|Name, Symbol, Number||astatine, At, 85|
|Group, Period, Block||17, 6, p|
|Standard atomic weight||(210) g·mol−1|
|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 (scale Pauling)|
|Ionization energies||1st: 890±40 kJ/mol|
|Magnetic ordering||no data|
|Thermal conductivity||(300 K) 1.7 W·m−1·K−1|
|CAS registry number||7440-68-8|
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), 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 product of the decay of astatine is an isotope of lead.
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. An earlier name for the element was alabamine (Ab).
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".
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.
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 8.3 hours; the shortest-lived known isotope is 213At, which has a half-life of 125 nanoseconds.
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