Информация - Иностранные языки

Другие материалы по предмету Иностранные языки

Скачать Бесплатно!
Для того чтобы скачать эту работу.
1. Пожалуйста введите слова с картинки:

2. И нажмите на эту кнопку.

ated during purification, but emerged again out of the pure actinium-227. Various tests eliminated the possibility of the unknown element being thorium, radium, lead, bismuth, or thallium. The new product exhibited chemical properties of an alkali metal (such as coprecipitating with caesium salts), which led Perey to believe that it was element 87, caused by the alpha decay of actinium-227.[21] Perey then attempted to determine the proportion of beta decay to alpha decay in actinium-227. Her first test put the alpha branching at 0.6%, a figure which she later revised to 1%.[26]

Perey named the new isotope actinium-K (now referred to as francium-223)[21] and in 1946, she proposed the name catium for her newly discovered element, as she believed it to be the most electropositive cation of the elements. Irne Joliot-Curie, one of Pereys supervisors, opposed the name due to its connotation of cat rather than cation.[21] Perey then suggested francium, after France. This name was officially adopted by the International Union of Pure and Applied Chemistry in 1949,[4] becoming the second element after gallium to be named after France. It was assigned the symbol Fa, but this abbreviation was revised to the current Fr shortly thereafter.[27] Francium was the last element discovered in nature, rather than synthesized, following rhenium in 1925.[21] Further research into franciums structure was carried out by, among others, Sylvain Lieberman and his team at CERN in the 1970s and 1980s.[28]

5. Occurrence


5.1 Natural


Francium-223 is the result of the alpha decay of actinium-227 and can be found in trace amounts in uranium and thorium minerals.[3] In a given sample of uranium, there is estimated to be only one francium atom for every 11018 uranium atoms.[15] It is also calculated that there is at most 30 g of francium in the earths crust at any time.[29] This makes it the second rarest element in the crust after astatine.[4][15]


Figure. 0. This sample of uraninite contains about 100,000 atoms (3.3 ? 10?20 g) of 223Fr at any given time.[15]


5.2 Synthesized


Francium can be synthesized in the nuclear reaction 197Au + 18O → 210Fr + 5n

This process, developed by Stony Brook Physics, yields francium isotopes with masses of 209, 210, and 211,[31] which are then isolated by the magneto-optical trap (MOT).[30] The production rate of a particular isotope depends on the energy of the oxygen beam. An 18O beam from the Stony Brook LINAC creates 210Fr in the gold target with the nuclear reaction 197Au + 18O = 210Fr + 5n. The production required some time to develop and understand. It was critical to operate the gold target very close to its melting point and to make sure that its surface was very clean. The nuclear reaction imbeds the francium atoms deep in the gold target, and they must be removed efficiently. The atoms diffuse fast to the surface of the gold target and are released as ions. The francium ions are guided by electrostatic lenses until they land into a surface of hot yttrium and become neutral again. The francium is then injected into a glass bulb. A magnetic field and laser beams cool and confine the atoms. Although the atoms remain in the trap for only about 20 seconds before escaping (or decaying), a steady stream of fresh atoms replaces those lost, keeping the number of trapped atoms roughly constant for minutes or longer. Initially, about 1000 francium atoms were trapped in the experiment. This was gradually improved and is capable of trapping over 300,000 neutral atoms of francium a time. Although these are neutral "metallic" atoms ("francium metal"), they in a gaseous unconsolidated state. Enough francium is trapped that a video camera can capture the light given off by the atoms as they fluoresce. The atoms appear as a glowing sphere about 1 millimeter in diameter. This was the very first time that anyone had ever seen francium. The researchers can now make extremely sensitive measurements of the light emitted and absorbed by the trapped atoms, providing the first experimental results on various transitions between atomic energy levels in francium. Initial measurements show very good agreement between experimental values and calculations based on quantum theory. Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions.[26] Francium has not yet, as of 2009[update], been synthesized in amounts large enough to weigh.[3][4][15][32]


Figure. 1. Neutral francium atoms can be trapped in the MOT using a magnetic field and laser beams.[30]

6. Isotopes


There are 34 known isotopes of francium ranging in atomic mass from 199 to 232.[3] Francium has seven metastable nuclear isomers.[3] Francium-223 and francium-221 are the only isotopes that occur in nature, though the former is far more common.[33]

Francium-223 is the most stable isotope with a half-life of 21.8 minutes,[3] and it is highly unlikely that an isotope of francium with a longer half-life will ever be discovered or synthesized.[26] Francium-223 is the fifth product of the actinium decay series as the daughter isotope of actinium-227.[17] Francium-223 then decays into radium-223 by beta decay (1149 keV decay energy), with a minor (0.006%) alpha decay path to astatine-219 (5.4 MeV decay energy).[34]

Francium-221 has a half-life of 4.8 minutes.[3] It is the ninth product of the neptunium decay series as a daughter isotope of actinium-225.[17] Francium-221 then decays into astatine-217 by alpha decay (6.457 MeV decay energy).[3]

The least stable ground state isotope is francium-215, with a half-life of 0.12 μs. (9.54 MeV alpha decay to astatine-211):[3] Its metastable isomer, francium-215m, is less stable still, with a half-life of only 3.5 ns.[35]

7. Conclusion


As an individual representative of the periodic table of chemical elements Dmitry Ivanovich Mendeleyev, the element has unique chemical and physical properties

Element is of great economic importance and plays a major role in world culture

8. References


  1. Actually the least unstable isotope, Fr-223
  2. Some synthetic elements, like technetium, have later been found in nature.
  3. a b c d e f g h i j k l CRC Handbook of Chemistry and Physics, 4, CRC, 2006, p. 12, ISBN 0-8493-0474-1
  4. a b c d e f g Price, Andy (2004-12-20). "Francium".
  5. Winter, Mark. "Electron Configuration". Francium. The University of Sheffield.
  6. Kozhitov, L. V.; Koltsov, V. B.; Koltsov, A. V. (2003). "Evaluation of the Surface Tension of Liquid Francium". Inorganic Materials 39 (11): 11381141. doi:10.1023/A:1027389223381.
  7. Pauling, Linus (1960). The Nature of the Chemical Bond (3rd Edn.). Cornell University Press. pp. 93.
  8. Allred, A. L. (1961). "Electronegativity values from thermochemical data". J. Inorg. Nucl. Chem. 17 (34): 215221. doi:10.1016/0022-1902(61)80142-5.
  9. Andreev, S.V.; Letokhov, V.S.; Mishin, V.I., (1987). "Laser resonance photoionization spectroscopy of Rydberg levels in Fr". Physical Review Letters 59: 127476. doi:10.1103/PhysRevLett.59.1274.
  10. Hyde, E. K. (1952). "Radiochemical Methods for the Isolation of Element 87 (Francium)". J. Am. Chem. Soc. 74 (16): 41814184. doi:10.1021/ja01136a066.
  11. 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.
  12. Maddock, A. G. (1951). "Radioactivity of the heavy elements". Q. Rev., Chem. Soc. 3: 270314. doi:10.1039/QR9510500270.
  13. Winter, Mark. "Uses". Francium. The University of Sheffield.
  14. Bentor, Yinon. "Chemical - Francium".
  15. a b c d e f Emsley, John (2001). Natures Building Blocks. Oxford: Oxford University Press. pp. 151153. ISBN 0-19-850341-5.
  16. Gagnon, Steve. "Francium". Jefferson Science Associates, LLC.
  17. a b c Considine, Glenn D., ed (2005). Chemical Elements, in Van Nostrands Encyclopedia of Chemistry. New York: Wiley-Interscience. p. 332. ISBN 0-471-61525-0.
  18. Haverlock, TJ (2003). "Selectivity of calix[4]arene-bis(benzocrown-6) in the complexation and transport of francium ion". J Am Chem Soc 125: 1126-7. doi:10.1021/ja0255251. PMID 12553788.
  19. Gomez, E; Orozco, L A, and Sprouse, G D (2005-11-07). "Spectroscopy with trapped francium: advances and perspectives for weak interaction studies". Rep. Prog. Phys. 69 (1): 79118. doi:10.1088/0034-4885/69/1/R02.
  20. Peterson, I (1996-05-11). "Creating, cooling, trapping francium atoms". Science News 149 (19): 294.
  21. a b c d e Adloff, Jean-Pierre; Kaufman, George B. (2005-09-25). Francium (Atomic Number 87), the Last Discovered Natural Element. The Chemical Educator 10 (5). Retrieved on 2007-03-26.
  22. a b c d e Fontani, Marco (2005-09-10). "The Twilight of the Naturally-Occurring Elements: Moldavium (Ml),