|Name , symbol , atomic number||Astatine, at, 85|
|Group , period , block||17 , 6 , p|
|Mass fraction of the earth's envelope||3 · 10 −21 ppm|
|Atomic mass||209.9871 u|
|Covalent radius||150 pm|
|Van der Waals radius||202 pm|
|Electron configuration||[ Xe ] 4 f 14 5 d 10 6 s 2 6 p 5|
|1. Ionization energy||9.31751 (8) eV ≈ 899.00 kJ / mol|
|2. Ionization energy||17th.880 (20) eV ≈ 1 725 kJ / mol|
|3. Ionization energy||26th.58 (5) eV ≈ 2 565 kJ / mol|
|4. Ionization energy||39.65 eV ≈ 3 826 kJ / mol|
|5. Ionization energy||50.39 eV ≈ 4 862 kJ / mol|
|6. Ionization energy||72.0 (2.0) eV ≈ 6 950 kJ / mol|
|7. Ionization energy||85.1 (2.0) eV ≈ 8 210 kJ / mol|
|Melting point||575 K (302 ° C)|
|boiling point||610 (337 ° C)|
|Heat of evaporation||approx. 40 kJ / mol|
|Heat of fusion||approx. 6 kJ mol −1|
|Thermal conductivity||2 W m −1 K −1|
|Oxidation states||± 1, 3, 5, 7|
|Electronegativity||2.2 ( Pauling scale )|
|For other isotopes see list of isotopes|
|Hazard and safety information|
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .
Astat [ asˈtaːt ] (from ancient Greek ἄστατος : "unstable, unsteady") is a radioactive chemical element with the element symbol At and the atomic number 85. In the periodic table it is in the 7th main group or the 17th IUPAC group and thus counts the halogens . Astatine is formed from the natural decay of uranium . Astatine is one of the rarest naturally occurring elements on earth that must be artificially created when needed.
When Dmitri Mendeleev established his periodic table in 1869, he predicted the existence of some elements not yet discovered at the time, including one that would take the place below iodine . As a result, some scientists tried to find this element, which was known as "eka-iodine".
In 1931, Fred Allison claimed that he and his coworkers at the Alabama Polytechnic Institute (now Auburn University ) discovered the missing element and named it Alabamine (Ab). However, their discovery could not be confirmed and was later found to be false.
Also searching for a member of the radioactive thorium family , the chemist De Rajendralal Mitra found two new elements in Dhaka , Bangladesh (then British India) in 1937 . The first he called Dakin (Eka-Iod), probably after the English name for Dhaka (Dacca), the other Gourium . However, both discoveries could not be confirmed.
The name Helvetium was in turn suggested by the Swiss chemist Walter Minder when he announced the discovery of element 85 in 1940. However, in 1942 he changed his proposal to Anglohelvetium .
The discovery of astatine ( ancient Greek ἀστατέω = "to be inconsistent", due to the radioactive decay) could be confirmed for the first time in 1940 by the scientists Dale Corson , Kenneth MacKenzie and Emilio Gino Segrè , who artificially made it in the University of California by bombarding it with bismuth made with alpha particles .
Extraction and presentation
Astatine is produced by bombarding bismuth with alpha particles in the energy range from 26 to 29 MeV. The relatively long-lived isotopes 209 At to 211 At are obtained, which are then sublimed in a stream of nitrogen at 450 to 600 ° C. and separated on a cooled platinum disk.
With the help of mass spectrometry , this radioactive element has been shown to behave chemically like the other halogens, especially iodine (it accumulates like this in the thyroid gland ). Astatine is more metallic than iodine. Researchers at Brookhaven National Laboratory have conducted experiments to identify and measure elemental chemical reactions that involve astatine.
Organic astatine compounds are used in nuclear medicine to irradiate malignant tumors. Because of their short half-lives, astatine isotopes are suitable when taken internally as radioactive preparations for marking the thyroid gland. The element is accumulated in the thyroid and stored in the liver.
The chemical properties of astatine could only be determined with tracer experiments due to the small amounts . They are very similar to those of iodine, but it is a weaker oxidizing agent . So far, various astatides, interhalogen compounds and organic compounds could be detected. The anions of the corresponding oxo acids are also known. Because of its more electropositive character compared to other halogens, it is only partially precipitated by silver . The complex-stabilized cation At (Py) 2 (Py = pyridine ) exists for this , which means that astatine can also be deposited cathodically . The hydride, astathydrogen HAt, was also detected.
Classifications according to the CLP regulation are not available because they only include chemical hazard and play a completely subordinate role compared to the hazards based on radioactivity . The latter also only applies if the amount of substance involved is relevant.
- AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 443.
- Eric Scerri : A tale of seven elements , Oxford University Press, Oxford, 2013.
- Harry H. Binder: Lexicon of the chemical elements , S. Hirzel Verlag, Stuttgart 1999, ISBN 3-7776-0736-3 .
- The values of the atomic and physical properties (info box) are (unless otherwise stated) taken from www.webelements.com (Astat) .
- Manjeera Mantina, Adam C. Chamberlin, Rosendo Valero, Christopher J. Cramer, Donald G. Truhlar: Consistent van der Waals Radii for the Whole Main Group. In: J. Phys. Chem. A. 2009, 113, pp. 5806-5812, doi: 10.1021 / jp8111556 .
- Entry on astatine in Kramida, A., Ralchenko, Yu., Reader, J. and NIST ASD Team (2019): NIST Atomic Spectra Database (ver. 5.7.1) . Ed .: NIST , Gaithersburg, MD. doi : 10.18434 / T4W30F ( https://physics.nist.gov/asd ). Retrieved June 13, 2020.
- Entry on astatine at WebElements, https://www.webelements.com , accessed on June 13, 2020.
- The hazards emanating from radioactivity do not belong to the properties to be classified according to the GHS labeling. With regard to other hazards, this element has either not yet been classified or a reliable and citable source has not yet been found.
- Wolfgang W. Merkel: Astatine is the rarest element on earth , in Welt.de Date: September 3, 2011; Retrieved September 4, 2011.
- Fred Allison, Edgar J. Murphy, Edna R. Bishop, Anna L. Sommer: Evidence of the Detection of Element 85 in Certain Substances . In: Phys. Rev. 1931, 37, pp. 1178-1180, doi: 10.1103 / PhysRev.37.1178 .
- RF Trimble: What happened to alabamine, virginium, and illinium? In: J. Chem. Educ. 1975, 52, p. 585, doi: 10.1021 / ed052p585 .
- 85 Astatine .
- Alice Leigh-Smith, Walter Minder: Experimental Evidence of the Existence of Element 85 in the Thorium Family. In: Nature . 1942, 150, pp. 767-768, doi: 10.1038 / 150767a0 .
- DR Corson, KR MacKenzie, E. Segrè: Artificially Radioactive Element 85. In: Phys. Rev. 1940, 58, pp. 672-678, doi: 10.1103 / PhysRev.58.672 .
- Berta Karlik, Traude Bernert: A new natural α radiation. In: Natural Sciences . 1943, 31, 25-26, pp. 289-299, doi: 10.1007 / BF01475613 .
- Berta Karlik, Traude Bernert: The element 85 in the natural decay series . In: Journal of Physics . 1943, 123, 1-2, pp. 51-72, doi: 10.1007 / BF01375144 .
- World of Physics: Fundamental properties of the rarest natural element measured ( Memento from June 18, 2013 in the Internet Archive ) (Deutsche Physikalische Gesellschaft eV on the ionization potential of astatine) 2013.
- Willhauck MJ, Samani BR, Wolf I, et al. : The potential of 211Astatine for NIS-mediated radionuclide therapy in prostate cancer . In: Eur. J. Nucl. Med. Mol. Imaging . 35, No. 7, July 2008, pp. 1272-1281. doi : 10.1007 / s00259-008-0775-4 . PMID 18404268 .