB). However, their discovery was not confirmed and was later recognized as wrong.

Also looking for a Member of the radioactive thorium family found the chemist De Rajendralal Mitra in 1937 in Dhaka, Bangladesh, (then British India ), two new elements. The first he called Dakin ( eka- iodine), well after the English name for Dhaka ( Dacca ), the other Gourium. However, both findings could not be confirmed.

The name Helvetium was again proposed by the Swiss chemist Walter Minder, when he announced the discovery of element 85 in 1940. He changed in 1942, but his proposal in Anglohelvetium.

Could be confirmed the discovery of the Astatine ( ancient Greek ἀστατέω = " fickle be," by radioactive decay ) for the first time in 1940 by the scientist Dale Corson, Kenneth MacKenzie and Emilio Gino Segrè, which is available in the University of California artificially by bombarding bismuth manufactured with alpha particles.

Three years later, the short-lived element of Berta Karlik and Bernert Traude as a product of the natural decay process of uranium are found.

Production and representation

Astatine is produced by bombarding bismuth with alpha particles in the energy range 26-29 MeV. Is obtained, the relatively long-lived isotope to 209At 211At, then sublimed in nitrogen stream at 450 to 600 ° C and separated on a cooled platinum disc.

Properties

This radioactive element was detected by means of mass spectrometry, that it behaves chemically like the other halogens, particularly as iodine ( as it accumulates in the thyroid at this ). Astatine is more metallic than iodine. Researchers at the Brookhaven National Laboratory have conducted experiments for identification and measurement of elementary chemical reactions carried out involving astatine.

With the on-line isotope mass separator ( ISOLDE ) at CERN, the ionization potential of astatine with 9.31751 ( 8) electron volts was determined in 2013.

Isotopes

Astatine has 20 known isotopes, all of which are radioactive; the longest-lived is 210At with a half-life of 8.3 hours.

Use

Organic Astatverbindungen used in nuclear medicine for irradiation of malignant tumors. Astatine isotopes are taken internally as radioactive preparations for marking the thyroid due to the short half-lives. The element is enriched in the thyroid, and stored in the liver.

Compounds

The chemical properties of astatine could previously only be determined with tracer experiments because of the small quantities. They are very similar to those of iodine, but it will be a weaker oxidant. So far, various Astatide, inter-halogen compounds and organic compounds could be detected. The anions of the corresponding oxygen acids are known. Due to the electropositive nature of halogens compared to the other it is only partially precipitated silver. For the complex- stabilized cation At ( Py) 2 exists ( Py = pyridine), which may be deposited cathodically astatine. Was also detected, the hydride, Astatwasserstoff HAt. However, it does not matter in industrial chemistry.

Safety

Classifications according to the Hazardous Substances Ordinance are not available because they only include the chemical danger and play a very minor role compared to the risks based on the radioactivity. Even the latter applies only if there is a relevant material for this amount.