Francium

Representation of distribution by layers of the electrons of the French.

Francium is a chemical element whose symbol is Fr and its atomic number is 87. Its electronegativity is the lowest known and it is the second least abundant element in nature (the first is astatine). Francium is a highly radioactive and reactive alkali metal that decays to generate astatine, radium, and radon. Like the rest of the alkali metals, it only has one electron in its valence shell.

Marguerite Perey discovered this element in 1939. Francium was the last chemical element discovered in nature before it was synthesized. Outside the laboratory, francium is extremely rare, being found in trace amounts in uranium and thorium ores, where ²²³Fr is continually forming and disintegrating. The amount of ²²³Fr in the earth's crust at any given time may not exceed 30 grams; the rest of the isotopes are synthetic. The largest amount of any of its isotopes recovered was a cluster of 10 billion atoms (of ²¹⁰Fr) synthesized as an ultracold gas at Stony Brook in 1996.

Physical and chemical properties

Francium is less stable than any element lighter than nobelium (element 102); its most stable isotope, ²²³Fr, has a half-life of less than 22 minutes. Astatine, the next least stable element, has a maximum half-life of 8.5 hours. All francium isotopes decay to produce astatine, radium, and radon.

Francium is an alkali metal with chemical properties similar to cesium. Since it is a very heavy element with only one valence electron, it has the largest equivalent weight of all chemical elements. Francium has the lowest electronegativity of all known elements, with a value of 0.7 on the Pauling scale. Cesium follows with a value of 0.79. Liquid francium, assuming it could be obtained, would have a surface tension of 0.05092 J·m^–2 at the melting point.

Francium coprecipitates, along with many cesium salts, such as cesium perchlorate, forming small amounts of francium perchlorate. This coprecipitation can be used to isolate francium, adapting the Glendenin and Nelson method of radiocesium precipitation. It also coprecipitates with other cesium salts such as iodate, picrate, tartrate (also rubidium tartrate), chloroplatinate, and silicotungstate. Other coprecipitations occur with silicotungstic acid and perchloric acid, without the need for another alkali metal to be present as a carrier, making other separation methods possible for francium. Almost all francium salts are soluble in water.

Francium Applications

There are no commercial applications for francium due to its scarcity and instability. It has only been used in research, both in the field of biology and atomic structure. It was thought that francium could be used as an aid in the diagnosis of cancer-related diseases; however, this application has ultimately been deemed impractical.

Francium's ability to synthesize, trap, and cool it, along with its relatively simple atomic structure, have made it a subject of specialized spectroscopy experimentation. These experiments have led to more specific information on the energy levels and coupling constants between subatomic particles. Studies of the light emitted by ²¹⁰Fr ions trapped by lasers have yielded precise data on transitions between atomic energy levels. These experimental results have been found to be quite similar to those predicted by Quantum Theory.

History

As early as 1870, chemists thought there must be an alkali metal beyond cesium, with an atomic number of 87. It was referred to by the provisional name eka-cesium. Investigative teams attempted to locate and isolate the item in question, and there are at least four false public announcements on record claiming to have discovered the item before it was actually discovered.

Wrong and incomplete discoveries

Russian chemist D. K. Dobroserdov was the first scientist to claim to have discovered eka-cesium. In 1925, he observed weak radioactivity in a sample of potassium, another alkali metal, and concluded that eka-cesium contaminated the sample. He published a thesis on his predictions of the properties of eka -cesium, in which he named the element russia, after its country of origin. Shortly thereafter, he began to focus on his teaching career at the Odessa Polytechnic Institute, completely abandoning his efforts to isolate eka-cesium.

The following year, in 1926, English chemists Gerald J. F. Druce and Frederick H. Loring analyzed an X-ray radiograph of manganese(II) sulfate. They observed spectral lines that they believed to belong to eka-cesium. They announced the discovery of element 87 and proposed the name alkalinium for what would be the heaviest alkali metal.

In 1930, Professor Fred Allison of the Alabama Polytechnic Institute announced that he had discovered element 87 by analyzing pollucite and lepidolite using his magneto-optical machine. Allison proposed that he be christened virginio, after his home state of Virginia, as well as that the symbols Vi and Vm. In 1934, however, UC Berkeley Professor MacPherson disproved the effectiveness of Allison's device and the validity of his false discovery.

In 1936, the Romanian chemist Horia Hulubei and his French colleague Yvette Cauchois also analyzed pollucite, this time using their high-resolution X-ray apparatus. They observed several faint emission lines that they assumed were due to element 87. Hulubei and Cauchois announced their discovery and proposed the name Moldavian, with the symbol Ml, after Moldavia., the now independent Romanian province where they carried out their work. In 1937, Hulubei's work was criticized by the American physicist F. H. Hirsh Jr., who rejected the Romanian chemist's research methods. Hirsh was convinced that eka-cesium could not be found in nature, and that the lines Hulubei had observed were due to mercury or bismuth. The Romanian chemist, however, insisted that his X-ray apparatus and his methods were too precise to make such mistakes. Jean Baptiste Perrin, Nobel Prize winner and Hulubei's mentor, endorsed Moldavian as the true eka-cesium instead of Marguerite Perey's newly discovered francium. Perey, however, continually criticized Hulubei's work until she was credited as the sole discoverer of element 87.

Perey analysis

eka-cesium was really discovered in 1939 by Marguerite Perey, of the Curie Institute in Paris (France), when she purified a sample of ²²⁷Ac that had an energy decay rate of 220 keV. However, Perey warned particles with an energy level below 80 keV to decay. She thought this activity must be caused by an unidentified previous decay product, a product separated during purification, but re-emerging from pure ²²⁷ Ac. Several tests eliminated the possibility that it was thorium, radium, lead, bismuth or thallium, thus being an unknown element. The new product showed chemical properties typical of an alkali metal (such as coprecipitation with cesium salts), which led Perey to believe that it was dealing with element 87, generated by the alpha decay of ²²⁷Ac. Perey then attempted to determine the ratio between the beta decay and alpha decay of ²²⁷Ac. The first test of it indicated that the alpha disintegration reached 0.6%, a result that was revised until reaching the value of 1%.

Perey named the new isotope actinium K, which referred to what we now know as ²²³Fr, and in 1946, he proposed the name catio for his newly discovered element, as he believed it to be the most electropositive cation of all the chemical elements. Irène Joliot-Curie, one of Perey's supervisors, objected to that name as it seemed to refer more to "cat" (cat in English) than a cation.So Perey suggested the name francium as a tribute to the country where he discovered it. This name was officially adopted by the International Union of Chemists in 1949, and was assigned the symbol Fa, but this abbreviation was changed to Fr soon after. Francium is the last of the naturally occurring elements to be discovered, the first being rhenium, in 1925. Further investigations into the structure of francium were carried out by carried out by Sylvain Lieberman and his team at CERN in the 1970s and 1980s, among others.

Abundance

This sample of uraninite contains about 100,000 atoms (3.3×10^{- 20}(g) Franciscan-223 at any given time.

Natural

²²³Fr results from the alpha decay of ²²⁷Ac and can be found in trace amounts in uranium and thorium ores. In a sample of uranium, it is estimates that there is only one francium atom for every 1×10¹⁸ uranium atoms. After astatine, francium is the least abundant element in the Earth's crust.

Synthesized

Francium can be synthesized in the nuclear reaction:

¹⁹⁷Au + ¹⁸O → ²¹⁰Fr + 5n.

This process, developed by Stony Brook Physics, generates francium isotopes with masses 209, 210, and 211, which can be isolated in a magneto-optical trap (MOT). The rate of production of a particular isotope depends on the energy of the oxygen beam. The Stony Brook LINAC beam produces ²¹⁰Fr on the gold target with the nuclear reaction ¹⁹⁷Au + ¹⁸O → ²¹⁰Fr + 5n. Production takes some time to develop and understand. This is critical to operating the gold target very close to its melting point and to ensure that its surface is very clean. The nuclear reaction deeply embeds the francium atoms in the gold target, and it must be removed efficiently. The atoms diffuse rapidly onto the surface of the gold target and are released as ions, however this does not happen all the time. The francium ions are guided by the electrostatic lenses until they land on a hot yttrium surface and become neutral again. Then the francium is injected into a glass ampoule. Laser beams and a magnetic field cool and confine atoms. Although the atoms remain in the trap for only about 20 seconds before they escape (or decay), a constant stream of fresh atoms replaces those lost, keeping the number of trapped atoms roughly constant for several minutes or longer. Initially, around 1000 francium atoms were trapped in the experiment. This was gradually improved and the facility is capable of trapping over 300,000 neutral francium atoms at a time. Although these are neutral "metallic" ("francium metals"), are in a state that is not considered gaseous. Enough Francium is trapped that the light emitted by the atoms can be captured by a video camera, since they fluoresce. The atoms appear as a shiny sphere about 1 millimeter in diameter. This was the first time anyone had seen francium. The researchers can now make very sensitive measurements of the light emitted and absorbed by the trapped atoms, thus providing the first experimental results on various transitions between atomic energy levels in francs. The initial measurements show very good agreement between the experimental values and the calculations based on quantum theory. Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions. Francium has not been, and probably will not be, synthesized in quantities large enough to be heavy.

Isotopes

Radioactive disintegration diagram in which you can see how the france is part of the actinium series and the plutonium series. (Pulsar image to see it increased).

Thirty-four isotopes of francium are known, ranging in atomic mass from 199 to 232. Francium has seven metastable nuclear isomers. ²²³Fr and ²²¹Fr are the only isotopes that occur in nature, although the former is much more common than the latter.

²²³Fr is the most stable isotope with a half-life of 21.8 minutes, and it is highly unlikely that a francium isotope with a longer period will ever be discovered or synthesized. ²²³Fr is the fifth product of the actinium decay series, from ²²⁷Ac. ²²³Fr then decays to generating ²²³Ra by beta decay (decay energy: 1149 keV), with a minor route (0.006%) of alpha decay generating ²¹⁹At (decay energy: 5 0.4 MeV).

²²¹Fr has a half-life of 4.8 minutes. It is the ninth product of the plutonium decay series, originating from ²²⁵Ac. The ²²¹Fr then decays to generate ²¹⁷At by alpha decay (decay energy: 6.457 MeV).

The least stable ground-state isotope is ²¹⁵Fr, with a half-life of 0.12 μs (decay energy up to ²¹¹At: 9.54 MeV). Its metastable isomer, ^215mFr, is even less stable, with a half-life of 3.5 ns.