Francium

Francium (IPA: /ˈfrænsiəm/), formerly known 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 (after Astatine). Francium is a highly radioactive alkali 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 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 a plasma at Stony Brook in 1996.