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.
Characteristics
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.
Natural
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.
PERIODIC TABLE
Check out this interactive Periodic Table.
Check out this Francium video. Prepared by The University of
Nottingham.