Mendelevium

Mendelevium is a synthetic chemical element; it has symbol Md (formerly Mv) and atomic number 101. A metallic radioactive transuranium element in the actinide series, it is the first element by atomic number that currently cannot be produced in macroscopic quantities by neutron bombardment of lighter elements. It is the thirteenth actinide, the ninth transuranic element, and the first transfermium; it is named after Dmitri Mendeleev, the father of the periodic table.

Like all the transfermiums, it can only be produced in particle accelerators by bombarding lighter elements with charged particles. The element was first produced in 1955 by bombarding einsteinium with alpha particles, the method still used today. Using commonly-available microgram quantities of einsteinium-253, over a million mendelevium atoms may be made each hour. The chemistry of mendelevium is typical for the late actinides, with a dominant +3 oxidation state but also a +2 oxidation state accessible in solution. All known isotopes of mendelevium have short half-lives; there are currently no uses for it outside basic scientific research, and only small amounts are produced.

Discovery

thumb|upright=0.7|left|The 60-inch cyclotron at the Lawrence Radiation Laboratory, [[University of California, Berkeley, in August 1939|alt=Black-and-white picture of heavy machinery with two operators sitting aside]]

Mendelevium was the ninth transuranic element to be synthesized. It was first synthesized by Albert Ghiorso, Glenn T. Seaborg, Gregory Robert Choppin, Bernard G. Harvey, and team leader Stanley G. Thompson in early 1955 at the University of California, Berkeley. The team produced ²⁵⁶Md (half-life 77.7 minutes) when they bombarded an ²⁵³Es target consisting of only a billion (10⁹) einsteinium atoms with alpha particles (helium nuclei) in the Berkeley Radiation Laboratory's 60-inch cyclotron, thus increasing the target's atomic number by two. ²⁵⁶Md thus became the first isotope of any element to be synthesized one atom at a time. In total, seventeen mendelevium atoms were detected. This discovery was part of a program, begun in 1952, that irradiated plutonium with neutrons to transmute it into heavier actinides. This method was necessary because of a lack of known beta decaying isotopes of fermium that might allow production by neutron capture; it is now known that such production is impossible at any possible reactor flux due to the very short half-life to spontaneous fission of ²⁵⁸Fm and subsequent isotopes, which still do not beta decay - the fermium gap that, as far as we know, sets a hard limit to the success of neutron capture processes.

To predict if the production of mendelevium would be possible, the team made use of a rough calculation. The number of atoms that would be produced would be approximately equal to the product of the number of atoms of target material, the target's cross section, the ion beam intensity, and the time of bombardment; this last factor was related to the half-life of the product when bombarding for a time on the order of its half-life. This gave one atom per experiment. Thus under optimum conditions, the preparation of only one atom of element 101 per experiment could be expected. This calculation demonstrated that it was feasible to go ahead with the experiment.

Initial experiments were carried out in September 1954. No alpha decay was seen from mendelevium atoms; thus, Ghiorso suggested that the mendelevium had all decayed by electron capture to fermium-256, correctly believed to decay primarily by fission, and that the experiment should be repeated, this time searching for those spontaneous fission events. This version of the experiment was performed in February 1955. The resultant drops entered a test tube, which Choppin and Ghiorso took in a car to get to the Radiation Laboratory as soon as possible. Thompson and Choppin used a cation-exchange resin column and the α-hydroxyisobutyric acid. The solution drops were collected on platinum disks and dried under heat lamps. The three disks were expected to contain, respectively, the fermium, no new elements, and the mendelevium. Finally, they were placed in their own counters, which were connected to recorders such that spontaneous fission events would be recorded as huge deflections in a graph showing the number and time of the decays. There thus was no direct detection, but by observation of spontaneous fission events arising from its electron-capture daughter ²⁵⁶Fm. The first one was identified with a "hooray" followed by a "double hooray" and a "triple hooray". The fourth one eventually officially proved the chemical identification of the 101st element, mendelevium. In total, five decays were reported up until 4 a.m. Seaborg was notified and the team left to sleep.|Glenn T. Seaborg

Being the first of the second hundred of the chemical elements, it was decided that the element would be named "mendelevium" after the Russian chemist Dmitri Mendeleev, father of the periodic table. Because this discovery came during the Cold War, Seaborg had to request permission from the government of the United States to propose that the element be named for a Russian, but it was granted. which was changed to "Md" in the next IUPAC General Assembly (Paris, 1957).

Characteristics

Physical

thumb|upright=1.6|right|Energy required to promote an f electron to the d subshell for the f-block [[lanthanides and actinides. Above around 210 kJ/mol, this energy is too high to be provided for by the greater crystal energy of the trivalent state and thus einsteinium, fermium, and mendelevium form divalent metals like the lanthanides europium and ytterbium. (Nobelium is also expected to form a divalent metal, but this has not yet been confirmed.)]]

In the periodic table, mendelevium is located to the right of the actinide fermium, to the left of the actinide nobelium, and below the lanthanide thulium. Mendelevium metal has not yet been prepared in bulk quantities, and bulk preparation is currently impossible. Nevertheless, a number of predictions and some preliminary experimental results have been done regarding its properties. The conclusion was that the increased binding energy of the [Rn]5f¹²6d¹7s² configuration over the [Rn]5f¹³7s² configuration for mendelevium was not enough to compensate for the energy needed to promote one 5f electron to 6d, as is true also for the very late actinides: thus einsteinium, fermium, mendelevium, and nobelium were expected to be divalent metals. Thermochromatographic studies with trace quantities of mendelevium by Zvara and Hübener from 1976 to 1982 confirmed this prediction. Its density is predicted to be around .

Before mendelevium's discovery, Seaborg and Katz predicted that it should be predominantly trivalent in aqueous solution and hence should behave similarly to other tripositive lanthanides and actinides. After the synthesis of mendelevium in 1955, these predictions were confirmed, first in the observation at its discovery that it eluted just after fermium in the trivalent actinide elution sequence from a cation-exchange column of resin, and later the 1967 observation that mendelevium could form insoluble hydroxides and fluorides that coprecipitated with trivalent lanthanide salts. In comparison, E°(Md³⁺→Md⁰) should be around −1.74 V, and E°(Md²⁺→Md⁰) should be around −2.5 V. In forming compounds, three valence electrons may be lost, leaving behind a [Rn]5f¹² core: this conforms to the trend set by the other actinides with their [Rn] 5fⁿ electron configurations in the tripositive state. The first ionization potential of mendelevium was measured to be at most (6.58 ± 0.07) eV in 1974, based on the assumption that the 7s electrons would ionise before the 5f ones; this value has not yet been refined further due to the lack to larger samples of mendelevium. The ionic radius of hexacoordinate Md³⁺ had been preliminarily estimated in 1978 to be around 91.2 pm; The longest-lived isotope is ²⁵⁸Md with a half-life of 51.6 days. Nevertheless, the shorter-lived ²⁵⁶Md (half-life 77.7 minutes) is more often used in chemical experiments because it can be produced in larger quantities from einsteinium, This eliminates the need for immediate chemical separation, which is both costly and prevents reusing of the expensive einsteinium target.

Notes

References

Bibliography

Further reading

• Hoffman, D.C., Ghiorso, A., Seaborg, G. T. The transuranium people: the inside story, (2000), 201–229
• Morss, L. R., Edelstein, N. M., Fuger, J., The chemistry of the actinide and transactinide element, 3, (2006), 1630–1636
• A Guide to the Elements – Revised Edition, Albert Stwertka, (Oxford University Press; 1998)

External links

• Los Alamos National Laboratory – Mendelevium
• It's Elemental – Mendelevium
• Mendelevium at The Periodic Table of Videos (University of Nottingham)
• Environmental Chemistry – Md info