Chemistry
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Francium

Perhaps one of only a few true images of Francium - this false-colour image displays the light given off by ~200,000 atoms of it caught in a light trap.

Francium (Fr) is element 87. It was discovered in 1939 by Marguerite Perey, of the Curie Institute.

The only isotope of Fr occurring naturally in significant amounts is 223Fr, a descendant of 235U. 223Fr decays mainly by beta emission to 223Ra, releasing 1.15 MeV worth of energy in the process. A bit of playing with numbers reveals that a 1 mm diameter sphere of 223Fr puts out an average of 245 watts during its first half-life, most of which remains inside the specimen. Within about 0.005 sec, the sphere will turn into a tiny nuclear fireball at a temperature approaching 2000 K. While no macroscopic sample can be obtained without high-order magic, a sample of some 200000 atoms (5E-16 micromole) emitted enough light to allow it to be photographed (see picture).


Often hailed as the most reactive Alkali Metal, it is thought perhaps it may be less reactive than Caesium, as Francium's lone 7s electron spends a part of its time close to the nucleus. In order to do that, the electron must have so much kinetic energy that it becomes more massive - causing its orbital to shrink. A 7s electron spends very little time near the nucleus, so its increasing bond strength and decreasing orbital radius are noticable but not dramatic.

Further Description[]

In one decay out of 16670, 223Fr decays by alpha emission to 219At instead of to 223Ra. Concentration of 219At

Concentration of Fr in earth as a whole is [Fr] = 5E-27 (molar, of course). Given that N(earth) = 1.4E50 atoms, earth's allotment works out to 1.2 mol or 260 g. As a component of the entire earth, Fr is around 100 times as abundant as Pu. (The situation is reversed in uranium ores; [Fr] is sensitive to [U], but [Pu] is very sensitive to [U].

Its electron configuration is [Rn] 7s1 - a single s electron over a filled shell, like the other Group 1 metals. Francium's estimated covalent and Van der Waals radii are greater than those of Cs - but Rcov(Fr)/Rcov(Cs) = 1.07 while Rcov(Cs)/Rcov(Rb) = 1.11 and RVdW(Fr)/RVdW(Cs) = 1.01 while RVdW(Cs)/RVdW(Rb) = 1.13. It's still the biggest alkali metal atom, but by a shrinking margin.

One occasionally sees data like:

  • Melting Point: 21°C, 70°F, 294°K
  • Boiling Point: 650°C, 1202°F, 923°C

Please note picture at this article's head. 200000 atoms is 5E-10 nanomoles, or 1.2E-16 g. It is difficult to think of the words "melting point" applying to something which throws off energy so fast that 0.0001 picogram of it gives off a detectable heat signature. Just because a property can be calculated does not mean the numbers describe anything real.

Applications[]

Francium's most dramatic use has been in the production of Bose-Einstein condensates.

Online References[]

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