Astatine ( from the Greek astatos ("unstable")) – is a radioactive chemical element with the chemical symbol At and atomic number 85.
Any astatine that was present at the Earth's formation has long since decayed, and the minute amounts of astatine existing currently have formed through the decay of heavier elements. It occurs on Earth only as the result of the radioactive decay of certain heavier elements.
There is no data indicating that astatine occurs in stars.
All of its isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours. Accordingly, much less is known about astatine than most other elements. Six astatine isotopes, with mass numbers of 214 to 219, are present in nature as the products of various decay routes of heavier elements, but neither the most stable isotope of astatine (with mass number 210) nor astatine-211 (which is used in medicine) is produced naturally.
The observed properties are consistent with it being a heavier analog of iodine; many other properties have been estimated based on this resemblance. Elemental astatine has never been viewed, because a mass large enough to be seen (by the naked human eye) would be immediately vaporized by the heat generated by its own radioactivity. Astatine may be dark, or it may have a metallic appearance and be a semiconductor, or it may even be a metal. It is likely to have a much higher melting point than does iodine, on par with those of bismuth and polonium.
Astatine was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley in 1940. Three years later it was found in nature, although it is the least abundant element in the Earth's crust among the non-transuranic elements after berkelium, with an estimated total amount of less than 28 grams (1 oz) at any given time.
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