Franciu

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Francium (IPA: /ˈfransiəm/) is a chemical element in the periodic table that has the symbol Fr and atomic number 87. This is a highly radioactive alkali metal that is found in very small amounts in uranium and thorium ores. It is also notable for having the lowest electronegativity and electron affinity of all the elements.

[modifică] Characteristics

This element, which was named for France, was discovered in 1939 by Marguerite Perey of the Curie Institute in Paris. An interesting sidenote: there was a British team working to be the first to discover this element at the same time as a French team. The British team wanted to name the element 'Britium' - the French team discovered it first and named it after their country.

Francium was the last element discovered in nature. Originally named Actinium K, it is the heaviest alkali metal and occurs as a result of actinium's alpha decay and can be artificially made by bombarding thorium with protons.

Even though it naturally occurs in uranium minerals, it has been estimated that there might be only from 340 to 550 grams [1] of francium in the earth's crust at any one time, making it the second rarest element in the crust, next to astatine. It is also the most unstable element among the first 101 and has the highest equivalent weight of any element. Francium is the least electronegative of all the known elements, with caesium as its runner up.

There are 34 known isotopes of francium (with the mass number between 199 and 232), and 11 metastable states. With a 22-minute half-life, the longest lived isotope of this element is ²²³Fr which is a daughter isotope of ²²⁷Ac and is one of two isotopes of francium that occur naturally. The second naturally occurring isotope of francium is ²²⁴Fr, being the member of the thorium radioactive series. All known isotopes of francium are highly unstable, therefore knowledge of the properties of this element only comes from radiochemical procedures.

A small number of pictures of francium have been taken, but only of at the most 350,000 atoms at a time. The images were made by trapping the atoms and using a special fluorescent imaging camera. The atoms were produced by a nuclear transformation with a particle accelerator at the State University of New York at Stony Brook. ¹⁸O nuclei are accelerated to an energy of 100 MeV, to have sufficient energy to fuse with a gold nucleus and create nuclei of francium. The Fr nuclei last for typically three minutes, and must be trapped and observed before they decay.

[modifică] References

Los Alamos National Laboratory - Francium
Stony Brook University Physics Dept.
Jean-Pierre Adloff and George B. Kauffman. Francium (Atomic Number 87), the Last Discovered Natural Element. The Chemical Educator, Vol. 10, No. 5, 2005 [2].