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Astatine could be described as the most rare element on Earth. All isotopes, atoms with the same number of protons in the nucleus and different numbers of neutrons, are radioactive; even its name is Greek for "unstable." When an atom decays its nucleus breaks into smaller atoms, subatomic particles, and energy. Astatine occurs naturally as one of the atoms produced when the uranium 235 isotope undergoes radioactive decay. However, astatine does not stay around long. Most of its identified isotopes have half-lives of less than one minute. That is, half of the unstable atoms will radioactively decay in that time.

Astatine was first synthesized in 1940 in cyclotron reactions by bombarding bismuth with alpha particles. The longest-lived isotope has a half-life of 8.3 hours. Therefore, weighable amounts of astatine have never been isolated, and little is known about its chemical or physical properties. In a mass spectrometer, an instrument that observes the masses of very small samples, astatine behaves much like the other halogens, especially iodine. There is evidence of compounds formed by its combining with other halogens, such as AtI, AtBr, and AtCl.

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Science EncyclopediaScience & Philosophy: Habit memory: to HeterodontHalogens - Chlorine, Bromine, Iodine, Astatine, Fluorine, Unexplored Sources And Problems