Francium formerly known as eka-caesium and actinium K, is a chemical element that has the symbol Fr and atomic number 87. It has one of the lowest electronegativities of all known elements, and is the second rarest naturally occurring element (after astatine). 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 synthesized. Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 continually forms and decays. As little as 20–30 g (one ounce) exists at any given time throughout the Earth's crust; the other isotopes are entirely synthetic. The largest amount produced in the laboratory was a cluster of more than 300,000 atoms.
There is "some" Francium in this uraninite sample, but not much.
This sample of uraninite contains about 100,000 atoms (3.3 × 10-20 g) of francium-223 at any given time.
Francium is the most unstable of the naturally occurring elements: its most stable isotope, francium-223, has a maximum half-life of only 22 minutes. In contrast, astatine, the second-least stable naturally occurring element, has a maximum half-life of 8.5 hours. All isotopes of francium decay into either astatine, radium, or radon. Francium is also less stable than all synthetic elements up to element 105.
Francium-223 is the result of the alpha decay of actinium-227 and can be found in trace amounts in uranium and thorium minerals. In a given sample of uranium, there is estimated to be only one francium atom for every 1×1018 uranium atoms. It is also calculated that there is at most 30 g of francium in the earth's crust at any time. This makes it the second rarest element in the crust after astatine.
Check out this interactive Periodic Table.
Check out this Francium video. Prepared by The University of Nottingham.