Period 7 element elements of the 7th period. Francium is a chemical element with symbol Fr and atomic number 87. It was formerly known as eka-caesium and actinium K. It is one of the two least electronegative elements, the other being caesium. Francium is a highly radioactive metal that decays into astatine, radium, and radon. As an alkali metal, it has one valence electron. Francium was discovered by Marguerite Perey in France (from which the element takes its name) in 1939. It was the last element discovered in nature, rather than by synthesis. Outside the laboratory, francium is extremely rare, with trace amounts
Uranium–uranium dating Uranium–uranium dating Uranium–uranium dating is a radiometric dating technique which compares two isotopes of uranium (U) in a sample: uranium-234 (U) and uranium-238 (U). It is one of several radiometric dating techniques exploiting the uranium radioactive decay series, in which U undergoes 14 alpha and beta decay events on the way to the stable isotope Pb. Other dating techniques using this decay series include uranium–thorium dating and uranium–lead dating. U, with a half-life of about 4.5 billion years, decays to U through emission of an alpha particle to thorium-234 (Th), which is comparatively unstable with a half-life of just 24