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Francium, 87Fr
Francium
Pronunciation/ˈfrænsiəm/ ​(FRAN-see-əm)
Mass number[223]
Francium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Cs
↑
Fr
↓
(Uue)
radon ← francium → radium
Atomic number (Z)87
Groupgroup 1: hydrogen and alkali metals
Periodperiod 7
Block  s-block
Electron configuration[Rn] 7s1
Electrons per shell2, 8, 18, 32, 18, 8, 1
Physical properties
Phase at STPsolid
Melting point300 K ​(27 °C, ​81 °F)
Boiling point950 K ​(677 °C, ​1251 °F)
Density (near r.t.)2.458 g/cm3 (estimated)[1]
Vapor pressure (extrapolated)
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 404 454 519 608 738 946
Atomic properties
Oxidation statescommon: +1
ElectronegativityPauling scale: >0.79
Ionization energies
  • 1st: 393 kJ/mol[2]
Covalent radius260 pm (extrapolated)
Van der Waals radius348 pm (extrapolated)
Other properties
Natural occurrencefrom decay
Crystal structure ​body-centered cubic (bcc)
(extrapolated)[1]
Lattice constant
Body-centered cubic crystal structure for francium
a = 670.4 pm (estimated)[1]
Thermal conductivity15 W/(m⋅K) (extrapolated)
Electrical resistivity3 µΩ⋅m (calculated)
Magnetic orderingParamagnetic
CAS Number7440-73-5
History
Namingafter France, homeland of the discoverer
Discovery and first isolationMarguerite Perey (1939)
Isotopes of francium
Main isotopes[3] Decay
abun­dance half-life (t1/2) mode pro­duct
212Fr synth 20.0 min β+ 212Rn
α 208At
221Fr trace 4.8 min α 217At
222Fr synth 14.2 min β− 222Ra
223Fr trace 22.00 min β− 223Ra
α 219At
 Category: Francium
| references

Francium (Template:PronEng), formerly known as eka-caesium and actinium K,[4] is a chemical element that has the symbol Fr and atomic number 87. It has the lowest electronegativity of all known elements, and is the second rarest naturally occurring element (after astatine). 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.[5] 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 are entirely synthetic. The largest amount ever collected of any isotope was a cluster of 10,000 atoms (of francium-210) created as an ultracold gas at Stony Brook in 1997.[6]

Characteristics

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.[7] All isotopes of francium decay into either astatine, radium, or radon.[7] Francium is also less stable than all synthetic elements up to element 105.[6]

Francium is an alkali metal whose chemical properties most resemble those of caesium.[6] A very heavy element with a single valence electron,[8] it has the highest equivalent weight of any element.[6] Liquid francium — if such a substance were to be created — should have a surface tension of 0.05092 N/m at its melting point.[9] 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 extreme rarity and 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.[10] 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.[citation needed] 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).[citation needed] Because radioactive elements give off heat Francium would almost certainly be a liquid if enough visible Francium were to be produced.

Linus Pauling estimated the electronegativity of francium at 0.7 on the Pauling scale, the same as caesium;[11] the value for caesium has since been refined to 0.79, although there are no experimental data to allow a refinement of the value for francium.[12] Francium has a slightly higher ionisation energy than caesium,[13] 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 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.[14][15] Nearly all francium salts are water-soluble.[16]

Applications

Due to its instability and rarity, there are no commercial applications for francium.[17][18][19][20][21] It has been used for research purposes in the fields of biology [22] and of atomic structure. Its use as a potential diagnostic aid for various cancers has also been explored,[7] but this application has been deemed impractical.[19]

Francium's ability to be synthesized, trapped, and cooled, along with its relatively simple atomic structure have made it the subject of specialized spectroscopy experiments. These experiments have led to more specific information regarding energy levels and the coupling constants between subatomic particles.[23] Studies on the light emitted by laser-trapped francium-210 ions have provided accurate data on transitions between atomic energy levels which are fairly similar to those predicted by quantum theory.[24]

History

As early as 1870, chemists thought that there should be an alkali metal beyond caesium, with an atomic number of 87.[7] It was then referred to by the provisional name eka-caesium.[25] Research teams attempted to locate and isolate this missing element, and at least four false claims were made that the element had been found before an authentic discovery was made.

Erroneous and incomplete discoveries

Russian chemist D. K. Dobroserdov was the first scientist to claim to have found eka-caesium, or francium. In 1925, he observed weak radioactivity in a sample of potassium, another alkali metal, and concluded that eka-caesium was contaminating the sample.[26] He then published a thesis on his predictions of the properties of eka-caesium, in which he named the element russium after his home country.[27] Shortly thereafter, Dobroserdov began to focus on his teaching career at the Polytechnic Institute of Odessa, and he did not pursue the element further.[26]

The following year, English chemists Gerald J. F. Druce and Frederick H. Loring analyzed X-ray photographs of manganese(II) sulfate.[27] They observed spectral lines which they presumed to be of eka-caesium. They announced their discovery of element 87 and proposed the name alkalinium, as it would be the heaviest alkali metal.[26]

In 1930, Fred Allison of the Alabama Polytechnic Institute claimed to have discovered element 87 when analyzing pollucite and lepidolite using his magneto-optical machine. Allison requested that it be named virginium after his home state of Virginia, along with the symbols Vi and Vm.[27][28] In 1934, however, H.G. MacPherson of UC Berkeley disproved the effectiveness of Allison's device and the validity of this false discovery.[29]

In 1936, Romanian chemist Horia Hulubei and his French colleague Yvette Cauchois also analyzed pollucite, this time using their high-resolution X-ray apparatus.[26] They observed several weak emission lines, which they presumed to be those of element 87. Hulubei and Cauchois reported their discovery and proposed the name moldavium, along with the symbol Ml, after Moldavia, the Romanian province where they conducted their work.[27] In 1937, Hulubei's work was criticized by American physicist F. H. Hirsh Jr., who rejected Hulubei's research methods. Hirsh was certain that eka-caesium would not be found in nature, and that Hulubei had instead observed mercury or bismuth X-ray lines. Hulubei, however, insisted that his X-ray apparatus and methods were too accurate to make such a mistake. Because of this, Jean Baptiste Perrin, Nobel Prize winner and Hulubei's mentor, endorsed moldavium as the true eka-caesium over Marguerite Perey's recently discovered francium. Perey, however, continuously criticized Hulubei's work until she was credited as the sole discoverer of element 87.[26]

Perey's analysis

Eka-caesium was discovered in 1939 by Marguerite Perey of the Curie Institute in Paris, France when she purified a sample of actinium-227 which had been reported to have a decay energy of 220 keV. However, Perey noticed decay particles with an energy level below 80 keV. Perey thought this decay activity might have been caused by a previously unidentified decay product, one which was separated 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.[25] 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%.[30]

Perey named the new isotope actinium-K (now referred to as francium-223)[25] 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. Irène Joliot-Curie, one of Perey's supervisors, opposed the name due to its connotation of cat rather than cation.[25] Perey then suggested francium, after France. This name was officially adopted by the International Union of Pure and Applied Chemistry in 1949,[7] 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.[31] Francium was the last element discovered in nature, rather than synthesized, following rhenium in 1925.[25] Further research into francium's structure was carried out by, among others, Sylvain Lieberman and his team at CERN in the 1970s and 1980s.[32]

This sample of uraninite contains about 100,000 atoms (3.3×10−20 g) of francium-223 at any given time.[19]

Occurrence

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.[6] In a given sample of uranium, there is estimated to be only one francium atom for every 1×1018 uranium atoms.[19] It is also calculated that there is at most 30 g of francium in the earth's crust at any time.[33] This makes it the second rarest element in the crust after astatine.[7][19]

Synthesized

In the MOT, a magnetic field is created by the copper solenoids. Neutral francium atoms enter the glass bulb from the left and are trapped by lasers.[34]

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,[35] which are then isolated by the magneto-optic trap (MOT).[34] 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 retroreflected 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.[30] Francium has not yet, as of 2009, been synthesized in amounts large enough to weigh.[6][7][19][36]

Isotopes

Main article: Isotopes of francium

There are 34 known isotopes of francium ranging in atomic mass from 199 to 232.[6] Francium has seven metastable nuclear isomers.[6] Francium-223 and francium-221 are the only isotopes that occur in nature, though the former is far more common.[37]

Francium-223 is the most stable isotope with a half-life of 21.8 minutes,[6] and it is highly unlikely that an isotope of francium with a longer half-life will ever be discovered or synthesized.[30] Francium-223 is the fifth product of the actinium decay series as the daughter isotope of actinium-227.[21] 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).[38]

Francium-221 has a half-life of 4.8 minutes.[6] It is the ninth product of the neptunium decay series as a daughter isotope of actinium-225.[21] Francium-221 then decays into astatine-217 by alpha decay (6.457 MeV decay energy).[6]

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):[6] Its metastable isomer, francium-215m, is less stable still, with a half-life of only 3.5 ns.[39]

See also

Notes

  1. ^ a b c Arblaster, John W. (2018). Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International. ISBN 978-1-62708-155-9.
  2. ^ ISOLDE Collaboration, J. Phys. B 23, 3511 (1990) (PDF online)
  3. ^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  4. ^ Actually the least unstable isotope, Fr-223
  5. ^ Some synthetic elements, like technetium, have later been found in nature.
  6. ^ a b c d e f g h i j k l CRC Handbook of Chemistry and Physics, vol. 4, CRC, 2006, p. 12, 0-8493-0474-1 Cite error: The named reference "CRC2006" was defined multiple times with different content (see the help page).
  7. ^ a b c d e f g Price, Andy (2004-12-20). "Francium". Retrieved 2007-03-25.
  8. ^ Winter, Mark. "Electron Configuration". Francium. The University of Sheffield. Retrieved 2007-04-18.
  9. ^ Kozhitov, L. V. (2003-02-21). "Evaluation of the Surface Tension of Liquid Francium". Inorganic Materials. 39 (11). Springer Science & Business Media B.V.: 1138–1141. doi:10.1023/A:1027389223381. Retrieved 2007-04-14. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ "What are the characteristics and properties of francium?". Yahoo! Answers. 2008. Retrieved 2009-01-04.
  11. ^ Pauling, Linus (1960). The Nature of the Chemical Bond (3rd Edn.). Cornell University Press. p. 93.
  12. ^ Allred, A. L. (1961). "Electronegativity values from thermochemical data". J. Inorg. Nucl. Chem. 17 (3–4): 215–221. doi:10.1016/0022-1902(61)80142-5. {{cite journal}}: Italic or bold markup not allowed in: |journal= (help)
  13. ^ Andreev, S.V.; Letokhov, V.S.; Mishin, V.I., (1987). "Laser resonance photoionization spectroscopy of Rydberg levels in Fr". Phys. Rev. Lett. 59: 1274–76. doi:10.1103/PhysRevLett.59.1274. {{cite journal}}: Italic or bold markup not allowed in: |journal= (help)CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  14. ^ Hyde, E. K. (1952). "Radiochemical Methods for the Isolation of Element 87 (Francium)". J. Am. Chem. Soc. 74 (16): 4181–4184. doi:10.1021/ja01136a066.
  15. ^ 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.
  16. ^ A. G. Maddock. Radioactivity of the heavy elements. Q. Rev., Chem. Soc., 1951, 3, 270–314. doi:10.1039/QR9510500270
  17. ^ Winter, Mark. "Uses". Francium. The University of Sheffield. Retrieved 2007-03-25.
  18. ^ Bentor, Yinon. "Chemical Element.com - Francium". Retrieved 2007-03-25.
  19. ^ a b c d e f Emsley, John (2001). Nature's Building Blocks. Oxford: Oxford University Press. pp. 151–153. ISBN 0-19-850341-5.
  20. ^ Gagnon, Steve. "Francium". Jefferson Science Associates, LLC. Retrieved 2007-04-01.
  21. ^ a b c Considine, Glenn D., ed. (2005), "Chemical Elements", Van Nostrand's Encyclopedia of Chemistry, New York: Wylie-Interscience, p. 332, ISBN 0-471-61525-0
  22. ^ Haverlock, TJ (2003). "Selectivity of calix[4]arene-bis(benzocrown-6) in the complexation and transport of francium ion". J Am Chem Soc. 125(5): 1126–7. doi:10.1021/ja0255251. PMID 12553788. {{cite journal}}: Unknown parameter |month= ignored (help)
  23. ^ Gomez, E (2005-11-07). "Spectroscopy with trapped francium: advances and perspectives for weak interaction studies". Rep. Prog. Phys. 69 (1): 79–118. doi:10.1088/0034-4885/69/1/R02. Retrieved 2007-04-11. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ Peterson, I (1996-05-11). "Creating, cooling, trapping francium atoms". Science News. p. 294. Retrieved 2007-04-11.
  25. ^ 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.
  26. ^ a b c d e Fontani, Marco (2005-09-10). "The Twilight of the Naturally-Occurring Elements: Moldavium (Ml), Sequanium (Sq) and Dor (Do)". International Conference on the History of Chemistry. Lisbon. pp. 1–8. Archived from the original on 2006-02-24. Retrieved 2007-04-08. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help)
  27. ^ a b c d Van der Krogt, Peter (2006-01-10). "Francium". Elementymology & Elements Multidict. Retrieved 2007-04-08.
  28. ^ "Alabamine & Virginium". TIME. 1932-02-15. Retrieved 2007-04-01.
  29. ^ MacPherson, H. G. (1934-12-21). "An Investigation of the Magneto-Optic Method of Chemical Analysis". Physical Review. 47 (4). American Physical Society: 310–315. doi:10.1103/PhysRev.47.310. Retrieved 2007-04-08.
  30. ^ a b c "Francium", McGraw-Hill Encyclopedia of Science & Technology, vol. 7, McGraw-Hill Professional, 2002, pp. 493–494, ISBN 0-07-913665-6
  31. ^ Grant, Julius (1969), "Francium", Hackh's Chemical Dictionary, McGraw-Hill, pp. 279–280
  32. ^ "History". Francium. SUNY Stony Brook Physics & Astronomy. 2007-02-20. Retrieved 2007-03-26.
  33. ^ Winter, Mark. "Geological information". Francium. The University of Sheffield. Retrieved 2007-03-26.
  34. ^ a b "Cooling and Trapping". Francium. SUNY Stony Brook Physics & Astronomy. 2007-02-20. Retrieved 2007-05-01.
  35. ^ "Production of Francium". Francium. SUNY Stony Brook Physics & Astronomy. 2007-02-20. Retrieved 2007-03-26.
  36. ^ "Francium". Los Alamos National Laboratory Chemistry Division. 2003-12-15. Retrieved 2007-03-29. {{cite web}}: External link in |publisher= (help)
  37. ^ Considine, Glenn D., ed. (2005), "Francium", Van Nostrand's Encyclopedia of Chemistry, New York: Wylie-Interscience, p. 679, ISBN 0-471-61525-0
  38. ^ National Nuclear Data Center (1990). "Table of Isotopes decay data". Brookhaven National Laboratory. Retrieved 2007-04-04.
  39. ^ National Nuclear Data Center (2003). "Fr Isotopes". Brookhaven National Laboratory. Retrieved 2007-04-04.
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