Rare element

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I read somewhere that francium is one of the rarest elements and that the entire planet contains only about 17 atoms of it . I really wondered if this was right and if so how could they know the number of atoms and also does this make it the most valuable substance i.e. the rarest or totally valueless since there is not enough to do anything with?
 

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Astronuc
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from http://www.webelements.com/webelements/elements/text/Fr/key.html
Francium occurs as a result of a disintegration of actinium. Francium is found in uranium minerals, and can be made artificially by bombarding thorium with protons. It is the most unstable of the first 101 elements. The longest lived isotope, 223Fr, a daughter of 227Ac, has a half-life of 22 minutes. This is the only isotope of francium occurring in nature, but at most there is only 20-30 g of the element present in the earth's crust at any one time. No weighable quantity of the element has been prepared or isolated. There are about 20 known isotopes.
As the article state, Fr is a daughter product of Actinium, which itself is a daughter product of another radionuclide, and they are part of the decay of uranium, U-235.

See U-235 decay series at - http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radser.html#c1
 

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