Astatine is the rarest, naturally occurring element in the Earth’s crust, occurring only as the decay product of various heavier elements. There are only about 25 grams of naturally-occurring astatine in Earth’s crust at any given time. It is a radioactive chemical element with symbol At and atomic number 85. It is the heaviest known halogen (a non-metal), but it has more metallic character than other elements in the halogen group.
The bulk properties of astatine are not known with certainty. Many of them have been estimated based on the element’s position on the periodic table as a heavier analog of iodine, and a member of the halogens (the group of elements including fluorine, chlorine, bromine, and iodine). Astatine is likely to have a dark or lustrous appearance and may be a semiconductor or possibly a metal; it probably has a higher melting point than that of iodine. Chemically, several anionic species of astatine are known and most of its compounds resemble those of iodine. It also shows some metallic behavior, including being able to form a stable monatomic cation in aqueous solution (unlike the lighter halogens).
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