Mendelevium

Mendelevium is a synthetic chemical element; it has symbol Md 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

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 when they bombarded an ²⁵³Es target consisting of only a billion einsteinium atoms with alpha particles 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. The target material, ²⁵³Es, could be produced readily from irradiating plutonium: one year of irradiation would give a billion atoms, and its three-week half-life meant that the element 101 experiments could be conducted in one week after the produced einsteinium was separated and purified to make the target. However, it was necessary to upgrade the cyclotron to obtain the needed intensity of 10¹⁴ alpha particles per second; Seaborg applied for the necessary funds.
While Seaborg applied for funding, Harvey worked on the einsteinium target, while Thomson and Choppin focused on methods for chemical isolation. Choppin suggested using α-hydroxyisobutyric acid to separate the mendelevium atoms from those of the lighter actinides. The initial separation was done by a recoil technique suggested by Albert Ghiorso: the einsteinium was placed on the opposite side of the target from the beam, so that the momentum of the recoiling mendelevium atoms would allow them to leave the target and be caught on a gold catcher foil behind it. This recoil target was made by an electroplating technique, developed by Alfred Chetham-Strode. This technique gave a very high yield, which was absolutely necessary when working with such a rare and valuable product as the einsteinium target material. The recoil target consisted of 10⁹ atoms of ²⁵³Es which were deposited electrolytically on a thin gold foil. It was bombarded by 41 MeV alpha particles in the Berkeley cyclotron with a very high beam density of 6×10¹³ particles per second over an area of 0.05 cm². The target was cooled by water or liquid helium, and the foil could be replaced.
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.
On the day of discovery, 19 February, alpha irradiation of the einsteinium target occurred in three three-hour sessions. The cyclotron was in the University of California campus, while the Radiation Laboratory was on the next hill. To deal with this situation, a complex procedure was used: Ghiorso took the catcher foils from the cyclotron to Harvey, who would use aqua regia to dissolve it and pass it through an anion-exchange resin column to separate the transuranium elements from the gold and other products. 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. Additional analysis and further experimentation showed the produced mendelevium isotope to have the expected mass of 256 and decay by electron capture to fermium-256, the source of the observed fission.
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. The name "mendelevium" was accepted by the International Union of Pure and Applied Chemistry in 1955 with symbol "Mv", which was changed to "Md" in the next IUPAC General Assembly.

Characteristics

Physical

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 lanthanides and actinides, in the metallic state, can exist as either divalent or trivalent metals. The former have fⁿs² configurations, whereas the latter have fⁿ⁻¹d¹s² configurations. In 1975, Johansson and Rosengren examined the measured and predicted values for the cohesive energies of the metallic lanthanides and actinides, both as divalent and trivalent metals. The conclusion was that the increased binding energy of the 5f¹²6d¹7s² configuration over the 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. The increasing predominance of the divalent state well before the actinide series concludes is attributed to the relativistic stabilization of the 5f electrons, which increases with increasing atomic number. Thermochromatographic studies with trace quantities of mendelevium by Zvara and Hübener from 1976 to 1982 confirmed this prediction. In 1990, Haire and Gibson estimated mendelevium metal to have an enthalpy of sublimation between 134 and 142 kJ/mol. Divalent mendelevium metal should have a metallic radius of around. Like the other divalent late actinides, metallic mendelevium should assume a face-centered cubic crystal structure. Mendelevium's melting point has been estimated at 800 °C, the same value as that predicted for the neighbouring element nobelium. Its density is predicted to be around.

Chemical

The chemistry of mendelevium is known largely in solution, in which it can take on the +3 or +2 oxidation states. The +1 state has also been reported, but has not yet been confirmed.
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. Cation-exchange and solvent extraction studies led to the conclusion that mendelevium was a trivalent actinide with an ionic radius somewhat smaller than that of the previous actinide, fermium. Mendelevium can form coordination complexes with 1,2-cyclohexanedinitrilotetraacetic acid.
In reducing conditions, mendelevium can be easily reduced to mendelevium, which is stable in aqueous solution. The standard reduction potential of the E° couple was variously estimated in 1967 as −0.10 V or −0.20 V: later 2013 experiments established the value as. In comparison, E° should be around −1.74 V, and E° should be around −2.5 V. Mendelevium's elution behavior has been compared with that of strontium and europium.
In 1973, mendelevium was reported to have been produced by Russian scientists, who obtained it by reducing higher oxidation states of mendelevium with samarium. It was found to be stable in neutral water–ethanol solution and be homologous to caesium. However, later experiments found no evidence for mendelevium and found that mendelevium behaved like divalent elements when reduced, not like the monovalent alkali metals. Nevertheless, the Russian team conducted further studies on the thermodynamics of cocrystallizing mendelevium with alkali metal chlorides, and concluded that mendelevium had formed and could form mixed crystals with divalent elements, thus cocrystallizing with them. The status of the +1 oxidation state is still tentative.
The electrode potential E° was predicted in 1975 to be +5.4 V; 1967 experiments with the strong oxidizing agent sodium bismuthate were unable to oxidize mendelevium to mendelevium.