Like all the transfermiums, it can only be produced inparticle accelerators by bombarding lighter elements with charged particles. The element was first produced in 1955 by bombardingeinsteinium withalpha particles, the method still used today. Using commonly-availablemicrogram 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 basicscientific research, and only small amounts are produced.
Mendelevium was the ninthtransuranic element to be synthesized. It was firstsynthesized byAlbert Ghiorso,Glenn T. Seaborg,Gregory Robert Choppin, Bernard G. Harvey, and team leaderStanley G. Thompson in early 1955 at the University of California, Berkeley. The team produced256Md (half-life 77.7 minutes[4]) when they bombarded an253Es target consisting of only abillion (109) einsteinium atoms withalpha particles (helium nuclei) in theBerkeley Radiation Laboratory's 60-inchcyclotron, 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.[5] This discovery was part of a program, begun in 1952, that irradiatedplutonium with neutrons to transmute it into heavier actinides.[6] This method was necessary because of a lack of knownbeta decayingisotopes 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 tospontaneous fission of258Fm[4] and subsequent isotopes, which still do not beta decay - thefermium 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.[5] The target material, einsteinium-253, could be produced readily from irradiatingplutonium: one year of irradiation would give a billion atoms, and its three-weekhalf-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.[6]
The data sheet, showing stylus tracing and notes, that proved the discovery of mendelevium.
While Seaborg applied for funding, Harvey worked on the einsteinium target, while Thomson and Choppin focused on methods for chemical isolation. Choppin suggested usingα-hydroxyisobutyric acid to separate the mendelevium atoms from those of the lighter actinides.[6] 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 themomentum 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.[5] The recoil target consisted of 109 atoms of253Es which were deposited electrolytically on a thin gold foil. It was bombarded by 41 MeValpha particles in theBerkeley 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 orliquid helium, and the foil could be replaced.[5][7]
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 byelectron capture tofermium-256, correctly believed to decay primarily by fission, and that the experiment should be repeated, this time searching for thosespontaneous fission events. This version of the experiment was performed in February 1955.[6]
On the day of discovery, 19 February, alpha irradiation of the einsteinium target occurred in three three-hour sessions. The cyclotron was in theUniversity 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 (there were three targets and three foils) from the cyclotron to Harvey, who would useaqua regia to dissolve it and pass it through ananion-exchangeresin column to separate thetransuranium elements from the gold and other products.[6][8] The resultant drops entered atest tube, which Choppin and Ghiorso took in a car to get to the Radiation Laboratory as soon as possible. Thompson and Choppin used acation-exchange resin column and the α-hydroxyisobutyric acid. The solution drops were collected onplatinum 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 daughter256Fm. 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.[6] 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 (half-life 157.6 minutes), the source of the observed fission.[4]
We thought it fitting that there be an element named for the Russian chemist Dmitri Mendeleev, who had developed the periodic table. In nearly all our experiments discovering transuranium elements, we'd depended on his method of predicting chemical properties based on the element's position in the table. But in the middle of the Cold War, naming an element for a Russian was a somewhat bold gesture that did not sit well with some American critics.[9]
— 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 chemistDmitri Mendeleev, father of theperiodic table. Because this discovery came during theCold War, Seaborg had to request permission from the government of theUnited States to propose that the element be named for a Russian, but it was granted.[6]The name "mendelevium" was accepted by theInternational Union of Pure and Applied Chemistry (IUPAC) in 1955 with symbol "Mv",[10] which was changed to "Md" in the next IUPAC General Assembly (Paris, 1957).[11]
Energy required to promote an f electron to the d subshell for the f-blocklanthanides andactinides. Above around 210 kJ/mol, this energy is too high to be provided for by the greatercrystal energy of the trivalent state and thuseinsteinium,fermium, and mendelevium form divalent metals like the lanthanideseuropium andytterbium. (Nobelium is also expected to form a divalent metal, but this has not yet been confirmed.)[12]
In theperiodic table, mendelevium is located to the right of the actinidefermium, to the left of the actinidenobelium, and below the lanthanidethulium. Mendelevium metal has not yet been prepared in bulk quantities, and bulk preparation is currently impossible.[13] Nevertheless, a number of predictions and some preliminary experimental results have been done regarding its properties.[13]
The lanthanides and actinides, in the metallic state, can exist as either divalent (such aseuropium andytterbium) or trivalent (most other lanthanides) 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 thecohesive energies (enthalpies of crystallization) of the metalliclanthanides andactinides, both as divalent and trivalent metals.[14][15] 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: thuseinsteinium,fermium, mendelevium, andnobelium were expected to be divalent metals.[14] The increasing predominance of the divalent state well before the actinide series concludes is attributed to therelativistic stabilization of the 5f electrons, which increases with increasing atomic number.[16]Thermochromatographic studies with trace quantities of mendelevium by Zvara and Hübener from 1976 to 1982 confirmed this prediction.[13] In 1990, Haire and Gibson estimated mendelevium metal to have anenthalpy of sublimation between 134 and 142 kJ/mol.[13] Divalent mendelevium metal should have ametallic radius of around194±10 pm.[13] Like the other divalent late actinides (except the once again trivalentlawrencium), metallic mendelevium should assume aface-centered cubic crystal structure.[1] Mendelevium's melting point has been estimated at 800 °C, the same value as that predicted for the neighbouring element nobelium.[17] Its density is predicted to be around10.3±0.7 g/cm3.[1]
The chemistry of mendelevium is known largely in solution (as available quantities do not allow the creation of pure compounds), in which it can take on the +3 or +2oxidation states. The +1 state has also been reported, but has not yet been confirmed.[18]
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 iteluted 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 insolublehydroxides andfluorides that coprecipitated with trivalent lanthanide salts.[18] 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.[18] Mendelevium can formcoordination complexes with 1,2-cyclohexanedinitrilotetraacetic acid (DCTA).[18]
Inreducing conditions, mendelevium(III) can be easily reduced to mendelevium(II), which is stable in aqueous solution.[18] Thestandard reduction potential of theE°(Md3+→Md2+) couple was variously estimated in 1967 as −0.10 V or −0.20 V:[18] later 2013 experiments established the value as−0.16±0.05 V.[19] In comparison,E°(Md3+→Md0) should be around −1.74 V, andE°(Md2+→Md0) should be around −2.5 V.[18] Mendelevium(II)'s elution behavior has been compared with that ofstrontium(II) andeuropium(II).[18]
In 1973, mendelevium(I) was reported to have been produced by Russian scientists, who obtained it by reducing higher oxidation states of mendelevium withsamarium(II). It was found to be stable in neutral water–ethanol solution and behomologous tocaesium(I). However, later experiments found no evidence for mendelevium(I) and found that mendelevium behaved like divalent elements when reduced, not like the monovalentalkali metals.[18] Nevertheless, the Russian team conducted further studies on thethermodynamics of cocrystallizing mendelevium with alkali metalchlorides, and concluded that mendelevium(I) had formed and could form mixed crystals with divalent elements, thus cocrystallizing with them. The status of the +1 oxidation state is still tentative.[18]
The electrode potentialE°(Md4+→Md3+) was predicted in 1975 to be +5.4 V; 1967 experiments with the strong oxidizing agentsodium bismuthate were unable to oxidize mendelevium(III) to mendelevium(IV).[18]
A mendelevium atom has 101 electrons. They are expected to be arranged in the configuration [Rn]5f137s2 (ground stateterm symbol2F7/2), although experimental verification of this electron configuration had not yet been made as of 2006. The fifteen electrons in the 5f and 7s subshells arevalence electrons.[20] 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 firstionization 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;[21] this value has not yet been refined further due to the lack to larger samples of mendelevium.[22] The ionic radius ofhexacoordinate Md3+ had been preliminarily estimated in 1978 to be around 91.2 pm;[18] 1988 calculations based on the logarithmic trend betweendistribution coefficients and ionic radius produced a value of 89.6 pm, as well as anenthalpy of hydration of−3654±12 kJ/mol.[18] Md2+ should have an ionic radius of 115 pm and hydration enthalpy −1413 kJ/mol; Md+ should have ionic radius 117 pm.[18]
Seventeen isotopes of mendelevium are known, with mass numbers from 244 to 260; all are radioactive.[23] The longest-lived isotope is258Md with a half-life of 51.6 days.[4] Nevertheless, the shorter-lived256Md (half-life 77.7 minutes) is more often used in chemical experiments because it can be produced in larger quantities from einsteinium,[23] as258Md would require255Es, of which significant quantities are available only as a minor component of an isotopic mixture.
The half-lives of mendelevium isotopes mostly increase smoothly (apart from odd/even effects) toward higher mass, up to258Md, then decrease (as indicated by what experimental data is available) asspontaneous fission becomes the dominant decay mode;[23] the second longest-living isotope is260Md, the heaviest known, with a half-life of 27.8 days.[4] Mendelevium is the last element that has any known isotope with a half-life longer than a day.[4]
Mendelevium-256, the currently most important isotope of mendelevium, decays about 90% throughelectron capture and 10% throughalpha decay.[23] It is most easily detected through thespontaneous fission of its electron capture daughterfermium-256, but in the presence of other nuclides that undergo spontaneous fission, alpha decays at the characteristic energies for mendelevium-256 (7.205 and 7.139 MeV) can provide more useful identification.[24]
The lightest isotopes (244Md to247Md) are mostly produced through bombardment ofbismuth targets withargon ions, while slightly heavier ones (248Md to253Md) are produced by bombardingplutonium andamericium targets with ions ofcarbon andnitrogen. The most important and most stable isotopes are in the range from254Md to258Md and are produced through bombardment ofeinsteinium with alpha particles: einsteinium-253, −254, and −255 can all be used.259Md is produced as adaughter of259No, and260Md can be produced in atransfer reaction between einsteinium-254 andoxygen-18.[23] Typically, the most commonly used isotope256Md is produced by bombarding either einsteinium-253 or −254 with alpha particles: einsteinium-254 is preferred when available because it has a longer half-life and therefore can be used as a target for longer.[23] Using available microgram quantities of einsteinium,femtogram quantities of mendelevium-256 may be produced.[23]
The recoilmomentum of the produced mendelevium-256 atoms is used to bring them physically far away from the einsteinium target from which they are produced, bringing them onto a thin foil of metal (usuallyberyllium,aluminium,platinum, orgold) just behind the target in a vacuum.[24] This eliminates the need for immediate chemical separation, which is both costly and prevents reusing of the expensive einsteinium target.[24] The mendelevium atoms are then trapped in a gas atmosphere (frequentlyhelium), and a gas jet from a small opening in the reaction chamber carries the mendelevium along.[24] Using a longcapillary tube, and includingpotassium chloride aerosols in the helium gas, the mendelevium atoms can be transported over tens ofmeters to be chemically analysed and have their quantity determined.[8][24] The mendelevium can then be separated from the foil material and otherfission products by applying acid to the foil and thencoprecipitating the mendelevium withlanthanum fluoride, then using acation-exchange resin column with a 10%ethanol solution saturated withhydrochloric acid, acting as aneluant. However, if the foil is made of gold and thin enough, it is enough to simply dissolve the gold inaqua regia before separating the trivalent actinides from the gold usinganion-exchangechromatography, the eluant being 6 M hydrochloric acid.[24]
Mendelevium can finally be separated from the other trivalent actinides using selective elution from a cation-exchange resin column, the eluant being ammonia α-HIB.< Using the gas-jet method often renders the first two steps unnecessary.[24]
Another possible way to separate the trivalent actinides is via solvent extraction chromatography using bis-(2-ethylhexyl) phosphoric acid (abbreviated as HDEHP) as the stationary organic phase andnitric acid as the mobile aqueous phase. The actinide elution sequence is reversed from that of the cation-exchange resin column, so that the heavier actinides elute later. The mendelevium separated by this method has the advantage of being free of organic complexing agent compared to the resin column; the disadvantage is that mendelevium then elutes very late in the elution sequence, after fermium.[8][24]
Another method to isolate mendelevium exploits the distinct elution properties of Md2+ from those of Es3+ and Fm3+. The initial steps are the same as above, and employs HDEHP for extraction chromatography, but coprecipitates the mendelevium with terbium fluoride instead of lanthanum fluoride. Then, 50 mg ofchromium is added to the mendelevium to reduce it to the +2 state in 0.1 M hydrochloric acid withzinc ormercury.[24] The solvent extraction then proceeds, and while the trivalent and tetravalent lanthanides and actinides remain on the column, mendelevium(II) does not and stays in the hydrochloric acid. It is then reoxidized to the +3 state usinghydrogen peroxide and then isolated by selective elution with 2 M hydrochloric acid (to remove impurities, including chromium) and finally 6 M hydrochloric acid (to remove the mendelevium).[24] It is also possible to use a column of cationite and zinc amalgam, using 1 M hydrochloric acid as an eluant, to effect the reduction.[24] Thermochromatographic chemical isolation could be achieved using the volatile mendeleviumhexafluoroacetylacetonate: the analogous fermium compound is known and similar.[24]
Though few people come in contact with mendelevium, theInternational Commission on Radiological Protection has set annual exposure limits for the most stable isotope. For mendelevium-258, the ingestion limit was set at 9×105becquerels (1 Bq = 1 decay per second). Given the half-life of this isotope, this is only 2.48 ng (nanograms). The inhalation limit is at 6000 Bq or 16.5 pg (picogram).[25]
^Johansson, Börje; Rosengren, Anders (1975). "Generalized phase diagram for the rare-earth elements: Calculations and correlations of bulk properties".Physical Review B.11 (8):2836–2857.Bibcode:1975PhRvB..11.2836J.doi:10.1103/PhysRevB.11.2836.
