Halogen  (German, Paperback, Quelle Wikipedia)

Aus Wikipedia. Nicht dargestellt. Auszug: Astatine (pronounced or ) is a radioactive chemical element with the symbol At and atomic number 85. It is the second-heaviest of the discovered halogens. Although astatine is produced by radioactive decay in nature, due to its short half life it is found only in minute amounts. Astatine was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè in 1940. Three years passed before traces of astatine were also found in natural minerals. Until recently most of the physical and chemical characteristics of astatine were inferred from comparison with other elements. Some astatine isotopes are used as alpha-particle emitters in science applications, and medical applications for astatine 211 have been tested. Astatine is currently the rarest naturally occurring element, with less than 30g estimated to be contained in the entire Earth\'s crust. This highly radioactive element has been confirmed by mass spectrometers to behave chemically much like other halogens, especially iodine (it would probably accumulate in the thyroid gland like iodine), though astatine is thought to be more metallic than iodine. Researchers at the Brookhaven National Laboratory have performed experiments that have identified and measured elementary reactions that involve astatine; however, chemical research into astatine is limited by its extreme rarity, which is a consequence of its extremely short half-life. Its most stable isotope has a half-life of around 8.3 hours. The final products of the decay of astatine are isotopes of lead. The halogens get darker in color with increasing molecular weight and atomic number. Thus, following the trend, astatine would be expected to be a nearly black solid, which, when heated, sublimes into a dark, purplish vapor (darker than iodine). Astatine is expected to form ionic bonds with metals such as sodium, like the other halogens, but it can be displaced from the salt...http://booksllc.net/?l=de