Berkelium

Berkelium is a transuranic radioactive chemical element with symbol Bk and atomic number 97. It is a member of the actinide and transuranium element series. It is named after the city of Berkeley, California, the location of the Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory) where it was discovered in December 1949. Berkelium was the fifth transuranium element discovered after neptunium, plutonium, curium and americium.

The major isotope of berkelium, 249Bk, is synthesized in minute quantities in dedicated high-flux nuclear reactors, mainly at the Oak Ridge National Laboratory in Tennessee, USA, and at the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. The production of the second-most important isotope 247Bk involves the irradiation of the rare isotope 244Cm with high-energy alpha particles.

Just over one gram of berkelium has been produced in the United States since 1967. There is no practical application of berkelium outside scientific research which is mostly directed at the synthesis of heavier transuranic elements and transactinides. A 22 milligram batch of berkelium-249 was prepared during a 250-day irradiation period and then purified for a further 90 days at Oak Ridge in 2009. This sample was used to synthesize the new element tennessine for the first time in 2009 at the Joint Institute for Nuclear Research, Russia, after it was bombarded with calcium-48 ions for 150 days. This was the culmination of the Russia–US collaboration on the synthesis of the heaviest elements on the periodic table.

Berkelium is a soft, silvery-white, radioactive metal. The berkelium-249 isotope emits low-energy electrons and thus is relatively safe to handle. It decays with a half-life of 330 days to californium-249, which is a strong emitter of ionizing alpha particles. This gradual transformation is an important consideration when studying the properties of elemental berkelium and its chemical compounds, since the formation of californium brings not only chemical contamination, but also free-radical effects and self-heating from the emitted alpha particles.

Berkelium,  97Bk
Berkelium metal
Berkelium
Pronunciation
  • /bərˈkɛliəm/
    (bər-KEL-ee-əm)
  • /ˈbɜːrkliəm/
    (BUR-klee-əm)
Appearancesilvery
Mass number247 (most stable isotope)
Berkelium 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
Tb

Bk

(Upu)
curiumberkeliumcalifornium
Atomic number (Z)97
Groupgroup n/a
Periodperiod 7
Blockf-block
Element category  actinide
Electron configuration[Rn] 5f9 7s2
Electrons per shell
2, 8, 18, 32, 27, 8, 2
Physical properties
Phase at STPsolid
Melting pointbeta: 1259 K ​(986 °C, ​1807 °F)
Boiling pointbeta: 2900 K ​(2627 °C, ​4760 °F)
Density (near r.t.)alpha: 14.78 g/cm3
beta: 13.25 g/cm3
Heat of fusion7.92 kJ/mol (calculated)
Atomic properties
Oxidation states+2, +3, +4, +5[1]
ElectronegativityPauling scale: 1.3
Ionization energies
  • 1st: 601 kJ/mol
Atomic radiusempirical: 170 pm
Color lines in a spectral range
Spectral lines of berkelium
Other properties
Natural occurrencesynthetic
Crystal structuredouble hexagonal close-packed (dhcp)
Double hexagonal close packed crystal structure for berkelium
Thermal conductivity10 W/(m·K)
Magnetic orderingparamagnetic
CAS Number7440-40-6
History
Namingafter Berkeley, California, where it was discovered
DiscoveryLawrence Berkeley National Laboratory (1949)
Main isotopes of berkelium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
245Bk syn 4.94 d ε 245Cm
α 241Am
246Bk syn 1.8 d α 242Am
ε 246Cm
247Bk syn 1380 y α 243Am
248Bk syn >300 y[2] α 244Am
249Bk syn 330 d α 245Am
SF
β 249Cf

Characteristics

Physical

Closest packing ABAC
Double-hexagonal close packing with the layer sequence ABAC in the crystal structure of α-berkelium (A: green, B: blue, C: red)

Berkelium is a soft, silvery-white, radioactive actinide metal. In the periodic table, it is located to the right of the actinide curium, to the left of the actinide californium and below the lanthanide terbium with which it shares many similarities in physical and chemical properties. Its density of 14.78 g/cm3 lies between those of curium (13.52 g/cm3) and californium (15.1 g/cm3), as does its melting point of 986 °C, below that of curium (1340 °C) but higher than that of californium (900 °C).[4] Berkelium is relatively soft and has one of the lowest bulk moduli among the actinides, at about 20 GPa (2×1010 Pa).[5]

Berkelium(III) ions shows two sharp fluorescence peaks at 652 nanometers (red light) and 742 nanometers (deep red – near infrared) due to internal transitions at the f-electron shell. The relative intensity of these peaks depends on the excitation power and temperature of the sample. This emission can be observed, for example, after dispersing berkelium ions in a silicate glass, by melting the glass in presence of berkelium oxide or halide.[6][7]

Between 70 K and room temperature, berkelium behaves as a Curie–Weiss paramagnetic material with an effective magnetic moment of 9.69 Bohr magnetonsB) and a Curie temperature of 101 K. This magnetic moment is almost equal to the theoretical value of 9.72 µB calculated within the simple atomic L-S coupling model. Upon cooling to about 34 K, berkelium undergoes a transition to an antiferromagnetic state.[8] Enthalpy of dissolution in hydrochloric acid at standard conditions is −600 kJ/mol, from which the standard enthalpy change of formationfH°) of aqueous Bk3+ ions is obtained as −601 kJ/mol. The standard potential Bk3+/Bk0 is −2.01 V.[9] The ionization potential of a neutral berkelium atom is 6.23 eV.[10]

Allotropes

At ambient conditions, berkelium assumes its most stable α form which has a hexagonal symmetry, space group P63/mmc, lattice parameters of 341 pm and 1107 pm. The crystal has a double-hexagonal close packing structure with the layer sequence ABAC and so is isotypic (having a similar structure) with α-lanthanum and α-forms of actinides beyond curium.[11] This crystal structure changes with pressure and temperature. When compressed at room temperature to 7 GPa, α-berkelium transforms to the beta modification, which has a face-centered cubic (fcc) symmetry and space group Fm3m. This transition occurs without change in volume, but the enthalpy increases by 3.66 kJ/mol.[12] Upon further compression to 25 GPa, berkelium transforms to an orthorhombic γ-berkelium structure similar to that of α-uranium. This transition is accompanied by a 12% volume decrease and delocalization of the electrons at the 5f electron shell.[13] No further phase transitions are observed up to 57 GPa.[5][14]

Upon heating, α-berkelium transforms into another phase with an fcc lattice (but slightly different from β-berkelium), space group Fm3m and the lattice constant of 500 pm; this fcc structure is equivalent to the closest packing with the sequence ABC. This phase is metastable and will gradually revert to the original α-berkelium phase at room temperature.[11] The temperature of the phase transition is believed to be quite close to the melting point.[15][16][17]

Chemical

Like all actinides, berkelium dissolves in various aqueous inorganic acids, liberating gaseous hydrogen and converting into the berkelium(III) state. This trivalent oxidation state (+3) is the most stable, especially in aqueous solutions[18][19], but tetravalent (+4)[20] and possibly divalent (+2) berkelium compounds are also known. The existence of divalent berkelium salts is uncertain and has only been reported in mixed lanthanum chloride-strontium chloride melts.[21][22] A similar behavior is observed for the lanthanide analogue of berkelium, terbium.[23] Aqueous solutions of Bk3+ ions are green in most acids. The color of Bk4+ ions is yellow in hydrochloric acid and orange-yellow in sulfuric acid.[21][24][25] Berkelium does not react rapidly with oxygen at room temperature, possibly due to the formation of a protective oxide layer surface. However, it reacts with molten metals, hydrogen, halogens, chalcogens and pnictogens to form various binary compounds.[8][15]

Isotopes

About twenty isotopes and six nuclear isomers (excited states of an isotope) of berkelium have been characterized with the mass numbers ranging from 235 to 254. All of them are radioactive. The longest half-lives are observed for 247Bk (1,380 years), 248Bk (over 300 years) and 249Bk (330 days); the half-lives of the other isotopes range from microseconds to several days. The isotope which is the easiest to synthesize is berkelium-249. This emits mostly soft β-particles which are inconvenient for detection. Its alpha radiation is rather weak – 1.45×103% with respect to the β-radiation – but is sometimes used to detect this isotope. The second important berkelium isotope, berkelium-247, is an alpha-emitter, as are most actinide isotopes.[26][27]

Occurrence

All berkelium isotopes have a half-life far too short to be primordial. Therefore, any primordial berkelium, that is, berkelium present on the Earth during its formation, has decayed by now.

On Earth, berkelium is mostly concentrated in certain areas, which were used for the atmospheric nuclear weapons tests between 1945 and 1980, as well as at the sites of nuclear incidents, such as the Chernobyl disaster, Three Mile Island accident and 1968 Thule Air Base B-52 crash. Analysis of the debris at the testing site of the first U.S. hydrogen bomb, Ivy Mike, (1 November 1952, Enewetak Atoll), revealed high concentrations of various actinides, including berkelium. For reasons of military secrecy, this result was published only in 1956.[28]

Nuclear reactors produce mostly, among the berkelium isotopes, berkelium-249. During the storage and before the fuel disposal, most of it beta decays to californium-249. The latter has a half-life of 351 years, which is relatively long when compared to the other isotopes produced in the reactor,[29] and is therefore undesirable in the disposal products.

The transuranic elements from americium to fermium, including berkelium, occurred naturally in the natural nuclear fission reactor at Oklo, but no longer do so.[30]

History

Berkeley 60-inch cyclotron
The 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley, in August 1939
The University of California Berkeley 1868
Berkelium is named after UC Berkeley

Although very small amounts of berkelium were possibly produced in previous nuclear experiments, it was first intentionally synthesized, isolated and identified in December 1949 by Glenn T. Seaborg, Albert Ghiorso, Stanley G. Thompson, and Kenneth Street, Jr.. They used the 60-inch cyclotron at the University of California, Berkeley. Similar to the nearly simultaneous discovery of americium (element 95) and curium (element 96) in 1944, the new elements berkelium and californium (element 98) were both produced in 1949–1950.[23][31][32][33][34]

The name choice for element 97 followed the previous tradition of the Californian group to draw an analogy between the newly discovered actinide and the lanthanide element positioned above it in the periodic table. Previously, americium was named after a continent as its analogue europium, and curium honored scientists Marie and Pierre Curie as the lanthanide above it, gadolinium, was named after the explorer of the rare earth elements Johan Gadolin. Thus the discovery report by the Berkeley group reads: "It is suggested that element 97 be given the name berkelium (symbol Bk) after the city of Berkeley in a manner similar to that used in naming its chemical homologue terbium (atomic number 65) whose name was derived from the town of Ytterby, Sweden, where the rare earth minerals were first found."[32] This tradition ended on berkelium, though, as the naming of the next discovered actinide, californium, was not related to its lanthanide analogue dysprosium, but after the discovery place.[35]

The most difficult steps in the synthesis of berkelium were its separation from the final products and the production of sufficient quantities of americium for the target material. First, americium (241Am) nitrate solution was coated on a platinum foil, the solution was evaporated and the residue converted by annealing to americium dioxide (AmO2). This target was irradiated with 35 MeV alpha particles for 6 hours in the 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley. The (α,2n) reaction induced by the irradiation yielded the 243Bk isotope and two free neutrons:[32]

After the irradiation, the coating was dissolved with nitric acid and then precipitated as the hydroxide using concentrated aqueous ammonia solution. The product was centrifugated and re-dissolved in nitric acid. To separate berkelium from the unreacted americium, this solution was added to a mixture of ammonium and ammonium sulfate and heated to convert all the dissolved americium into the oxidation state +6. Unoxidized residual americium was precipitated by the addition of hydrofluoric acid as americium(III) fluoride (AmF
3
). This step yielded a mixture of the accompanying product curium and the expected element 97 in form of trifluorides. The mixture was converted to the corresponding hydroxides by treating it with potassium hydroxide, and after centrifugation, was dissolved in perchloric acid.[32]

Elutionskurven Tb Gd Eu und Bk Cm Am
Chromatographic elution curves revealing the similarity between the lanthanides terbium (Tb), gadolinium (Gd), and europium (Eu) (top graph) and their corresponding actinides berkelium (Bk), curium (Cm), and americium (Am) (bottom graph)[32]

Further separation was carried out in the presence of a citric acid/ammonium buffer solution in a weakly acidic medium (pH≈3.5), using ion exchange at elevated temperature. The chromatographic separation behavior was unknown for the element 97 at the time, but was anticipated by analogy with terbium. The first results were disappointing because no alpha-particle emission signature could be detected from the elution product. With further analysis, searching for characteristic X-rays and conversion electron signals, a berkelium isotope was eventually detected. Its mass number was uncertain between 243 and 244 in the initial report,[23] but was later established as 243.[32]

Synthesis and extraction

Preparation of isotopes

Berkelium is produced by bombarding lighter actinides uranium (238U) or plutonium (239Pu) with neutrons in a nuclear reactor. In a more common case of uranium fuel, plutonium is produced first by neutron capture (the so-called (n,γ) reaction or neutron fusion) followed by beta-decay:[36]

(the times are half-lives)

Plutonium-239 is further irradiated by a source that has a high neutron flux, several times higher than a conventional nuclear reactor, such as the 85-megawatt High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory in Tennessee, USA. The higher flux promotes fusion reactions involving not one but several neutrons, converting 239Pu to 244Cm and then to 249Cm:

Curium-249 has a short half-life of 64 minutes, and thus its further conversion to 250Cm has a low probability. Instead, it transforms by beta-decay into 249Bk:[27]

The thus-produced 249Bk has a long half-life of 330 days and thus can capture another neutron. However, the product, 250Bk, again has a relatively short half-life of 3.212 hours and thus does not yield any heavier berkelium isotopes. It instead decays to the californium isotope 250Cf:[37][38]

Although 247Bk is the most stable isotope of berkelium, its production in nuclear reactors is very difficut because its potential progenitor 247Cm has never been observed to undergo beta decay.[39] Thus, 249Bk is the most accessible isotope of berkelium, which still is available only in small quantities (only 0.66 grams have been produced in the US over the period 1967–1983[40]) at a high price of the order 185 USD per microgram.[4] It is the only berkelium isotope available in bulk quantities, and thus the only berkelium isotope whose properties can be extensively studied.[41]

The isotope 248Bk was first obtained in 1956 by bombarding a mixture of curium isotopes with 25 MeV α-particles. Although its direct detection was hindered by strong signal interference with 245Bk, the existence of a new isotope was proven by the growth of the decay product 248Cf which had been previously characterized. The half-life of 248Bk was estimated as 23±5 hours,[42] though later 1965 work gave a half-life in excess of 300 years (which may be due to an isomeric state).[43] Berkelium-247 was produced during the same year by irradiating 244Cm with alpha-particles:[44]

Berkelium-242 was synthesized in 1979 by bombarding 235U with 11B, 238U with 10B, 232Th with 14N or 232Th with 15N. It converts by electron capture to 242Cm with a half-life of 7.0±1.3 minutes. A search for an initially suspected isotope 241Bk was then unsuccessful;[45] 241Bk has since been synthesized.[46]

Separation

The fact that berkelium readily assumes oxidation state +4 in solids, and is relatively stable in this state in liquids greatly assists separation of berkelium away from many other actinides. These are inevitably produced in relatively large amounts during the nuclear synthesis and often favor the +3 state. This fact was not yet known in the initial experiments, which used a more complex separation procedure. Various inorganic oxidation agents can be applied to the berkelium(III) solutions to convert it to the +4 state, such as bromates (BrO
3
), bismuthates (BiO
3
), chromates (CrO2−
4
and Cr
2
O2−
7
), silver(I) thiolate (Ag
2
S
2
O
8
), lead(IV) oxide (PbO
2
), ozone (O
3
), or photochemical oxidation procedures. More recently, it has been discovered that some organic and bio-inspired molecules, such as the chelator called 3,4,3-LI(1,2-HOPO), can also oxidize Bk(III) and stabilize Bk(IV) under mild conditions.[20] Berkelium(IV) is then extracted with ion exchange, extraction chromatography or liquid-liquid extraction using HDEHP (bis-(2-ethylhexyl) phosphoric acid), amines, tributyl phosphate or various other reagents. These procedures separate berkelium from most trivalent actinides and lanthanides, except for the lanthanide cerium (lanthanides are absent in the irradiation target but are created in various nuclear fission decay chains).[47]

A more detailed procedure adopted at the Oak Ridge National Laboratory was as follows: the initial mixture of actinides is processed with ion exchange using lithium chloride reagent, then precipitated as hydroxides, filtered and dissolved in nitric acid. It is then treated with high-pressure elution from cation exchange resins, and the berkelium phase is oxidized and extracted using one of the procedures described above.[47] Reduction of the thus-obtained berkelium(IV) to the +3 oxidation state yields a solution, which is nearly free from other actinides (but contains cerium). Berkelium and cerium are then separated with another round of ion-exchange treatment.[48]

Bulk metal preparation

In order to characterize chemical and physical properties of solid berkelium and its compounds, a program was initiated in 1952 at the Material Testing Reactor, Arco, Idaho, US. It resulted in preparation of an eight-gram plutonium-239 target and in the first production of macroscopic quantities (0.6 micrograms) of berkelium by Burris B. Cunningham and Stanley G. Thompson in 1958, after a continuous reactor irradiation of this target for six years.[40][49] This irradiation method was and still is the only way of producing weighable amounts of the element, and most solid-state studies of berkelium have been conducted on microgram or submicrogram-sized samples.[15][50]

The world's major irradiation sources are the 85-megawatt High Flux Isotope Reactor at the Oak Ridge National Laboratory in Tennessee, USA,[51] and the SM-2 loop reactor at the Research Institute of Atomic Reactors (NIIAR) in Dimitrovgrad, Russia,[52] which are both dedicated to the production of transcurium elements (atomic number greater than 96). These facilities have similar power and flux levels, and are expected to have comparable production capacities for transcurium elements,[53] although the quantities produced at NIIAR are not publicly reported. In a "typical processing campaign" at Oak Ridge, tens of grams of curium are irradiated to produce decigram quantities of californium, milligram quantities of berkelium-249 and einsteinium, and picogram quantities of fermium.[54][55] In total, just over one gram of berkelium-249 has been produced at Oak Ridge since 1967.[15]

The first berkelium metal sample weighing 1.7 micrograms was prepared in 1971 by the reduction of berkelium(III) fluoride with lithium vapor at 1000 °C; the fluoride was suspended on a tungsten wire above a tantalum crucible containing molten lithium. Later, metal samples weighing up to 0.5 milligrams were obtained with this method.[11][56]

Similar results are obtained with berkelium(IV) fluoride.[13] Berkelium metal can also be produced by the reduction of berkelium(IV) oxide with thorium or lanthanum.[56][57]

Compounds

Oxides

Two oxides of berkelium are known, with the berkelium oxidation state of +3 (Bk2O3) and +4 (BkO2).[58] Berkelium(IV) oxide is a brown solid,[59] while berkelium(III) oxide is a yellow-green solid with a melting point of 1920 °C[60][59] and is formed from BkO2 by reduction with molecular hydrogen:

Upon heating to 1200 °C, the oxide Bk2O3 undergoes a phase change; it undergoes another phase change at 1750 °C. Such three-phase behavior is typical for the actinide sesquioxides. Berkelium(II) oxide, BkO, has been reported as a brittle gray solid but its exact chemical composition remains uncertain.[61]

Halides

In halides, berkelium assumes the oxidation states +3 and +4.[62] The +3 state is the most stable, especially in solutions, while the tetravalent halides BkF4 and Cs2BkCl6 are only known in the solid phase.[63] The coordination of berkelium atom in its trivalent fluoride and chloride is tricapped trigonal prismatic, with the coordination number of 9. In trivalent bromide, it is bicapped trigonal prismatic (coordination 8) or octahedral (coordination 6),[64] and in the iodide it is octahedral.[65]

Oxidation
number
F Cl Br I
+4 BkF4
(yellow[65])
Cs2BkCl6
(orange[61])
+3 BkF3
(yellow[65])
BkCl3
(green[65])
Cs2NaBkCl6[66]
BkBr3[64][67]
(yellow-green[65])
BkI3
(yellow[65])

Berkelium(IV) fluoride (BkF4) is a yellow-green ionic solid and is isotypic with uranium tetrafluoride or zirconium(IV) fluoride.[66][68][69] Berkelium(III) fluoride (BkF3) is also a yellow-green solid, but it has two crystalline structures. The most stable phase at low temperatures is isotypic with yttrium(III) fluoride, while upon heating to between 350 and 600 °C, it transforms to the structure found in lanthanum(III) fluoride.[66][68][70]

Visible amounts of berkelium(III) chloride (BkCl3) were first isolated and characterized in 1962, and weighed only 3 billionths of a gram. It can be prepared by introducing hydrogen chloride vapors into an evacuated quartz tube containing berkelium oxide at a temperature about 500 °C.[71] This green solid has a melting point of 600 °C,[62] and is isotypic with uranium(III) chloride.[72][73] Upon heating to nearly melting point, BkCl3 converts into an orthorhombic phase.[74]

Two forms of berkelium(III) bromide are known: one with berkelium having coordination 6, and one with coordination 8.[50] The latter is less stable and transforms to the former phase upon heating to about 350 °C. An important phenomenon for radioactive solids has been studied on these two crystal forms: the structure of fresh and aged 249BkBr3 samples was probed by X-ray diffraction over a period longer than 3 years, so that various fractions of berkelium-249 had beta decayed to californium-249. No change in structure was observed upon the 249BkBr3249CfBr3 transformation. However, other differences were noted for 249BkBr3 and 249CfBr3. For example, the latter could be reduced with hydrogen to 249CfBr2, but the former could not – this result was reproduced on individual 249BkBr3 and 249CfBr3 samples, as well on the samples containing both bromides.[64] The intergrowth of californium in berkelium occurs at a rate of 0.22% per day and is an intrinsic obstacle in studying berkelium properties. Beside a chemical contamination, 249Cf, being an alpha emitter, brings undesirable self-damage of the crystal lattice and the resulting self-heating. The chemical effect however can be avoided by performing measurements as a function of time and extrapolating the obtained results.[63]

Other inorganic compounds

The pnictides of berkelium-249 of the type BkX are known for the elements nitrogen,[75] phosphorus, arsenic and antimony. They crystallize in the rock-salt structure and are prepared by the reaction of either berkelium(III) hydride (BkH3) or metallic berkelium with these elements at elevated temperature (about 600 °C) under high vacuum.[76]

Berkelium(III) sulfide, Bk2S3, is prepared by either treating berkelium oxide with a mixture of hydrogen sulfide and carbon disulfide vapors at 1130 °C, or by directly reacting metallic berkelium with elemental sulfur. These procedures yield brownish-black crystals.[77]

Berkelium(III) and berkelium(IV) hydroxides are both stable in 1 molar solutions of sodium hydroxide. Berkelium(III) phosphate (BkPO4) has been prepared as a solid, which shows strong fluorescence under excitation with a green light.[78] Berkelium hydrides are produced by reacting metal with hydrogen gas at temperatures about 250 °C.[75] They are non-stoichiometric with the nominal formula BkH2+x (0 < x < 1).[77] Several other salts of berkelium are known, including an oxysulfide (Bk2O2S), and hydrated nitrate (Bk(NO
3
)
3
·4H
2
O
), chloride (BkCl
3
·6H
2
O
), sulfate (Bk
2
(SO
4
)
3
·12H
2
O
) and oxalate (Bk
2
(C
2
O
4
)
3
·4H
2
O
).[63] Thermal decomposition at about 600 °C in an argon atmosphere (to avoid oxidation to BkO
2
) of Bk
2
(SO
4
)
3
·12H
2
O
yields the crystals of berkelium(III) oxysulfate (Bk
2
O
2
SO
4
). This compound is thermally stable to at least 1000 °C in inert atmosphere.[79]

Organoberkelium compounds

Berkelium forms a trigonal (η5–C5H5)3Bk metallocene complex with three cyclopentadienyl rings, which can be synthesized by reacting berkelium(III) chloride with the molten beryllocene (Be(C5H5)2) at about 70 °C. It has an amber color and a density of 2.47 g/cm3. The complex is stable to heating to at least 250 °C, and sublimates without melting at about 350 °C. The high radioactivity of berkelium gradually destroys the compound (within a period of weeks).[71][80] One cyclopentadienyl ring in (η5–C5H5)3Bk can be substituted by chlorine to yield [Bk(C5H5)2Cl]2. The optical absorption spectra of this compound are very similar to those of (η5–C5H5)3Bk.[56][79]

Applications

Berkelium
The berkelium target used for the synthesis of tennessine (in dissolved state)[81]

There is currently no use for any isotope of berkelium outside basic scientific research.[15] Berkelium-249 is a common target nuclide to prepare still heavier transuranic elements and transactinides, such as lawrencium, rutherfordium and bohrium.[15] It is also useful as a source of the isotope californium-249, which is used for studies on the chemistry of californium in preference to the more radioactive californium-252 that is produced in neutron bombardment facilities such as the HFIR.[15][82]

A 22 milligram batch of berkelium-249 was prepared in a 250-day irradiation and then purified for 90 days at Oak Ridge in 2009. This target yielded the first 6 atoms of tennessine at the Joint Institute for Nuclear Research (JINR), Dubna, Russia, after bombarding it with calcium ions in the U400 cyclotron for 150 days. This synthesis was a culmination of the Russia—US collaboration between JINR and Lawrence Livermore National Laboratory on the synthesis of elements 113 to 118 which was initiated in 1989.[83][84]

Nuclear fuel cycle

The nuclear fission properties of berkelium are different from those of the neighboring actinides curium and californium, and they suggest berkelium to perform poorly as a fuel in a nuclear reactor. Specifically, berkelium-249 has a moderately large neutron capture cross section of 710 barns for thermal neutrons, 1200 barns resonance integral, but very low fission cross section for thermal neutrons. In a thermal reactor, much of it will therefore be converted to berkelium-250 which quickly decays to californium-250.[85][86][87] In principle, berkelium-249 can sustain a nuclear chain reaction in a fast breeder reactor. Its critical mass is relatively high at 192 kg; it can be reduced with a water or steel reflector but would still exceed the world production of this isotope.[88]

Berkelium-247 can maintain chain reaction both in a thermal-neutron and in a fast-neutron reactor, however, its production is rather complex and thus the availability is much lower than its critical mass, which is about 75.7 kg for a bare sphere, 41.2 kg with a water reflector and 35.2 kg with a steel reflector (30 cm thickness).[88]

Health issues

Little is known about the effects of berkelium on human body, and analogies with other elements may not be drawn because of different radiation products (electrons for berkelium and alpha particles, neutrons, or both for most other actinides). The low energy of electrons emitted from berkelium-249 (less than 126 keV) hinders its detection, due to signal interference with other decay processes, but also makes this isotope relatively harmless to humans as compared to other actinides. However, berkelium-249 transforms with a half-life of only 330 days to the strong alpha-emitter californium-249, which is rather dangerous and has to be handled in a glove box in a dedicated laboratory.[89]

Most available berkelium toxicity data originate from research on animals. Upon ingestion by rats, only about 0.01% berkelium ends in the blood stream. From there, about 65% goes to the bones, where it remains for about 50 years, 25% to the lungs (biological half-life about 20 years), 0.035% to the testicles or 0.01% to the ovaries where berkelium stays indefinitely. The balance of about 10% is excreted.[90] In all these organs berkelium might promote cancer, and in the skeletal system its radiation can damage red blood cells. The maximum permissible amount of berkelium-249 in the human skeleton is 0.4 nanograms.[4][91]

References

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External links

Actinide

The actinide or actinoid (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.Strictly speaking, both actinium and lawrencium have been labeled as group 3 elements, but both elements are often included in any general discussion of the chemistry of the actinide elements. Actinium is the more often omitted of the two, because its placement as a group 3 element is somewhat more common in texts and for semantic reasons: since "actinide" means "like actinium", it has been argued that actinium cannot logically be an actinide, even though IUPAC acknowledges its inclusion based on common usage.The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, with the exception being either actinium or lawrencium. The series mostly corresponds to the filling of the 5f electron shell, although actinium and thorium lack any f-electrons, and curium and lawrencium have the same number as the preceding element. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. They all have very large atomic and ionic radii and exhibit an unusually large range of physical properties. While actinium and the late actinides (from americium onwards) behave similarly to the lanthanides, the elements thorium, protactinium, and uranium are much more similar to transition metals in their chemistry, with neptunium and plutonium occupying an intermediate position.

All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in nuclear reactors and nuclear weapons. Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern smoke detectors.

Of the actinides, primordial thorium and uranium occur naturally in substantial quantities. The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements. Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).

Berkelium compounds

Berkelium forms a number of chemical compounds, where it normally exists in an oxidation state of +3 or +4, and behaves similarly to its lanthanide analogue, terbium. Like all actinides, berkelium easily dissolves in various aqueous inorganic acids, liberating gaseous hydrogen and converting into the trivalent oxidation state. This trivalent state is the most stable, especially in aqueous solutions, but tetravalent berkelium compounds are also known. The existence of divalent berkelium salts is uncertain and has only been reported in mixed lanthanum chloride-strontium chloride melts. Aqueous solutions of Bk3+ ions are green in most acids. The color of the Bk4+ ions is yellow in hydrochloric acid and orange-yellow in sulfuric acid. Berkelium does not react rapidly with oxygen at room temperature, possibly due to the formation of a protective oxide surface layer; however, it reacts with molten metals, hydrogen, halogens, chalcogens and pnictogens to form various binary compounds. Berkelium can also form several organometallic compounds.

Californium

Californium is a radioactive chemical element with symbol Cf and atomic number 98. The element was first synthesized in 1950 at the Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory), by bombarding curium with alpha particles (helium-4 ions). It is an actinide element, the sixth transuranium element to be synthesized, and has the second-highest atomic mass of all the elements that have been produced in amounts large enough to see with the unaided eye (after einsteinium). The element was named after the university and the state of California.

Two crystalline forms exist for californium under normal pressure: one above and one below 900 °C (1,650 °F). A third form exists at high pressure. Californium slowly tarnishes in air at room temperature. Compounds of californium are dominated by the +3 oxidation state. The most stable of californium's twenty known isotopes is californium-251, which has a half-life of 898 years. This short half-life means the element is not found in significant quantities in the Earth's crust. Californium-252, with a half-life of about 2.645 years, is the most common isotope used and is produced at the Oak Ridge National Laboratory in the United States and the Research Institute of Atomic Reactors in Russia.

Californium is one of the few transuranium elements that have practical applications. Most of these applications exploit the property of certain isotopes of californium to emit neutrons. For example, californium can be used to help start up nuclear reactors, and it is employed as a source of neutrons when studying materials using neutron diffraction and neutron spectroscopy. Californium can also be used in nuclear synthesis of higher mass elements; oganesson (element 118) was synthesized by bombarding californium-249 atoms with calcium-48 ions. Users of californium must take into account radiological concerns and the element's ability to disrupt the formation of red blood cells by bioaccumulating in skeletal tissue.

Curium

Curium is a transuranic radioactive chemical element with symbol Cm and atomic number 96. This element of the actinide series was named after Marie and Pierre Curie – both were known for their research on radioactivity. Curium was first intentionally produced and identified in July 1944 by the group of Glenn T. Seaborg at the University of California, Berkeley. The discovery was kept secret and only released to the public in November 1947. Most curium is produced by bombarding uranium or plutonium with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains about 20 grams of curium.

Curium is a hard, dense, silvery metal with a relatively high melting point and boiling point for an actinide. Whereas it is paramagnetic at ambient conditions, it becomes antiferromagnetic upon cooling, and other magnetic transitions are also observed for many curium compounds. In compounds, curium usually exhibits valence +3 and sometimes +4, and the +3 valence is predominant in solutions. Curium readily oxidizes, and its oxides are a dominant form of this element. It forms strongly fluorescent complexes with various organic compounds, but there is no evidence of its incorporation into bacteria and archaea. When introduced into the human body, curium accumulates in the bones, lungs and liver, where it promotes cancer.

All known isotopes of curium are radioactive and have a small critical mass for a sustained nuclear chain reaction. They predominantly emit α-particles, and the heat released in this process can serve as a heat source in radioisotope thermoelectric generators, but this application is hindered by the scarcity and high cost of curium isotopes. Curium is used in production of heavier actinides and of the 238Pu radionuclide for power sources in artificial pacemakers. It served as the α-source in the alpha particle X-ray spectrometers installed on several space probes, including the Sojourner, Spirit, Opportunity and Curiosity Mars rovers and the Philae lander on comet 67P/Churyumov–Gerasimenko, to analyze the composition and structure of the surface.

Einsteinium

Einsteinium is a synthetic element with symbol Es and atomic number 99. A member of the actinide series, it is the seventh transuranic element.

Einsteinium was discovered as a component of the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein. Its most common isotope einsteinium-253 (half-life 20.47 days) is produced artificially from decay of californium-253 in a few dedicated high-power nuclear reactors with a total yield on the order of one milligram per year. The reactor synthesis is followed by a complex process of separating einsteinium-253 from other actinides and products of their decay. Other isotopes are synthesized in various laboratories, but at much smaller amounts, by bombarding heavy actinide elements with light ions. Owing to the small amounts of produced einsteinium and the short half-life of its most easily produced isotope, there are currently almost no practical applications for it outside basic scientific research. In particular, einsteinium was used to synthesize, for the first time, 17 atoms of the new element mendelevium in 1955.

Einsteinium is a soft, silvery, paramagnetic metal. Its chemistry is typical of the late actinides, with a preponderance of the +3 oxidation state; the +2 oxidation state is also accessible, especially in solids. The high radioactivity of einsteinium-253 produces a visible glow and rapidly damages its crystalline metal lattice, with released heat of about 1000 watts per gram. Difficulty in studying its properties is due to einsteinium-253's decay to berkelium-249 and then californium-249 at a rate of about 3% per day. The isotope of einsteinium with the longest half-life, einsteinium-252 (half-life 471.7 days) would be more suitable for investigation of physical properties, but it has proven far more difficult to produce and is available only in minute quantities, and not in bulk. Einsteinium is the element with the highest atomic number which has been observed in macroscopic quantities in its pure form, and this was the common short-lived isotope einsteinium-253.Like all synthetic transuranic elements, isotopes of einsteinium are very radioactive and are considered highly dangerous to health on ingestion.

Extended periodic table

An extended periodic table theorizes about chemical elements beyond those currently known in the periodic table and proven up through oganesson, which completes the seventh period (row) in the periodic table at atomic number (Z) 118.

If further elements with higher atomic numbers than this are discovered, they will be placed in additional periods, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned. Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to have an additional so-called g-block, containing at least 18 elements with partially filled g-orbitals in each period.An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969. The first element of the g-block may have atomic number 121, and thus would have the systematic name unbiunium. Despite many searches, no elements in this region have been synthesized or discovered in nature.According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially filled g-orbitals, but spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number. While Seaborg's version of the extended period had the heavier elements following the pattern set by lighter elements, as it did not take into account relativistic effects, models that take relativistic effects into account do not. Pekka Pyykkö and Burkhard Fricke used computer modeling to calculate the positions of elements up to Z = 172, and found that several were displaced from the Madelung rule. As a result of uncertainty and variability in predictions of chemical and physical properties of elements beyond 120, there is currently no consensus on their placement in the extended periodic table.

Elements in this region are likely to be highly unstable with respect to radioactive decay and undergo alpha decay or spontaneous fission with extremely short half-lives, though element 126 is hypothesized to be within an island of stability that is resistant to fission but not to alpha decay. Other islands of stability beyond the known elements may also be possible, including one theorized around element 164, though the extent of stabilizing effects from closed nuclear shells is uncertain. It is not clear how many elements beyond the expected island of stability are physically possible, whether period 8 is complete, or if there is a period 9. The International Union of Pure and Applied Chemistry (IUPAC) defines an element to exist if its lifetime is longer than 10−14 seconds, which is the time it takes for the nucleus to form an electron cloud.As early as 1940, it was noted that a simplistic interpretation of the relativistic Dirac equation runs into problems with electron orbitals at Z > 1/α ≈ 137, suggesting that neutral atoms cannot exist beyond element 137, and that a periodic table of elements based on electron orbitals therefore breaks down at this point. On the other hand, a more rigorous analysis calculates the analogous limit to be Z ≈ 173 where the 1s subshell dives into the Dirac sea, and that it is instead not neutral atoms that cannot exist beyond element 173, but bare nuclei, thus posing no obstacle to the further extension of the periodic system. Atoms beyond this critical atomic number are called supercritical atoms.

Fissile material

In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.

Inorganic compounds by element

This is a list of common inorganic and organometallic compounds of each element. Compounds are listed alphabetically by their chemical element name rather than by symbol, as in list of inorganic compounds.

Isotopes of berkelium

Berkelium (97Bk) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was 243Bk in 1949. There are 21 known radioisotopes, from 233Bk to 254Bk (with the exception of 235Bk), and 6 nuclear isomers. The longest-lived isotope is 247Bk with a half-life of 1,380 years.

Kenneth Street Jr.

Kenneth Street Jr. (1920 – 13 March 2006) was an American chemist. He was part of the team that discovered elements 97 and 98 (berkelium and californium) in 1949 and 1950.Street was born in 1920 in Berkeley, California. He obtained his degree in chemistry in 1943 from the University of California, Berkeley. He then served in World War II as a fighter pilot, with his awards including the Air Medal and the Distinguished Flying Cross. After the war, he returned to Berkeley, obtaining his PhD in nuclear chemistry in 1949, with a thesis titled 'Isotopes of americium and curium'.The work on berkelium and californium was carried out at the Lawrence Radiation Laboratory (now part of the Lawrence Berkeley National Laboratory) with Stanley G. Thompson, Glenn T. Seaborg and Albert Ghiorso. Street joined the faculty at Berkeley in 1949, and became Deputy Director of the Lawrence Radiation Laboratory, and later a professor of chemistry. His specialities and interests were in the areas of nuclear chemistry, geochemistry and geothermal energy.Street retired in 1986, and moved to Taylorsville, California in 1997 with his wife Jane (née Armitage). They had married in 1944 and had three children, two sons and a daughter. Street's interests included walking in the mountains, backpacking and sailing. Street died on 13 March 2006, in Paradise, California.

Major actinide

Major actinides is a term used in the nuclear power industry that refers to the plutonium and uranium present in used nuclear fuel, as opposed to the minor actinides neptunium, americium, curium, berkelium, and californium.

Minor actinide

The minor actinides are the actinide elements in used nuclear fuel other than uranium and plutonium, which are termed the major actinides. The minor actinides include neptunium (element 93), americium (element 95), curium (element 96), berkelium (element 97), californium (element 98), einsteinium (element 99), and fermium (element 100). The most important isotopes in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.

Plutonium and the minor actinides will be responsible for the bulk of the radiotoxicity and heat generation of used nuclear fuel in the medium term (300 to 20,000 years in the future).The plutonium from a power reactor tends to have a greater amount of Pu-241 than the plutonium generated by the lower burnup operations designed to create weapons-grade plutonium. Because the reactor-grade plutonium contains so much Pu-241 the presence of americium-241 makes the plutonium less suitable for making a nuclear weapon. The ingrowth of americium in plutonium is one of the methods for identifying the origin of an unknown sample of plutonium and the time since it was last separated chemically from the americium.

Americium is commonly used in industry as both an alpha particle and as a low photon energy gamma radiation source. For instance it is used in many smoke detectors. Americium can be formed by neutron capture of Pu-239 and Pu-240 forming Pu-241 which then beta decays to Am-241. In general, as the energy of the neutrons increases, the ratio of the fission cross section to the neutron capture cross section changes in favour of fission. Hence if MOX is used in a thermal reactor such as a boiling water reactor (BWR) or pressurized water reactor (PWR) then more americium can be expected in the used fuel than that from a fast neutron reactor.Some of the minor actinides have been found in fallout from bomb tests. See Actinides in the environment for details.

Period 7 element

A period 7 element is one of the chemical elements in the seventh row (or period) of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The seventh period contains 32 elements, tied for the most with period 6, beginning with francium and ending with oganesson, the heaviest element currently discovered. As a rule, period 7 elements fill their 7s shells first, then their 5f, 6d, and 7p shells, in that order; however, there are exceptions, such as plutonium.

Plutonium-241

Plutonium-241 (Pu-241) is an isotope of plutonium formed when plutonium-240 captures a neutron. Like Pu-239 but unlike 240Pu, 241Pu is fissile, with a neutron absorption cross section about 1/3 greater than 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission case, neutron capture produces plutonium-242. In general, isotopes with an odd number of neutrons are both more likely to absorb a neutron, and more likely to undergo fission on neutron absorption, than isotopes with an even number of neutrons.

Plutonium-242

Plutonium-242 is one of the isotopes of plutonium, the second longest-lived, with a half-life of 373,300 years.

242Pu's halflife is about 15 times as long as Pu-239's halflife; therefore, it is one-fifteenth as radioactive and not one of the larger contributors to nuclear waste radioactivity.

242Pu's gamma ray emissions are also weaker than those of the other isotopes.It is not fissile (though it is fissionable by fast neutrons) and its neutron capture cross section is also low.

Synthetic element

In chemistry, a synthetic element is a chemical element that does not occur naturally on Earth, and can only be created artificially. So far, 24 synthetic elements have been created (those with atomic numbers 95–118). All are unstable, decaying with half-lives ranging from 15.6 million years to a few hundred microseconds.

Seven other elements were first created artificially and thus considered synthetic, but later discovered to exist naturally (in trace quantities) as well; among them plutonium—first synthesized in 1940—the one best known to laypeople, because of its use in atomic bombs and nuclear reactors.

Tennessine

Tennessine is a synthetic chemical element with symbol Ts and atomic number 117. It is the second-heaviest known element and the penultimate element of the 7th period of the periodic table.

The discovery of tennessine was officially announced in Dubna, Russia, by a Russian–American collaboration in April 2010, which makes it the most recently discovered element as of 2019. One of its daughter isotopes was created directly in 2011, partially confirming the results of the experiment. The experiment itself was repeated successfully by the same collaboration in 2012 and by a joint German–American team in May 2014. In December 2015, the Joint Working Party of the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics, which evaluates claims of discovery of new elements, recognized the element and assigned the priority to the Russian–American team. In June 2016, the IUPAC published a declaration stating that the discoverers had suggested the name tennessine after Tennessee, United States. In November 2016, they officially adopted the name "tennessine".

Tennessine may be located in the "island of stability", a concept that explains why some superheavy elements are more stable compared to an overall trend of decreasing stability for elements beyond bismuth on the periodic table. The synthesized tennessine atoms have lasted tens and hundreds of milliseconds. In the periodic table, tennessine is expected to be a member of group 17, all other members of which are halogens. Some of its properties may significantly differ from those of the halogens due to relativistic effects. As a result, tennessine is expected to be a volatile metal that neither forms anions nor achieves high oxidation states. A few key properties, such as its melting and boiling points and its first ionization energy, are nevertheless expected to follow the periodic trends of the halogens.

Transuranium element

The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements.

UC Berkeley College of Chemistry

The UC Berkeley College of Chemistry is one of 14 schools and colleges at the University of California, Berkeley. It houses the departments of Chemistry, Chemical and Biomolecular Engineering, and Chemical Biology and occupies six buildings flanking a central plaza.UC Berkeley's College of Chemistry has been listed as the best global university for chemistry in the 2019 U.S. News and World Report Education rankings. The college's Chemical and Biomolecular Engineering program was ranked number two in a tie with Caltech and Stanford among U.S. News best engineering schools in the United States in 2018. Its faculty and graduates have won numerous awards, including the Wolf Prize, the National Medal of Science, the National Medal of Technology, the Presidential Medal of Freedom, as well as fourteen Nobel Prizes. As of 2018-19, it has 760 undergraduates, 523 graduate students and 164 postdoctoral students.The Department of Chemistry is one of the largest and most productive in the world, graduating about 80 doctoral students per year. The College hosts 10 recognized world-class researchers by production of multiple highly cited papers that rank in the top 1% by citations for field and year in Web of Science. Scientists affiliated with the department and the nearby Lawrence Berkeley National Laboratory are responsible for the discovery of sixteen elements, including berkelium, named after the city, and seaborgium, named after Nobel laureate and former department chair Glenn Seaborg.First established in 1872, the college awarded its first Ph.D. in 1885 to John Stillman, who later founded the chemistry department at Stanford University. A Division of Chemical Engineering was established in 1946, becoming a department in 1957. The Department of Chemical Engineering changed its name to Chemical and Biomolecular Engineering in 2010 to reflect the research focus of its faculty in the 21st century.

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