An extremely rare radioactive chemical element; symbol Fr. Atomic number 87; atomic weight of most stable isotope 223; melting point about 27°C; boiling point about 677°C. Its most stable isotope (half-life about 22 minutes) occurs naturally, to a very limited extent, in uranium minerals. More than 30 other isotopes of francium are known; some are prepared by bombarding thorium with protons, deuterons, or alpha particles.
From France, where the French physicist Marguerite Perey (1919-1975) discovered it in 1939 in the alpha particle decay of actinium.