Mendelevium

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Mendelevium, 101Md
Mendelevium
Pronunciation
Mass number[258]
Mendelevium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Tm

Md

(Upp)
fermiummendeleviumnobelium
Discovery
Lawrence Berkeley National Laboratory (1955)
Isotopes of mendelevium
Main isotopes[3] Decay
abun­dance half-life (t1/2) mode pro­duct
256Md synth 1.17 h ε
256Fm
257Md synth 5.52 h ε
257Fm
α
253Es
SF
258Md synth 51.5 d α
254Es
ε
258Fm
β
258No
259Md synth 1.60 h SF
α
255Es
260Md synth 31.8 d SF
α
256Es
ε
260Fm
β
260No
 Category: Mendelevium
| references

Mendelevium is a

transuranic element. It can only be produced in particle accelerators by bombarding lighter elements with charged particles. Seventeen isotopes are known; the most stable is 258Md with half-life 51 days; however, the shorter-lived 256Md (half-life 1.17 hours
) is most commonly used in chemistry because it can be produced on a larger scale.

Mendelevium was discovered by bombarding

scientific research
, and only small amounts are produced.

Discovery

Black-and-white picture of heavy machinery with two operators sitting aside
The 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley, in August 1939

Mendelevium was the ninth

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 produced.[5] This discovery was part of a program, begun in 1952, that irradiated plutonium with neutrons to transmute it into heavier actinides.[6] This method was necessary as the previous method used to synthesize transuranic elements, neutron capture, could not work because of a lack of known beta decaying isotopes of fermium that would produce isotopes of the next element, mendelevium, and also due to the very short half-life to spontaneous fission of 258Fm that thus constituted a hard limit to the success of the neutron capture process.[4]

External videos
video icon Reenactment of the discovery of mendelevium at Berkeley

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 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.[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 using

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.[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 by electron capture to fermium and that the experiment should be repeated to search instead for spontaneous fission events.[6] The repetition of the experiment happened in February 1955.[6]

The element was named after Dmitri Mendeleev.

On the day of discovery, 19 February, alpha irradiation of the einsteinium target occurred in three three-hour sessions. The cyclotron was in the

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.[6] Additional analysis and further experimentation showed the produced mendelevium isotope to have mass 256 and to decay by electron capture to fermium-256 with a half-life of 1.5 h.[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 chemist Dmitri Mendeleev, father of the periodic table. Because this discovery came during the Cold War, Seaborg had to request permission of the government of the United States to propose that the element be named for a Russian, but it was granted.[6] The name "mendelevium" was accepted by the International 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]

Characteristics

Physical

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.)[12]

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.[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 as

face-centered cubic crystal structure.[1] Mendelevium's melting point has been estimated at 800 °C, the same value as that predicted for the neighboring element nobelium.[17] Its density is predicted to be around 10.3±0.7 g/cm3.[1]

Chemical

The chemistry of mendelevium is mostly known only 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.[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 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.[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 form coordination complexes with 1,2-cyclohexanedinitrilotetraacetic acid (DCTA).[18]

In

standard reduction potential of the E°(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, and E°(Md2+→Md0) should be around −2.5 V.[18] Mendelevium(II)'s elution behavior has been compared with that of strontium(II) and europium(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 with samarium(II). It was found to be stable in neutral water–ethanol solution and be homologous to caesium(I). However, later experiments found no evidence for mendelevium(I) and found that mendelevium behaved like divalent elements when reduced, not like the monovalent alkali metals.[18] Nevertheless, the Russian team conducted further studies on the thermodynamics of cocrystallizing mendelevium with alkali metal chlorides, 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 potential E°(Md4+→Md3+) was predicted in 1975 to be +5.4 V; 1967 experiments with the strong oxidizing agent sodium bismuthate were unable to oxidize mendelevium(III) to mendelevium(IV).[18]

Atomic

A mendelevium atom has 101 electrons. They are expected to be arranged in the configuration [Rn]5f137s2 (ground state

distribution coefficients and ionic radius produced a value of 89.6 pm, as well as an enthalpy 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]

Isotopes

Seventeen isotopes of mendelevium are known, with mass numbers from 244 to 260; all are radioactive.[23] Additionally, five nuclear isomers are known: 245mMd, 247mMd, 249mMd, 254mMd, and 258mMd.[4][24] Of these, the longest-lived isotope is 258Md with a half-life of 51.5 days, and the longest-lived isomer is 258mMd with a half-life of 58.0 minutes.[4][24] Nevertheless, the shorter-lived 256Md (half-life 1.17 hours) is more often used in chemical experimentation because it can be produced in larger quantities from alpha particle irradiation of einsteinium.[23] After 258Md, the next most stable mendelevium isotopes are 260Md with a half-life of 31.8 days, 257Md with a half-life of 5.52 hours, 259Md with a half-life of 1.60 hours, and 256Md with a half-life of 1.17 hours. All of the remaining mendelevium isotopes have half-lives that are less than an hour, and the majority of these have half-lives that are less than 5 minutes.[4][23][24]

The half-lives of mendelevium isotopes mostly increase smoothly from 244Md onwards, reaching a maximum at 258Md.[4][23][24] Experiments and predictions suggest that the half-lives will then decrease, apart from 260Md with a half-life of 31.8 days,[4][23][24] as spontaneous fission becomes the dominant decay mode[4] due to the mutual repulsion of the protons posing a limit to the island of relative stability of long-lived nuclei in the actinide series.[25]

Mendelevium-256, the chemically most important isotope of mendelevium, decays through

fermium-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.[26]

Production and isolation

The lightest isotopes (244Md to 247Md) are mostly produced through bombardment of

femtogram quantities of mendelevium-256 may be produced.[23]

The recoil

anion-exchange chromatography, the eluant being 6 M hydrochloric acid.[26]

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.[26] Using the gas-jet method often renders the first two steps unnecessary.[26] The above procedure is the most commonly used one for the separation of transeinsteinium elements.[26]

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 and nitric 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][26]

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 of

hexafluoroacetylacetonate: the analogous fermium compound is also known and is also volatile.[26]

Toxicity

Though few people come in contact with mendelevium, the International Commission on Radiological Protection has set annual exposure limits for the most stable isotope. For mendelevium-258, the ingestion limit was set at 9×105 becquerels (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).[27]

Notes

  1. ^ The density is calculated from the predicted metallic radius (Silva 2006, p. 1635) and the predicted close-packed crystal structure (Fournier 1976).

References

  1. ^ .
  2. .
  3. .
  4. ^
  5. ^
    ISBN 9789810214401. {{cite book}}: |journal= ignored (help
    )
  6. ^ a b c d e f g h Choppin, Gregory R. (2003). "Mendelevium". Chemical and Engineering News. 81 (36).
  7. .
  8. ^ .
  9. ^ 101. Mendelevium - Elementymology & Elements Multidict. Peter van der Krogt.
  10. ^ Chemistry, International Union of Pure and Applied (1955). Comptes rendus de la confèrence IUPAC.
  11. ^ Chemistry, International Union of Pure and Applied (1957). Comptes rendus de la confèrence IUPAC.
  12. ISBN 978-1-4020-3555-5. Archived from the original
    (PDF) on 2010-07-17. Retrieved 2014-08-04.
  13. ^ a b c d e Silva, pp. 1634–5
  14. ^ a b Silva, pp. 1626–8
  15. .
  16. .
  17. .
  18. ^ a b c d e f g h i j k l m n Silva, pp. 1635–6
  19. ^ Toyoshima, Atsushi; Li, Zijie; Asai, Masato; Sato, Nozomi; Sato, Tetsuya K.; Kikuchi, Takahiro; Kaneya, Yusuke; Kitatsuji, Yoshihiro; Tsukada, Kazuaki; Nagame, Yuichiro; Schädel, Matthias; Ooe, Kazuhiro; Kasamatsu, Yoshitaka; Shinohara, Atsushi; Haba, Hiromitsu; Even, Julia (11 October 2013). "Measurement of the Md3+/Md2+ Reduction Potential Studied with Flow Electrolytic Chromatography". Inorganic Chemistry. 52 (21): 12311–3.
    PMID 24116851
    .
  20. ^ Silva, pp. 1633–4
  21. doi:10.1063/1.3253147. Archived from the original
    (PDF) on 2014-02-11. Retrieved 2013-10-19.
  22. ^ David R. Lide (ed), CRC Handbook of Chemistry and Physics, 84th Edition. CRC Press. Boca Raton, Florida, 2003; Section 10, Atomic, Molecular, and Optical Physics; Ionization Potentials of Atoms and Atomic Ions
  23. ^ a b c d e f g h i Silva, pp. 1630–1
  24. ^ a b c d e Nucleonica (2007–2014). "Universal Nuclide Chart". Nucleonica. Retrieved 22 May 2011.
  25. .
  26. ^ a b c d e f g h i j k l m n Silva, pp. 1631–3
  27. .

Bibliography

Further reading

External links