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Francium (IPA: ), also referred to as eka-caesium and actinium K, is a chemical element in the periodic table that has the symbol Fr and atomic number 87. It has the lowest known electronegativity and is the second rarest naturally occurring element. Francium is a highly radioactive alkali metal that decays into astatine, radium, and radon; as an alkali metal, it has one valence electron.