francium

francium 化学特性,用途語,生産方法

• 物理的性質

  Not a great deal is known about francium’s properties, but some measurements of its moststable isotope have been made. Its melting point is 27°C and its boiling point is 677°C, butits density is unknown. It is assumed to have a +1 oxidation state (similar to all the otheralkali metals)Francium’s atoms are the largest and heaviest of the alkali metals in group 1 (IA). It islocated just below cesium on the periodic table, and thus it is assumed to be an extremelyreactive reducing agent even though it is the most scarce of the alkali metals. Its most stableisotope (Fr-223) exists for about 21 or 22 minutes. No one has figured out how to refinefrancium from natural minerals (ores) because the atoms of the most stable isotope found innature (Fr-223) are scattered very thinly over the Earth’s crust. All of the other 30 isotopes areproduced for study by nuclear decay of other radioactive elements.

• 同位体

  There are no stable isotopes of francium found on Earth. All of its 33 isotopes(ranging from Fr-201 to Fr-232) are radioactive; therefore, the one with the longest halflife of about 20 minutes (Fr-223) is the one used to determine its atomic weight. Fr-223is the only radioisotope of francium that is found naturally as a decay product from otherunstable elements.

• 名前の由来

  In 1939 Marguerite Perey (1909–1975), a French physicist who worked for the Curie institute in Paris, named the newly discovered element after her country—France.

• 天然物の起源

  Only about one ounce of natural francium exists in the Earth’s crust. All the other isotopesof francium are artificially produced in very small amounts (just a few atoms at a time) thatexist for a few seconds to minutes.Two methods to secure very small samples of francium for examination use the decay processes of other radioactive elements. One is to bombard thorium with protons. The second isto start with radium in an accelerator, where, through a series of decay processes, the radium isconverted to actinium, which in turn rapidly decays into thorium, and finally, thorium decaysnaturally into francium.

• 特性

  Francium is the only element from atomic numbers 1 to 92 that has a half-life of less than30 minutes. It is the only known element with more identified isotopes than identified compounds. All samples of francium available for study are produced artificially.

• 来歴

  Discovered in 1939 by Mlle. Marguerite Perey of the Curie Institute, Paris. Francium, the heaviest known member of the alkali metal series, occurs as a result of an alpha disintegration of actinium. It can also be made artificially by bombarding thorium with protons. While it occurs naturally in uranium minerals, there is probably less than an ounce of francium at any time in the total crust of the earth. It has the highest equivalent weight of any element, and is the most unstable of the first 101 elements of the periodic system. Thirtysix isotopes and isomers of francium are recognized. The longest lived ²²³Fr(Ac, K), a daughter of ²²⁷Ac, has a half-life of 21.8 min. This is the only isotope of francium occurring in nature. Because all known isotopes of francium are highly unstable, knowledge of the chemical properties of this element comes from radiochemical techniques. No weighable quantity of the element has been prepared or isolated. The chemical properties of francium most closely resemble cesium. In 1996, researchers Orozco, Sprouse, and co-workers at the State University of New York, Stony Brook, reported that they had produced francium atoms by bombarding 18O atoms at a gold target heated almost to its melting point. Collisions between gold and oxygen nuclei created atoms of francium-210 which had 87 protons and 123 neutrons. This team reported they had generated about 1 million francium-210 ions per second and held 1000 or more atoms at a time for about 20 secs in a magnetic trap they had devised before the atoms decayed or escaped. Enough francium was trapped so that a videocamera could capture the light given off by the atoms as they fluoresced. A cluster of about 10,000 francium atoms appeared as a glowing sphere about 1 mm in diameter. It is thought that the francium atoms could serve as miniature laboratories for probing interactions between electrons and quarks.

• 使用

  Given that all isotopes of francium are radioactive with relatively short half-lives, thereare few practical uses for it—except as a source of radiation to study the radioactive decayprocess.

• 定義

  A radioactive element of the alkali-metal group. It is found on Earth only as a short-lived product of radioactive decay, occurring in uranium ores in minute quantities. A large number of radioisotopes of francium are known.

• 危険性

  Because all of francium’s isotopes are radioactive, they are a potential hazard to humans andshould be handled only by experienced laboratory personnel. On the other hand, the scarcityof these isotopes makes it almost impossible to consider them harmful in our everyday lives.

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