Francium previously known as eka-caesium and actinium K is a chemical element that has the symbol Fr and atomic number 87. It has the lowest electronegativity of all known elements, and is the second rarest naturally occurring element. Francium is a highly radioactive metal that decays into astatine, radium, and radon.
Due to its volatility and rarity, there are no profitable applications for francium. It has been used for study purposes in the fields of biology and of atomic structure. Its use as a potential diagnostic aid for a variety of cancers has also been explored, but this application has been deemed impractical. Francium's ability to be synthesized, trapped, and cooled, along with its relatively simple atomic structure have made it the subject of specialized spectroscopy experiments.