Mendelevium

fermium ← mendelevium → nobelium Tm ↑ Md ↓ –

Atomic properties Atomic number 101 Electron configuration [Rn] 5f13 7s2 Chemical properties Electronegativity 1.3 (Pauling) Ionization energy 635 kJ mol−1 6.58 eV Atomic radii Metallic radius 194(10) pm Ionic radius 90 pm (Md3+) 115 pm (Md2+) Thermodynamic properties 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).

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.

25399Es(α,n)256101Md

The new element was identified by its elution time in ion-exchange chromatography: 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. A larger-scale preparation in 1958 demostrated a parent–daughter relationship between ²⁵⁵101 and the previously-described ²⁵⁵Fm.

The preparation of mendelevium still follows the same principles, although production rates of about a million atoms per hour can now be obtained. 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. Final purification is acheived again by ion-exchange chromatography, this time using aqueous ammonium α-hydroxyisobutyrate as the eluant.

Isotopes

There are 18 isotopes of mendelevium listed in NUBASE 2003, 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 (t_½ = 77(2) min) as this is more readily produced by the 25399Es(α,n)256101Md or 25499Es(α,2n)256101Md reactions.

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, and the enthalpy change of hydration was calculated to be −3654(12) kJ mol⁻¹. Consistent with this behaviour is the quantitative precipitation of mendelevium from solution when carried by the insoluble lanthanoid(III) fluorides and hydroxides.

Mendelevium was also observed to co-precipitate with barium sulfate in the presence of mild reducing agents, indicating the facile formation of a +2 cation. The reduction potential from Md³⁺ to Md²⁺ has been estimated at −0.1 V to −0.2 V. 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. Reports of the formation of mendelevium(I) species could not be confirmed, and appear to be refuted by polarographic measurements.

Other properties

The ground state of gaseous mendelevium is predicted to be the ²F_7/2 level of the [Rn] 5f¹³ 7s² electron configuration: this has yet to be confirmed experimentally. A theoretical estimate of the first ionization energy is 6.58 eV. It is believed that only the two 7s electrons participate in metallic bonding (as is the case for einsteinium and fermium): the enthalpy change of vaporization has been estimated at 134–142 kJ mol⁻¹.

Notes and references

Further reading

• Hulet, E. K. Chemistry of the Heaviest Actinides: Fermium, Mendelevium, Nobelium, and Lawrencium. In Lanthanide and Actinide Chemistry and Spectroscopy; Edelstein, Norman M., Ed.; American Chemical Society: Washington, D.C., 1980; Chapter 12, pp 239–63. ACS Symposium Series, Vol. 131. ISBN 9780841205680. DOI: 10.1021/bk-1980-0131.ch012.
• Seaborg, Glenn T. The transcalifornium elements. J. Chem. Educ. 1959, 36 (1), 38–44. DOI: 10.1021/ed036p38.

External links

• WebElements