astatine is highly radioactive and all its isotopes have half-lives of approximately 8 hours or less, the longest being astatine-210 with a half-life of a bit over eight hours. some of its isotopes may be found in nature, but because it is so unstable, only about an ounce exists in the earth's crust at any point in time. chemists say it is the rarest element in the earth's crust. less reactive than iodine, astatine is the least reactive of the halogens. only a few compounds with metals have been found, in the form of astatides of sodium, palladium, silver, thallium, and lead.