Astatine is the rarest naturally-occurring chemical element. Its symbol is At and it has the atomic number 85.
It's highly radioactive, and only occurs as a decay product of other radioactive compounds. Because of its short half-life (the most stable isotope has a half-life of around 8hrs) it has been very difficult to study. Its estimated that less than 1 gram is present in the Earth's crust at any given time.
It can be synthesized in the laboratory in small quantities, but decays so rapidly that Its 'bulk properties' are still generally unknown. It's assumed to be somewhat similar to iodine, but possibly more metallic.
At and compounds derived from the element are under investigation for possible use in radioactive cancer therapy.
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