After bismuth, which has a half-life or more than 1019 years, polonium, astatine, and radon are some of the shortest-lived and rarest elements known; less than a gram of astatine is estimated to exist on earth at any given time. a Note that lanthanum (or, alternatively, lutetium) is also considered to be a transition metal, but marked as a lanthanide, as it is considered so by IUPAC. b An exception to the Madelung rule.
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