Astatine


from Wikipedia, the free encyclopedia
properties
General
Name , symbol , atomic number Astatine, at, 85
Element category Halogens
Group , period , block 17 , 6 , p
Appearance metallic
CAS number 7440-68-8
Mass fraction of the earth's envelope 3 · 10 −21  ppm
Atomic
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) eV899.00 kJ / mol
2. Ionization energy 17th.880 (20) eV1 725 kJ / mol
3. Ionization energy 26th.58 (5) eV2 565 kJ / mol
4. Ionization energy 39.65 eV3 826 kJ / mol
5. Ionization energy 50.39 eV4 862 kJ / mol
6. Ionization energy 72.0 (2.0) eV6 950 kJ / mol
7. Ionization energy 85.1 (2.0) eV8 210 kJ / mol
Physically
Physical state firmly
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
Chemically
Oxidation states ± 1, 3, 5, 7
Electronegativity 2.2 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
209 at {syn.} 5.41 h ε 3.486 209 Po
α 5.757 205 bi
210 at {syn.} 8.3 h ε 3,981 210 Po
α 5.631 206 bi
211 at {syn.} 7.124 h ε 0.786 211 Po
α 5,982 207 bi
212 at {syn.} 0.314 s ε 1.754 212 Po
α 7,829 208 Bi
β - 0.043 212 para
213 at {syn.} 125 ns α 9,254 209 bi
214 at {syn.} 558 ns α 8,987 210 bi
215 at in traces 0.10 ms α 8.178 211 bi
... ... ... ... ... ...
218 at in traces 1.5 s α (99.90%) 6.874 214 Bi
β - (0.10%) 2.883 218 para
219 at in traces 56 s α (97%) 6.390 215 bi
β - (3%) 1,700 219 para
220 at {syn.} 3.71 min β - 9,900 220 para
For other isotopes see list of isotopes
Hazard and safety information
Radioactive
Radioactive
GHS hazard labeling
no classification available
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.

history

Emilio Gino Segrè, one of the discoverers of astatine

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 .

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

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.

properties

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.

With the online isotope mass separator ( ISOLDE ) at CERN , the ionization potential of astatine was determined in 2013 to be 9.31751 (8) electron volts .

Isotopes

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

use

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.

links

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.

safety instructions

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.

literature

Individual evidence

  1. ^ Harry H. Binder: Lexicon of the chemical elements , S. Hirzel Verlag, Stuttgart 1999, ISBN 3-7776-0736-3 .
  2. The values ​​of the atomic and physical properties (info box) are (unless otherwise stated) taken from www.webelements.com (Astat) .
  3. 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 .
  4. a b c d e f g 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.
  5. a b c d e f g Entry on astatine at WebElements, https://www.webelements.com , accessed on June 13, 2020.
  6. 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.
  7. Wolfgang W. Merkel: Astatine is the rarest element on earth , in Welt.de Date: September 3, 2011; Retrieved September 4, 2011.
  8. Jump up ↑ 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 .
  9. RF Trimble: What happened to alabamine, virginium, and illinium? In: J. Chem. Educ. 1975, 52, p. 585, doi: 10.1021 / ed052p585 .
  10. 85 Astatine .
  11. 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 .
  12. ^ DR Corson, KR MacKenzie, E. Segrè: Artificially Radioactive Element 85. In: Phys. Rev. 1940, 58, pp. 672-678, doi: 10.1103 / PhysRev.58.672 .
  13. Berta Karlik, Traude Bernert: A new natural α radiation. In: Natural Sciences . 1943, 31, 25-26, pp. 289-299, doi: 10.1007 / BF01475613 .
  14. 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 .
  15. 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.
  16. 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 .

Web links

Wiktionary: Astat  - explanations of meanings, word origins, synonyms, translations
Commons : Astat  - album with pictures, videos and audio files