About this schools Wikipedia selection
The articles in this Schools selection have been arranged by curriculum topic thanks to SOS Children volunteers. SOS Children has looked after children in Africa for forty years. Can you help their work in Africa?
|Name, symbol, number||mendelevium, Md, 101|
|Pronunciation|| / /
or / /
|Group, period, block||n/a, 7, f|
|Standard atomic weight||(258)|
|Electron configuration||[Rn] 5f13 7s2
2, 8, 18, 32, 31, 8, 2
|Discovery||Lawrence Berkeley National Laboratory (1955)|
|Melting point||1100 K, 827 °C, 1521 °F|
|Oxidation states||2, 3|
|Electronegativity||1.3 (Pauling scale)|
|Ionization energies||1st: 635 kJ·mol−1|
|Magnetic ordering||no data|
|CAS registry number||7440-11-1|
|Most stable isotopes|
|Main article: Isotopes of mendelevium|
Mendelevium is a synthetic element with the symbol Md (formerly Mv) and the atomic number 101. A metallic radioactive transuranic element in the actinide series, mendelevium is usually synthesized by bombarding einsteinium with alpha particles. It was named after Dmitri Ivanovich Mendeleev, who created the Periodic Table. Mendeleev's periodic system is the fundamental way to classify all the chemical elements. The name "mendelevium" was accepted by the International Union of Pure and Applied Chemistry (IUPAC). On the other hand, the proposed symbol "Mv" submitted by the discoverers was not accepted, and IUPAC changed the symbol to "Md" in 1963.
Researchers have shown that mendelevium has a moderately stable dipositive (II) oxidation state in addition to the more characteristic (for actinide elements) tripositive (III) oxidation state, the latter being the more dominantly exhibited state in an aqueous solution (chromatography being the process used). Sometimes, mendelevium can even be shown to exhibit a monopositive (I) state. 256Md has been used to find out some of the chemical properties of this element while in an aqueous solution. There are no other known uses of mendelevium and only trace amounts of the element have ever been produced. Other isotopes of mendelevium, all radioactive, have been discovered, with 258Md being the most stable with a two-month half-life (about 55 days). Other isotopes range from 248 to 258 mass numbers and half-lives from a few seconds to about 51 days. The original 256Md had a half-life of 87 minutes.
The trivalent element is radioactive. It was expected that the reaction would be253Es (α,n) 255Md, where 255Md was α-active with a t½ of 5 minutes and the corresponding α-energy. No such α-activity was observed, but the 101 fraction showed spontaneous fission representing a t½ less than 3 hours. Because spontaneous fission was also observed in the fraction containing fermium, the α-bombardment of 253Es produced 256Md. The latter underwent electron capture to become 256Fm, which then decayed by spontaneous fission. So 256Fm was produced by the decays of cyclotron-synthesized mendelevium.
Johansson and Rosengren predicted in 1975 that Md would prefer a divalent metallic state, similar to europium (Eu) and ytterbium (Yb), rather than a trivalent one. Thermochromatographic studies conducted with trace amounts of Md concluded that Md forms a divalent metal. With the aid of empirical correlation method, a divalent metallic radius of (0.194 ± 0.010) nm has been estimated. The estimated enthalpy of sublimation is in the range of 134-142 kJ/mol.
Before the actual discovery of mendelevium, the trivalent state was the most stable one in aqueous solution. Accordingly, a similar chemical behaviour to the other 3+ actinides and lanthanides was expected. The elution of Md just before Fm in the elution sequence of the trivalent actinides from the cation-exchange resin column, confirmed this prediction. Afterwards, Md in the form of insoluble hydroxides and fluorides that are quantitatively coprecipitated with trivalent lanthanides was found. The cation-exchange resin column as well as the HDEHP solvent extraction column elution date is consistent with a trivalent state for Md and an ionic radius smaller than Fm. An ionic radius of 0.0192 nm and a coordination number of 6 for Md3+ was predicted using empirical correlations. Using the known ionic radii for the trivalent rare earths and the linear correlation of log distribution coefficient with ionic radius, an average ionic radius of 0.089 nm was estimated for Md3+ and a heat of hydration of –(3654 ± 12) kJ/mol calculated using empirical models and the Born-Haber cycle. In reducing conditions, an anomalous chemical behaviour of Md was found. Coprecipitation with BaSO4 and solvent extraction chromatography experiments using HDEHP were carried out in different reducing agents. These showed that Md3+ could easily be reduced to a stable Md2+ in aqueous solution. Mendelevium can also be reduced to the monovalent state in water-ethanol solutions. The cocrystallization of Md+ with salts of divalent ions is due to the formation of mixed crystals. For Md+, an ionic radius of 0.117 nm was found. The oxidation of Md3+ to Md4+ was rather unsuccessful.
Mendelevium (for Dimitri Ivanovich Mendeleev, surname commonly transliterated into Latin script as Mendeleev, Mendeleyev, Mendeléef, or even Mendelejeff, and first name sometimes transliterated as Dmitry or Dmitriy) was first synthesized by Albert Ghiorso, Glenn T. Seaborg, Gregory R. Choppin, Bernard G. Harvey, and Stanley G. Thompson (team leader) in early 1955 at the University of California, Berkeley. The team produced 256Md (half-life of 87 minutes) when they bombarded an 253Es target with alpha particles (helium nuclei) in the Berkeley Radiation Laboratory's 60-inch cyclotron (256Md was the first isotope of any element to be synthesized one atom at a time). Element 101 was the ninth transuranic element synthesized. The first 17 atoms of this element were created and analyzed using the ion-exchange adsorption-elution method. During the process, mendelevium behaved very much like thulium, its naturally occurring homologue.
Discovery in detail
The discovery was based on a grand total of only 17 atoms. It is synthesized via the 253Es (α,n) 256101 reaction in the 60-Inch-Cyclotron (= 152 cm) at Berkeley (California). The target can be produced by irradiation of lighter isotopes as plutonium in the Materials Testing Reactor at the Arco Reactor Station in Idaho. Remarkable is that this target consisted of only 109 atoms of highly radioactive 253Es (with a half-life of 20.5 days). By elution through a calibrated cation exchange resin column, mendelevium was separated and chemically identified.
To predict if this method would be possible, they made use of a rough calculation. The number of atoms that would be produced, would be approximately equal to the number of atoms of target material times its cross section times the ion beam intensity times the time of bombardment related to the half-life of the product when bombarding for a time of the order of its half-life). This gave 1 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 actual synthesis was done by a recoil technique, introduced by Albert Ghiorso. In this technique, the target element was placed on the opposite side of the target from the beam and caught the recoiling atoms on a catcher foil. This recoil target was made by an electroplating technique, developed by Alfred Chetham-Strode. This technique gave a very high yield, which is absolutely necessary when working with such a rare product as the einsteinium target material.
The recoil target consisted of 10−9 of 253Es which were deposited electrolytically on a thin gold foil (also Be, Al and Pt can be used). It was bombarded by 41 eV α-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. The use of helium, in a gaseous atmosphere, slowed down the recoil atoms. This gas could be pumped out of the reaction chamber through a small orifice to form a ‘gas-jet’. Some fraction of the nonvolatile product atoms carried along with the gas, were deposited permanently on the foil surface. The foil could be removed periodically and a new foil could be installed. The next reaction was used for the mendelevium discovery experiment: 253Es + 4He → 256Md + 1n.
Purification and isolation
The removal of the Md atoms from the collector foil was done by acid etching or total dissolution of the thin gold foil. They can be purified and isolated from other product activities by several techniques. Separation of trivalent actinides from lanthanide fission products and La carrier can be done by a cation-exchange resin column using a 90% water/10% ethanol solution saturated with HCl as eluant. To separate Md rapidly from the catcher foil, an anion-exchange chromatography using 6M HCl as eluant can be used. The gold remained on the column while the Md and other actinides passed through. A final isolation of Md3+ from other trivalent actinides was also required. To separate fractions containing elements 99, 100 and 101, a cation-exchange resin column (Dowex-50 exchange column) treated with ammonium salts was used. A chemical identification was made on the basis of its elution position just before Fm. In series of repetitive experiments, they made use of the eluant: α-hydroxyisobutyrate solution (α-HIB). Using the ‘gas-jet’ method, the first two steps can be eliminated. There was shown that in this method it is possible to transport and collect individual product atoms in a fraction of a second some tens of meters away from the target area. Effective transport over long distances requires the presence of large clusters (KCl aerosols) in the ‘carrier’ gas. It is used frequently in the production and isolation of transeinsteinium elements.
Another possible way to separate the 3+ actinides can be achieved by solvent extraction chromatography using bis-(2-ethylhexyl) phosphoric acid (abbreviated as HDEHP) as the stationary organic phase and HNO3 as the mobile aqueous phase. The actinide elution sequence is reversed from that of the cation-exchange resin column. The Md separated by this method has the advantage to be free of organic complexing agent compared to the resin column. The disadvantage of this method is that Md elutes after Fm late in the sequence.
The first "Hooray!"
There was no direct detection, but by observation of spontaneous fission events arising from its electron-capture daughter 256Fm. These events were recorded during the night of February 19, 1955. 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. Additional analysis and further experimentation, showed the isotope to have mass 256 and to decay by electron capture with a half-life of 1.5 h.
Sixteen isotopes of mendelevium from mass 245 to 260 have been characterized, with the most stable being 258Md with a half-life of 51.5 days, 260Md with a half-life of 31.8 days, and 257Md with a half-life of 5.52 hours. All of the remaining radioactive isotopes have half-lives that are less than 97 minutes, and the majority of these have half-lives that are less than 5 minutes. This element also has 5 meta states, with the longest-lived being 258mMd (t½ = 58 minutes). The isotopes of mendelevium range in atomic weight from 245.091 u (245Md) to 260.104 u (260Md).