Astatine

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Astatine is a chemical element. Its atomic number (which is the number of protons in it) is 85, and its atomic weight (which is approximately the number of protons plus the number of neutrons in it) is 210. It is part of the Group 7 (halogens) on the periodic table of elements. Some of its isotopes may be in nature. But because it is extremely unstable (roughly speaking, the atoms fall apart), only about an ounce exists in the Earth's crust at one point in time. It is radioactive. The longest-living isotope 210At has a half-life of a bit over eight hours.