The francium is a chemical element of Atomic number 87 known during a time under the names of eka-cesium or actinium K . It is a alkaline Métal radioactive of Atomic mass 233 and symbol Fr .

It is the second rarest element, after the Astate: there would be only one about thirty Gram S in the Earth's crust.
This scarcity is due to its transitory existence, as a product of Désintégration of the Actinium. The Demi-vie of the Isotope most stable Fr is of 23 minute S, then it transforms itself into Radium by Beta decay or into Astate by Rayonnement alpha.
And it is this double radioactivity which makes astatine even rarer than francium, even if astatine has a half-life higher than that of the francium (8.1 H > 22 min).

The francium was discovered in 1939 with the Institut Curie of Paris by Marguerite Perey, by purifying Lanthane containing actinium. It is about the last element discovered in nature and not synthesized (certain elements the such Technétium were discovered in nature after being synthesized in laboratory).

History

As of the Years 1870, the community of the chemists thought that it was to exist a Métal of the alkaline type of Atomic number 87, in lower part of the Césium in the periodic Classification. It was then known under the provisional name eka-cesium. The research teams tried to discover and isolate this missing element. At least four premature advertisements of its discovery were made before it is indeed discovered.

Erroneous or incomplete discoveries

The first scientist to announce to have discovered the eka-cesium was the Russian chemist D.K. Dobroserdov. In 1925, this one observed traces of Radioactivité in a sample of Potassium (another metal alkaline), and concludes some with a contamination from the sample by eka-cesium. It then published a thesis about its predictions of the properties of this element, in which it baptized it russium , of the name of its country. A little later Dobroserdov started to concentrate on its career of teaching at the Polytechnic Institute of Odessa and did not continue its front work. However in 1934, professor MacPherson, of the the University of California, showed that this discovery was erroneous and related to problems of equipment.

In 1936, the Rumanian chemist Horia Hulubei and his French colleague Yvette Cauchois also studied pollucite, this time by using an apparatus of spectroscopy of rays-X high-resolution.

Perey baptized new the Isotope actinium-K , known at present under the name of francium-223. The francium was the last element existing in a state natural to be discovered, after the Rhénium in 1925.

Properties

The francium is the least stable of the elements lighter than the nobelium (Atomic number 102). Its the most stable Isotope, francium 223, has a Demi-vie lower than 22 minutes. As comparison the second least stable element, astatine, has a half-life for the most stable isotope lower than 8,5 hours. All the isotopes of francium disintegrate by forming Astate, Radium or Radon.

The francium is an alkaline whose chemical properties approach those of the Césium, it is the element having the equivalent mass largest (the second least electronegative element is the cesium, 0,79 on the scale of Pauling). So liquid francium could be obtained, it would have a Surface stress of 0.05092 [[Joule J]] [[meter m]] -2 with its Melting point, which is relatively weak.

The francium coprécipite with several salts of cesium, like the Perchlorate of cesium with which it forms small quantities of Perchlorate of francium. The coprecipitation can be used to isolate francium, by adapting the method of coprecipitation of cesium of Glendenin and Nelson. It precipitates in particular with cesium salts of which the Iodate, the picrate, the tartrate (it also precipitates with tartrate of Rubidium), the Chloroplatinate, and the Silicotungstate. It also precipitates with the acid silicotungstic and the Perchloric acid, which makes possible of other techniques of separation. The majority of francium salts are soluble in the Eau.

Applications

Because of its unstable character and of its scarcity, there is no commercial application of francium. It is used only in research, at the same time in the field of the Biologie and that of the atomic Physique. It was imagined that it can be a help for the diagnosis of cancerous diseases. The study of the Rayonnement S emitted by Ion S of francium-210 confined by Laser made it possible to obtain precise data as for the transitions between atomic energy levels. The experimental results are close to those predicted by the Quantum physics.

Abundance

Natural

The francium is the decay product of the alpha type of actinium 227 and exists with the state of traces in the ores of Uranium and Thorium. For this reason, it is about the second rarest element in the earth's crust after the Astate, which can be then isolated by exploiting a magneto-optical effect. Among the other methods of synthesis appear in particular the bombardment of radium atoms by Neutron S or that of thorium atoms by Proton S, or Deutérium or Hélium ionized. At present, it produced forever in significant amount.

See too

Related articles

• alkaline Metal

External bonds

• WebElements.com - Francium

• Los Alamos National Laboratory - Francium
• Stony Brook University Physics Dept.

References

the data of the table result from the site of Faculty from chemistry and technology of Split (Coatie).

Simple: Francium