2007 Schools Wikipedia Selection. Related subjects: Chemical elements
|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−1|
|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−1·K−1|
|CAS registry number||7440-68-8|
Astatine ( IPA: /ˈastətiːn/) is a chemical element in the periodic table that has the symbol At and atomic number 85. This radioactive element occurs naturally from uranium-235 and uranium-238 decay; it is the heaviest of the halogens.
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). 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 result of its extremely short half-life.
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. The Guinness Book of 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 wrote a 1955 essay on large numbers, scientific notation, and the size of the atom, in which he stated that the number of astatine atoms on Earth at any time was "only a trillion".
The existence of "eka-iodine" had been predicted by Mendeleev. Astatine (after Greek αστατος astatos, meaning "unsteady") 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. An earlier name for the element was alabamine (Ab).
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.
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; however, they are also being studied for potential use in nuclear medicine.
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.