|Name, Symbol, Number||astatine, At, 85|
|Group, Period, Block||17, 6, p|
|Atomic mass||(210) g/mol|
|Electron configuration||[Xe] 4f14 5d10 6s2 6p5|
|Electrons per shell||2, 8, 18, 32, 18, 7|
|Melting point||575 K
(302 °C, 576 °F)
|Boiling point|| ? 610 K
(? 337 °C, ? 639 °F)
|Heat of vaporization||ca. 40 kJ/mol|
|Crystal structure||no data|
|Oxidation states||±1, 3, 5, 7|
|Electronegativity||2.2 (Pauling scale)|
|Ionization energies||1st: (est.) 920 kJ/mol|
|Magnetic ordering||no data|
|Thermal conductivity||(300 K) 1.7 W/(m·K)|
|CAS registry number||7440-68-8|
Astatine (chemical symbol At, atomic number 85) is the rarest naturally occurring chemical element. It is a member of the halogen family of elements and is the heaviest halogen. Its chemical properties appear to resemble those of iodine. All its isotopes are radioactive, and a few of them are produced by the natural radioactive decay of uranium-235 and uranium-238. Various compounds of astatine have been prepared in minute amounts, and the possibility of their use for nuclear medicine is being studied.
The total amount of astatine in the Earth's crust has been estimated to be less than one ounce (28 grams) at any given time—corresponding to no more than one teaspoonful in volume. It is produced in nature by the radioactive decay of uranium and thorium, and it is therefore present in trace amounts in minerals of these elements.
Astatine can be artificially produced by bombarding bismuth with energetic alpha particles. This method generates the relatively long-lived isotopes 209At, 210At, and 211At. These isotopes can then be separated from the bismuth by a process of distillation, which involves heating the mixture in the presence of air, and condensing the vapors in a separate container.
The name astatine was derived from the Greek word αστατος (astatos), meaning "unsteady." Long before it was discovered, Dmitri Mendeleev had predicted its existence based on his analysis of the periodic table. He called it "eka-iodine."
This element was discovered in 1940 by Dale R. Corson, K. R. MacKenzie, and Emilio Segrè at the University of California, Berkeley. They found it during experiments in which they subjected bismuth to a barrage of alpha particles. An earlier name for the element was alabamine (Ab).
According to experiments done with a mass spectrometer, the chemical properties of this highly radioactive element probably resemble those of the other halogens, especially iodine. (Like iodine, it would be expected to accumulate in the thyroid gland.) Yet astatine is thought to be more metallic than iodine and is classified as a metalloid. Researchers at the Brookhaven National Laboratory have been able to study elementary reactions that involve astatine, but chemical research into this element is limited by its extreme rarity, which is a result of its extremely short half-life.
Astatine has many known isotopes, all of which are radioactive. Their mass numbers range from 191 to 223. There exist also 23 metastable excited states. Among the various known isotopes, the longest-lived member is 210At, which has a half-life of 8.1 hours; the shortest-lived member is 213At, which has a half-life of 125 nanoseconds.
Multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. These compounds are primarily of theoretical interest. They are, however, also being studied for their potential use in nuclear medicine.
All links retrieved November 19, 2012.
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