Astatine

Astatine (pronEng|ˈæstətiːn) is a radioactive chemical element with the symbol At and atomic number 85. It is the heaviest of the discovered halogens.

Characteristics

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 [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 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. Following the color trend 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 metals such as sodium, like the other halogens, but it can be displaced from the salts by lighter, more reactive halogens. Astatine can also react with hydrogen to form hydrogen astatide, which when dissolved in water, forms hydroastatic acid. Astatine is the least reactive of the halogens, being less reactive than iodine.

History

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 Segrè at the University of California, Berkeley by barraging bismuth with 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
]

As the periodic table of elements was 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
]

Occurrence

Astatine occurs naturally in three natural radioactive decay series, but because of its short half-life is 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 bismuth with energetic alpha particles to obtain relatively long-lived 209At - 211At, which can then be distilled from the target by heating in the presence of air.

Compounds

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 metals such as sodium, like the other halogens, but it can be displaced from the salts by lighter, more reactive halogens. Astatine can also react with hydrogen to form hydrogen astatide, which when dissolved in water, forms hydroastatic acid.

Some examples of astatic compounds are:

NaAt or sodium astatide

MgAt2 or magnesium astatide

CAt4 or carbon tetrastatide (tetraastatide)

Isotopes

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-life of 8.1 hours; the shortest-lived known isotope is 213At, which has a half-life of 125 nanoseconds.

Applications

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-32 as 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"]

Precautions

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.

References

External links

* [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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  • astatine — Symbol: At Atomic number: 85 Atomic weight: (210) Radioactive halogen element. Occurs naturally from uranium and thorium decay. At least 20 known isotopes. At 210, the most stable, has a half life of 8.3 hours. Synthesized by nuclear bombardment… …   Elements of periodic system

  • astatine — radioactive element, named 1947, from Gk. astatos unstable (see ASTATIC (Cf. astatic)) + chemical suffix INE (Cf. ine) (2). So called for its short half life and lack of stable isotopes. The element appears not to have a stable form and probably… …   Etymology dictionary

  • astatine — ☆ astatine [as′tə tēn΄, as′tətin΄ ] n. [< Gr astatos, unstable + INE3] a radioactive chemical element, one of the halogens, formed from bismuth when it is bombarded by alpha particles: it resembles iodine in its chemical properties: symbol,… …   English World dictionary

  • astatine — /as teuh teen tin/, n. Chem. a rare element of the halogen family. Symbol: At; at. no.: 85. [1945 50; < Gk ástat(os) not steadfast, unstable (see ASTATIC) + INE2] * * * ▪ chemical element Introduction  radioactive chemical element and the… …   Universalium

  • astatine — astatas statusas T sritis chemija apibrėžtis Cheminis elementas. simbolis( iai) At atitikmenys: lot. astatinum angl. astatine rus. астат; астатин ryšiai: sinonimas – astatinas …   Chemijos terminų aiškinamasis žodynas

  • astatine — astatinas statusas T sritis fizika atitikmenys: angl. astatine vok. Astat, n; Astatin, n rus. астат, m pranc. astate, m …   Fizikos terminų žodynas

  • astatine — noun Etymology: Greek astatos unsteady, from a + statos standing, from histanai to cause to stand more at stand Date: 1947 a radioactive halogen element discovered by bombarding bismuth with alpha particles and also formed by radioactive decay… …   New Collegiate Dictionary

  • Astatine — Astate Astate …   Wikipédia en Français

  • astatine — noun /ˈæstətiːn,ˈæstətɪn/ A highly radioactive chemical element (symbol At) with atomic number 85 and one of the halogens.<!encyclopedic info It occurs in only one isotope with an atomic mass of 210. It is intensely radioactive with a half… …   Wiktionary

  • astatine — An artificial radioactive element of the halogen series; atomic no. 85, atomic wt. 211. [G. astatos, unstable] * * * as·ta·tine as tə .tēn n a radioactive halogen element discovered by bombarding bismuth with helium nuclei and also formed by… …   Medical dictionary

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