Francium formerly known as eka-caesium and actinium K, is a chemical element that has the symbol Fr and atomic number 87. It has one of the lowest electronegativities of all known elements. Francium is a highly radioactive metal that decays into astatine, radium, and radon. As an alkali metal, it has one valence electron.
Francium was discovered by Marguerite Perey in France (from which the element takes its name) in 1939. It was the last element discovered in nature, rather than synthesized. Trace amounts found can be found in uranium and thorium ores, where the isotope francium-223 continually forms and decays, the other isotopes are entirely synthetic. The largest amount ever collected of any isotope was a cluster of about 10,000 atoms (of francium-210) created as an ultracold gas at Stony Brook in 1997.
Francium is the least stable of the naturally occurring elements: its most stable isotope, francium-223, has a maximum half-life of only 22 minutes. In contrast, astatine, the second-least stable naturally occurring element, has a maximum half-life of 8.5 hours. Francium is also less stable than all synthetic elements up to element 105.
Francium is an alkali metal whose chemical properties most resemble those of cesium. A very heavy element with a single valence electron, it has the highest equivalent weight of any element. Liquid francium—if such a substance were to be created—should have a surface tension of 0.05092 N/m at its melting point. Francium’s melting point was claimed to have been calculated to be around 27 °C (80 °F, 300 K). However, the melting point is uncertain because of the element’s radioactivity. This melting point may have been in limited precision, or so much heat produced from radioactivity that its calculated melting point may have been overestimated. However, the melting point of francium is estimated to be about 22 °C (71 °F, 295 K), based from the periodic trends in melting points with other alkali metals. Also the boiling point may have been overestimated at around 677 °C (1250 °F, 950 K). Based from the periodic trends with other alkali metals, the boiling point of francium is estimated to be between 660 to 665 °C (1220 to 1230 °F, 935 to 940 K). Because radioactive elements give off heat, francium would almost certainly be a liquid if enough were to be produced.
Linus Pauling estimated the electronegativity of francium at 0.7 on the Pauling scale, the same as cesium; the value for cesium has since been refined to 0.79, although there are no experimental data to allow a refinement of the value for francium. Francium has a slightly higher ionisation energy than cesium, 392.811(4) kJ/mol as opposed to 375.7041(2) kJ/mol for cesium, as would be expected from relativistic effects, and this would imply that cesium is the less electronegative of the two.
Francium coprecipitates with several cesium salts, such as cesium perchlorate, which results in small amounts of francium perchlorate. This coprecipitation can be used to isolate francium, by adapting the radiocesium coprecipitation method of Glendenin and Nelson. It will additionally coprecipitate with many other cesium salts, including the iodate, the picrate, the tartrate (also rubidium tartrate), the chloroplatinate, and the silicotungstate. It also coprecipitates with silicotungstic acid, and with perchloric acid, without another alkali metal as a carrier, which provides other methods of separation. Nearly all francium salts are water-soluble.
The base value of each unit of ranges between 5 and 35Ð per unit, with up to 3 units being found at any one time.
Presence on Mars: Very Rare
|Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6|
|Group 3|||Antimony | Astatine | Barium | Bismuth | Cesium | Francium | Hafnium | Indium | Iodine | Iridium | Lanthanum | Lead | Mercury | |Osmium | Platinum | Polonium | Radium | Radon | Rhenium | Tantalum | Tellurium | Thallium | Tin | Tungsten | Xenon||