Mendelevium

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fermium ← mendelevium → nobelium
Tm
↑
Md
↓
–
Atomic properties
Atomic number 101
Electron configuration [Rn] 5f13 7s2
Chemical properties[1][note 1]
Electronegativity 1.3 (Pauling)
Ionization energy[2][note 1]
635 kJ mol−1
6.58 eV
Atomic radii[3][4][5][6][note 1]
Metallic radius 194(10) pm
Ionic radius 90 pm (Md3+)
115 pm (Md2+)
Thermodynamic properties[7][note 1]
Enthalpy change of vaporization 134–142 kJ mol−1
Miscellaneous
CAS number 7440-11-1
Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).
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Mendelevium (symbol: Md) is a synthetic chemical element and a member of the actinoid series. It cannot be formed by neutron bombardment in a nuclear reactor, but must instead be produced in particle accelerators. Mendelevium cannot be produced in macroscopic quantities.

It was first prepared in 1955, and named after Dmitri Mendeleev in recognition of his development of the periodic table. Its chemistry is typical of the late actinoids, with both the +3 and +2 oxidation states stable in aqueous solution.

Preparation

Mendelevium was first prepared at the University of California Radiation Laboratory in Berkeley, California, in 1955 by the bombardment of 25399Es targets (roughly 109 atoms) with 41 MeV α-particles: just two atoms of the new element, later confirmed to be the A = 256 isotope, were produced per three-hour bombardment session, and only seventeen atoms in total were detected.[8]

25399Es(α,n)256101Md

The new element was identified by its elution time in ion-exchange chromatography:[note 2] a spontaneous fission activity was observed both in the fermium fraction and in a fraction that eluted before the fermium, and this was taken to be a fermium daughter isotope produced by electron-capture decay of an isotope of element 101.[8] A larger-scale preparation in 1958 demostrated a parent–daughter relationship between 255101 and the previously-described 255Fm.[9]

The preparation of mendelevium still follows the same principles, although production rates of about a million atoms per hour can now be obtained.[10] The mendelevium atoms are collected on a thin metal film placed behind the einsteinium target (either 253Es or the longer-lived 254Es), or are carried in a stream of inert gas to a separate collection area. The collector foil is then dissolved in acid and the mendelevium is coprecipitated with lanthanum fluoride. The mendelevium is then separated from the lanthanum carrier and any lanthanoid fission products by ion-exchange chromatography using 10% aqueous ethanol saturated with hydrogen chloride as the eluant.[11] Final purification is acheived again by ion-exchange chromatography, this time using aqueous ammonium α-hydroxyisobutyrate as the eluant.[12]

Isotopes

Main article: Isotopes of mendelevium

There are 18 isotopes of mendelevium listed in NUBASE 2003,[13] with A = 245–262, of which 258Md is the longest lived with a half-life of 51.5(3) days. Nevertheless, most studies of mendelevium use 256Md (t½ = 77(2) min) as this is more readily produced by the 25399Es(α,n)256101Md or 25499Es(α,2n)256101Md reactions.[14]

Chemistry

The chemistry of mendelevium has only been studied in solution using tracer techniques, and no solid compounds have been prepared. Its behavour in ion-exchange chromatography after preparation shows that the initial solutions contain the Md3+ ion, and that this is slightly smaller than Fm3+, as expected from periodic trends. The ionic radius of Md3+ has been estimated as 89.6 pm (intermediate between Er3+ and Ho3+ in the lanthanoid series) on the basis of its behaviour in ion-exchange chromatography,[note 2] and the enthalpy change of hydration was calculated to be −3654(12) kJ mol−1.[3][4] Consistent with this behaviour is the quantitative precipitation of mendelevium from solution when carried by the insoluble lanthanoid(III) fluorides and hydroxides.[10]

Mendelevium was also observed to co-precipitate with barium sulfate in the presence of mild reducing agents,[10][15] indicating the facile formation of a +2 cation. The reduction potential from Md3+ to Md2+ has been estimated at −0.1 V[16] to −0.2 V.[10] Based on the behaviour of divalent mendelevium in ion-exchange chromatography compared to Eu2+ and Sr2+, the ionic radius of Md2+ has been estimated to be 115 pm, implying an enthalpy change of hydration of about −1413 kJ mol−1.[5] Reports of the formation of mendelevium(I) species could not be confirmed, and appear to be refuted by polarographic measurements.[14]

Notes and references

Notes

  1. ↑ 1.0 1.1 1.2 1.3 The properties of mendelevium are only known through estimation and/or extrapolation.
  2. ↑ 2.0 2.1 In the ion-exchange chromatography of actinoids (and lanthanoids), the sample is loaded onto a column of cation-exchange polymer and eluted with a solution containing a suitable ligand (such as ammonium α-hydroxyisobutyrate). The elution time depends on the relative stabilities of the complex in solution compared to the bound complex on the cation-exchange polymer. The later actinoids (lanthanoids) form smaller ions, due to the greater effective nuclear charge, and these smaller ions form more stable complexes with the ligand in the eluant, so elements of the smae series (actinoid or lanthanoid) elute from the column in reverse order of atomic number. The distribution coefficient between stationary and mobile phases is related to the ionic radius of the ion, so the ionic radii of the later actinoids can be calculated by ocmparison to the known ionic radii of the lanthanoids.

References

  1. ↑ Pauling, Linus The Nature of the Chemical Bond, 3rd ed.; Ithaca, NY, 1960; pp 88–95. ISBN 0-8014-0333-2.
  2. ↑ Sugar, Jack Revised ionization energies of neutral actinides. J. Chem. Phys. 1974, 60 (10), 4103. DOI: 10.1063/1.1680874.
  3. ↑ 3.0 3.1 Brüchle, W.; Schädel, M.; Scherer, U. W.; Kratz, J. V.; Gregorich, K. E.; Lee, D.; Nurmia, M.; Chasteler, R. M., et al. The hydration enthalpies of Md3+ and Lr3+. Inorg. Chim. Acta 1988, 146 (2), 267–76. DOI: 10.1016/S0020-1693(00)80619-2.
  4. ↑ 4.0 4.1 Hoffman, D. C.; Henderson, R. A.; Gregorich, K. E.; Bennett, D. A.; Chasteler, R. M.; Gannett, C. M.; Hall, H. L.; Lee, D. M., et al. Atom-at-a-time radiochemical separations of the heaviest elements: Lawrencium chemistry. J. Radioanal. Nucl. Chem. 1988, 124 (1), 135–44. DOI: 10.1007/BF02035512.
  5. ↑ 5.0 5.1 Guseva, L. I.; Tikhomirova, G. S.; Buklanov, G. V.; Pkhar, Z. Z.; Lebedev, I. A.; Katargin, N. V.; Myasoedov, B. F. Radiokhimiya 1988, 30 (1), 21–25.
  6. ↑ David, F.; Samhoun, K.; Guillaumont, R.; Edelstein, N. Thermodynamic properties of 5f elements. J. Inorg. Nucl. Chem. 1978, 40 (1), 69–74. DOI: 10.1016/0022-1902(78)80309-1.
  7. ↑ Haire, R. G.; Gibson, J. K. Selected systematic properties and some recent investigations of actinide metals and alloys. J. Radioanal. Nucl. Chem. 1990, 143 (1), 35–51. DOI: 10.1007/BF02117545.
  8. ↑ 8.0 8.1 Ghiorso, A.; Harvey, B. G.; Choppin, G. R.; Thompson, S. G.; Seaborg, G. T. New Element Mendelevium, Atomic Number 101. Phys. Rev. 1955, 98 (5), 1518–19. DOI: 10.1103/PhysRev.98.1518.
  9. ↑ Phillips, L.; Gatti, R.; Chesne, A.; Muga, L.; Thompson, S. Discovery of a new Mendelevium Isotope. Phys. Rev. Lett. 1958, 1 (6), 215–21. DOI: 10.1103/PhysRevLett.1.215.
  10. ↑ 10.0 10.1 10.2 10.3 Hulet, E. K.; Lougheed, R. W.; Brady, J. D.; Stone, R. E.; Coops, M. S. Mendelevium: Divalency and Other Chemical Properties. Science 1967, 158, 486–88. DOI: 10.1126/science.158.3800.486.
  11. ↑ Thompson, S. G.; Harvey, B. G.; Choppin, G. R.; Seaborg, G. T. Chemical Properties of Elements 99 and 100. J. Am. Chem. Soc. 1954, 76 (24), 6229–36. DOI: 10.1021/ja01653a004.
  12. ↑ Choppin, G. R.; Harvey, B. G.; Thompson, S. G. A new eluant for the separation of the actinide elements. J. Inorg. Nucl. Chem. 1956, 2 (1), 66–68. DOI: 10.1016/0022-1902(56)80105-X.
  13. ↑ Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. The NUBASE evaluation of nuclear and decay properties. Nucl. Phys. A 2003, 729, 3–128. doi:10.1016/j.nuclphysa.2003.11.001, <http://amdc.in2p3.fr/nubase/Nubase2003.pdf>.
  14. ↑ 14.0 14.1 Silva, Robert J. Fermium, Mendelevium, Nobelium, and Lawrencium. In The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean, Eds.; Springer: Dordrecht, the Netherlands, 2006; Vol. 3, Chapter 13, pp 1621–51. doi:10.1007/1-4020-3598-5_13, <http://radchem.nevada.edu/classes/rdch710/files/Fm%20to%20Lr.pdf>.
  15. ↑ Malý, Jaromír The amalgamation behavior of heavy elements—IV the tracer chemistry of divalent mendelevium, 1969; Vol. 31, pp 741–54. DOI: 10.1016/0022-1902(69)80021-7.
  16. ↑ Malý, Jaromír; Cunningham, Burris B. The amalgamation behavior of heavy elements. 2. Dipositive state of mendelevium. Inorg. Nucl. Chem. Lett. 1967, 3 (10), 445–51. DOI: 10.1016/0020-1650(67)80103-X.

Further reading

External links

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