Francium is a chemical element that has the symbol Fr and atomic number 87. It has the lowest known electronegativity and is the second rarest naturally occurring element on Earth (after astatine)<!-- Cf and others occur naturally in supernovae; Universe-wide abundance is difficult to ascertain --. Francium is a highly radioactive metal that decays into astatine, radium, and radon. As an alkali metal, it has one valence electron. Marguerite Perey discovered francium in
1939. Francium was the last element discovered in nature, rather than synthesized. Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 is continually formed and continually decays. Perhaps an ounce (30 g) exists at any given time throughout the Earth's crust; the other isotopes are entirely synthetic. The largest amount ever collected of any isotope was a cluster of 10,000 atoms (of francium-210) created as an ultracold gas at Stony Brook in 1996.
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