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Francium is a chemical element with symbol Fr and atomic number 87. It used to be known as eka-caesium and actinium K.〔Actually the least unstable isotope, francium-223〕 It is the second-least electronegative element, behind only caesium. Francium is a highly radioactive metal that decays into astatine, radium, and radon. As an alkali metal, it has one valence electron. Bulk francium has never been viewed. Because of the general appearance of the other elements in its periodic table column, it is assumed that francium would appear as a highly reflective metal, if enough could be collected together to be viewed as a bulk solid or liquid. Obtaining such a sample is highly improbable, since the extreme heat of decay (the half-life of its longest-lived isotope is only 22 minutes) would immediately vaporize any viewable quantity of the element. Francium was discovered by Marguerite Perey in France (from which the element takes its name) in 1939. It was the last element first discovered in nature, rather than by synthesis.〔Some synthetic elements, like technetium and plutonium, have later been found in nature.〕 Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 continually forms and decays. As little as 20–30 g (one ounce) exists at any given time throughout the Earth's crust; the other isotopes (except for francium-221) are entirely synthetic. The largest amount produced in the laboratory was a cluster of more than 300,000 atoms.〔 ==Characteristics== Francium is the most unstable of the naturally occurring elements: its most stable isotope, francium-223, has a half-life of only 22 minutes. In contrast, astatine, the second-least stable naturally occurring element, has a half-life of 8.5 hours.〔 All isotopes of francium decay into astatine, radium, or radon. Francium is also less stable than all synthetic elements up to element 105. Francium is an alkali metal whose chemical properties mostly resemble those of caesium.〔 A heavy element with a single valence electron, it has the highest equivalent weight of any element.〔 Liquid francium—if created—should have a surface tension of 0.05092 N/m at its melting point. Francium's melting point was calculated to be around 27 °C (80 °F, 300 K).〔 The melting point is uncertain because of the element's extreme rarity and radioactivity. Thus, the estimated boiling point value of 677 °C (1250 °F, 950 K) is also uncertain. Linus Pauling estimated the electronegativity of francium at 0.7 on the Pauling scale, the same as caesium; the value for caesium has since been refined to 0.79, but there are no experimental data to allow a refinement of the value for francium. Francium has a slightly higher ionization energy than caesium, 392.811(4) kJ/mol as opposed to 375.7041(2) kJ/mol for caesium, as would be expected from relativistic effects, and this would imply that caesium is the less electronegative of the two. Francium should also have a higher electron affinity than caesium and the Fr− ion should be more polarizable than the Cs− ion. The CsFr molecule is predicted to have francium at the negative end of the dipole, unlike all known heterodiatomic alkali metal molecules. Francium superoxide (FrO2) is expected to have a more covalent character than its lighter congeners; this is attributed to the 6p electrons in francium being more involved in the francium–oxygen bonding.〔 Francium coprecipitates with several caesium salts, such as caesium perchlorate, which results in small amounts of francium perchlorate. This coprecipitation can be used to isolate francium, by adapting the radiocaesium coprecipitation method of Glendenin and Nelson. It will additionally coprecipitate with many other caesium 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.〔E. N K. Hyde ''Radiochemistry of Francium'',Subcommittee on Radiochemistry, National Academy of Sciences-National Research Council; available from the Office of Technical Services, Dept. of Commerce, 1960.〕 Nearly all francium salts are water-soluble. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Francium」の詳細全文を読む スポンサード リンク
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