Astatine (pronEng|ˈæstətiːn) is a
radioactive chemical elementwith the symbol At and atomic number85. It is the heaviest of the discovered halogens.
radioactiveelement has been confirmed by mass spectrometers to behave chemically much like other halogens, especially iodine(it would probably accumulate in the thyroidgland like iodine [cite web
url = http://periodic.lanl.gov/elements/85.html
title = Astatine
accessdate = 2008-07-10
publisher = Los Alamos Laboratories] ), though astatine is thought to be more
metallic than iodine. Researchers at the Brookhaven National Laboratoryhave 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. Following the colortrend of the halogens, the elements 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 metalssuch as sodium, like the other halogens, but it can be displaced from the saltsby lighter, more reactive halogens. Astatine can also react with hydrogento form hydrogen astatide, which when dissolved in water, forms hydroastatic acid. Astatine is the least reactive of the halogens, being less reactive than iodine.
The existence of "eka-iodine" had been predicted by
Mendeleev. Astatine (after Greek αστατος "astatos", meaning "unstable") was first synthesized in 1940 by Dale R. Corson, K. R. MacKenzie, and Emilio Segrèat the University of California, Berkeleyby barraging bismuthwith alpha particles. [cite journal
title = Artificially Radioactive Element 85
author = D. R. Corson, K. R. MacKenzie, and E. Segrè
journal = Phys. Rev.
volume = 58
pages = 672–678
year = 1940
doi = 10.1103/PhysRev.58.672]
periodic table of elementswas long known, several scientists tried to find the element following iodine in the halogen group. The unknown substance was called Eka-iodine before its discovery because the name of the element was to be suggested by the discoverer. The claimed discovery in 1931 at the Alabama Polytechnic Institute (now Auburn University) led to the name for the element "alabamine" (Ab). [cite journal
title = Evidence of the Detection of Element 85 in Certain Substances
author = Fred Allison, Edgar J. Murphy, Edna R. Bishop, and Anna L. Sommer
journal = Phys. Rev.
volume = 37
pages = 1178–1180
year = 1931
doi = 10.1103/PhysRev.37.1178 ] [ cite web
url = http://www.time.com/time/magazine/article/0,9171,743159,00.html
title = Alabamine & Virginium
publisher = time
accessdate = 2008-07-10 ] .
The name "Dakin" was proposed for this element in 1937 by chemist Rajendralal De working in
Dhaka. [ [http://elements.vanderkrogt.net/elem/at.html 85 Astatine ] ]
The name Helvetium was chosen by the Swiss chemist Walter Minder, when he announced the discovery of element 85 in 1940, but changed his suggested name to Anglohelvetium in 1942. [cite journal
title = Experimental Evidence of the Existence of Element 85 in the Thorium Family
author = Alice Leigh-Smith, Walter Minder
journal = nature
volume = 150
pages = 767–768
year = 1942
doi = 10.1038/150767a0]
Astatine occurs naturally in three natural radioactive decay series, but because of its short
half-lifeis only found in minute amounts. Astatine-218 (218At) is found in the uranium series, 216At is in the thorium series, and 215At as well as 219At are in the actinium series [cite web
title = astatine (At)
publisher = Encyclopedia Britannica online
url = http://www.britannica.com/eb/article-9009963/astatine
accessdate = 2008-06-22 ] . The most long-lived of these naturally-occurring astatine isotopes is 210At with a half-life of 8.3 hours.
Astatine is the rarest naturally-occurring element, with the total amount in Earth's crust estimated to be less than 1 oz (28 g) at any given time. This amounts to less than one teaspoon of the element. "
Guinness World Records" has dubbed the element the rarest on Earth, stating: "Only around 0.9 oz (25 g) of the element astatine (At) occurring naturally". Isaac Asimov, in a 1957 essay on large numbers, scientific notation, and the size of the atom, wrote that in "all of North and South America to a depth of ten miles", the number of astatine atoms at any time is "only a trillion". [http://ia331335.us.archive.org/1/items/onlyatrillion017765mbp/onlyatrillion017765mbp_djvu.txt]
Astatine is produced by bombarding
bismuthwith energetic alpha particlesto obtain relatively long-lived 209At - 211At, which can then be distilled from the target by heating in the presence of air.
Multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. While these compounds are primarily of theoretical interest, they are being studied for potential use in
nuclear medicine. [ [http://www.medicalnewstoday.com/articles/80581.php Nuclear Nanocapsules, The New Cancer Weapon ] ] Astatine is expected to form ionic bonds with metalssuch as sodium, like the other halogens, but it can be displaced from the saltsby lighter, more reactive halogens. Astatine can also react with hydrogento form hydrogen astatide, which when dissolved in water, forms hydroastatic acid.
Some examples of astatic compounds are:
Astatine has 33 known
isotopes, all of which are radioactive; the range of their mass numbers is from 191 to 223. There exist also 23 metastable excited states. The longest-lived isotope is 210At, which has a half-lifeof 8.1 hours; the shortest-lived known isotope is 213At, which has a half-life of 125 nanoseconds.
The least stable isotopes of astatine have no practical applications other than scientific study due to their extremely short life, but heavier isotopes have medical uses. The isotope 211 of astatine is used for treating different types of tumorsFact|date=May 2008. Astatine 211 is an alpha emitter with a physical halflife of 7.2 h. These features have led to its use in
radiation therapy. An investigation of the efficacy of astatine-211--tellurium colloid for the treatment of experimental malignant ascites in mice reveals that this alpha-emitting radiocolloid can be curative without causing undue toxicity to normal tissue. By comparison, beta-emitting phosphorus-32as colloidal chromic phosphate had no antineoplastic activity. The most compelling explanation for this striking difference is the dense ionization and short range of action associated with alpha-emission. These results have important implications for the development and use of alpha-emitters as radiocolloid therapy for the treatment of human tumors. [" http://www.sciencemag.org/cgi/content/abstract/212/4492/340"]
Since astatine is radioactive, it should be handled with care. Because of its extreme rarity, it is not likely that the general public will be exposed.
Astatine is a halogen, and standard precautions apply. It is less reactive than iodine, but they share similar characteristics.
* [http://www.webelements.com/webelements/elements/text/At/index.html WebElements.com - Astatine]
* [http://periodic.lanl.gov/elements/85.html Los Alamos National Laboratory - Astatine]
* [http://www.docbrown.info/page03/The_Halogens.htm Doc Brown's Chemistry Clinic - Group 7 The Halogens]
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