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Occurrence and Production of Astatine
Natural Occurence
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 25g of the element astatine (At) occurring naturally"; Isaac Asimov, in a 1955 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 was "only a trillion".
Its most stable isotope has a half-life of around 8.3 hours. Also, when astatine finishes decaying it becomes an isotope of lead.
Synthesis
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.