Astatine is the rarest naturally occurring element that is not a transuranic element, with the total amount in Earth’s crust estimated to be less than one gram at any given time. 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. While it was previously thought to be the rarest element occurring on the Earth, astatine has lost this status to berkelium, atoms of which can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits.
Six astatine isotopes occur naturally (astatine-214 to astatine-219). Because of their short half-lives, they are found only in trace amounts. There is no data indicating that astatine occurs in stars.
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 iodine, on par with those of bismuth and polonium. Chemically, astatine behaves more or less as a halogen (the group including chlorine and fluorine), being expected to form ionic astatides with alkali or alkaline earth metals; it is known to form covalent compounds with nonmetals, including other halogens. It does, however, also have a notable cationic chemistry that distinguishes it from the lighter halogens. The second longest-lived isotope of astatine, astatine-211, is the only one currently having any commercial application, being employed in medicine to diagnose and treat some diseases via its emission of alpha particles (helium-4 nuclei). Only extremely small quantities are used, however, due to its intense radioactivity.
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