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{{Use dmy dates|date=September 2020}}
{{infobox berkelium}}
'''Berkelium''' is a [[synthetic element|synthetic chemical element]]; it has [[Chemical symbol|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, Berkeley|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, <sup>249</sup>Bk, is synthesized in minute quantities in dedicated high-flux [[nuclear reactor]]s, mainly at the [[Oak Ridge National Laboratory]] in [[Tennessee]], United States, and at the [[Research Institute of Atomic Reactors]] in [[Dimitrovgrad, Russia]]. The longest-lived and second-most important isotope, <sup>247</sup>Bk, can be synthesized via irradiation of [[Isotopes of curium|<sup>244</sup>Cm]] with high-energy [[alpha particle]]s.

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 [[transuranium element]]s and [[superheavy element]]s. 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 decay|radioactive]] metal. The berkelium-249 isotope emits low-energy [[electron]]s 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.

==Characteristics==

===Physical===
[[File:Closest packing ABAC.png|thumb|Double-hexagonal close packing with the layer sequence ABAC in the crystal structure of α-berkelium (A: green, B: blue, C: red)|alt=Sequential layers of spheres arranged from top to bottom: GRGBGRGB (G=green, R=red, B=blue)]]

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&nbsp;g/cm<sup>3</sup> lies between those of curium (13.52&nbsp;g/cm<sup>3</sup>) and californium (15.1&nbsp;g/cm<sup>3</sup>), as does its melting point of 986&nbsp;°C, below that of curium (1340&nbsp;°C) but higher than that of californium (900&nbsp;°C).<ref name="CRC" /> Berkelium is relatively soft and has one of the lowest [[bulk modulus|bulk moduli]] among the actinides, at about 20 [[Pascal (unit)|GPa]] (2{{e|10}}&nbsp;Pa).<ref name="pressure" />

{{Not a typo|Berkelium(III)}} ions shows two sharp [[fluorescence]] peaks at 652&nbsp;[[nanometre|nanometer]]s (red light) and 742&nbsp;nanometers (deep red – [[Infrared#Regions within the infrared|near-infrared]]) due to internal transitions at the [[Electron configuration|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.<ref>{{cite journal|last1=Assefa|first1=Z.|last2=Haire|first2=R. G.|last3=Stump|first3=N. A.|title=Emission profile of Bk(III) in a silicate matrix: anomalous dependence on excitation power|journal=Journal of Alloys and Compounds|volume=271-273|pages=854–858|date=1998|doi=10.1016/S0925-8388(98)00233-3|url=https://zenodo.org/record/1260149}}</ref><ref>Rita Cornelis, Joe Caruso, Helen Crews, Klaus Heumann [https://books.google.com/books?id=1PmjurlE6KkC&pg=PA553 Handbook of elemental speciation II: species in the environment, food, medicine & occupational health. Volume 2 of Handbook of Elemental Speciation], John Wiley and Sons, 2005, {{ISBN|0-470-85598-3}} p. 553</ref>

Between 70&nbsp;K and room temperature, berkelium behaves as a [[Curie–Weiss law|Curie–Weiss]] paramagnetic material with an effective magnetic moment of 9.69&nbsp;[[Bohr magneton]]s (μ<sub>B</sub>) and a [[Curie temperature]] of 101&nbsp;K. This magnetic moment is almost equal to the theoretical value of 9.72&nbsp;μ<sub>B</sub> calculated within the simple atomic [[Angular momentum coupling|L-S coupling model]]. Upon cooling to about 34&nbsp;K, berkelium undergoes a transition to an [[antiferromagnetism|antiferromagnetic]] state.{{sfn|Peterson|Hobart|1984|p=45}} The [[Enthalpy change of solution|enthalpy of dissolution]] in [[hydrochloric acid]] at standard conditions is −600&nbsp;kJ/mol, from which the [[standard enthalpy of formation]] (Δ<sub>f</sub>''H''°) of aqueous {{chem2|Bk(3+)}} ions is obtained as −601&nbsp;kJ/mol. The [[standard electrode potential]] {{chem2|Bk(3+)}}/Bk is −2.01&nbsp;V.<ref>{{cite journal|last1=Fuger|first1=J.|title=A new determination of the enthalpy of solution of berkelium metal and the standard enthalpy of formation of Bk3+ (aq)|journal=Journal of Inorganic and Nuclear Chemistry|volume=43|pages=3209|date=1981|doi=10.1016/0022-1902(81)80090-5|issue=12|last2=Haire|first2=R. G.|last3=Peterson|first3=J. R.}}</ref> The [[Ionization energy|ionization potential]] of a neutral berkelium atom is 6.23&nbsp;eV.{{sfn|Peterson|Hobart|1984|p=34}}

===Allotropes===
At ambient conditions, berkelium assumes its most stable α form which has a [[hexagonal crystal family|hexagonal]] symmetry, [[space group]] ''P6<sub>3</sub>/mmc'', lattice parameters of 341&nbsp;[[picometre|pm]] and 1107&nbsp;pm. The crystal has a double-[[Close-packing of equal spheres|hexagonal close packing]] structure with the layer sequence ABAC and so is [[isostructural|isotypic]] (having a similar structure) with α-lanthanum and α-forms of actinides beyond curium.<ref name = "Peterson" /> This crystal structure changes with pressure and temperature. When compressed at room temperature to 7&nbsp;GPa, α-berkelium transforms to the β modification, which has a [[Cubic crystal system|face-centered cubic]] (''fcc'') symmetry and space group ''Fm{{overline|3}}m''. This transition occurs without change in volume, but the [[enthalpy]] increases by 3.66&nbsp;kJ/mol.{{sfn|Peterson|Hobart|1984|p=44}} Upon further compression to 25&nbsp;GPa, berkelium transforms to an [[Orthorhombic crystal system|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 [[electron shell|5f electron shell]].<ref name="pressure2" /> No further phase transitions are observed up to 57 GPa.<ref name="pressure">{{cite journal|last1=Benedict|first1=U.|title=Study of actinide metals and actinide compounds under high pressures|journal=Journal of the Less Common Metals|volume=100|pages=153–170|date=1984|doi=10.1016/0022-5088(84)90061-4}}</ref><ref>Young, David A. [https://books.google.com/books?id=F2HVYh6wLBcC&pg=PA228 Phase diagrams of the elements], University of California Press, 1991, {{ISBN|0-520-07483-1}} p. 228</ref>

Upon heating, α-berkelium transforms into another phase with an ''fcc'' lattice (but slightly different from β-berkelium), space group ''Fm{{overline|3}}m'' and the lattice constant of 500&nbsp;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]].<ref name="Peterson" /> The temperature of the phase transition is believed to be quite close to the melting point.<ref name="H&P" /><ref name="Fahey">{{cite journal|last1 = Fahey|first1 = J. A.|last2 = Peterson|first2 = J. R.|last3 = Baybarz|first3 = R. D.|date = 1972|title = Some properties of berkelium metal and the apparent trend toward divalent character in the transcurium actinide metals|journal = Inorg. Nucl. Chem. Lett.|volume = 8|issue = 1|pages = 101–7|doi = 10.1016/0020-1650(72)80092-8}}</ref><ref name="Ward">{{cite journal|last1 = Ward|first1 = John W.|last2 = Kleinschmidt|first2 = Phillip D.|last3 = Haire|first3 = Richard G.|date = 1982|title = Vapor pressure and thermodynamics of Bk-249 metal|journal = J. Chem. Phys.|volume = 77|issue = 3|pages = 1464–68|doi = 10.1063/1.443975|bibcode = 1982JChPh..77.1464W }}</ref>

===Chemical===
Like all [[actinide]]s, berkelium dissolves in various aqueous inorganic acids, liberating gaseous [[hydrogen]] and converting into the {{Not a typo|berkelium(III)}} state. This [[Valence (chemistry)#trivalent|trivalent]] [[oxidation state]] (+3) is the most stable, especially in aqueous solutions,<ref>{{Cite journal|last1=Deblonde|first1=Gauthier J.-P.|last2=Kelley|first2=Morgan P.|last3=Su|first3=Jing|last4=Batista|first4=Enrique R.|last5=Yang|first5=Ping|last6=Booth|first6=Corwin H.|last7=Abergel|first7=Rebecca J.|date=2018|title=Spectroscopic and Computational Characterization of Diethylenetriaminepentaacetic Acid/Transplutonium Chelates: Evidencing Heterogeneity in the Heavy Actinide(III) Series|journal=Angewandte Chemie International Edition|language=en|volume=57|issue=17|pages=4521–4526|doi=10.1002/anie.201709183|pmid=29473263|issn=1521-3773|doi-access=free}}</ref><ref>{{Cite journal|last1=Kelley|first1=Morgan P.|last2=Deblonde|first2=Gauthier J.-P.|last3=Su|first3=Jing|last4=Booth|first4=Corwin H.|last5=Abergel|first5=Rebecca J.|last6=Batista|first6=Enrique R.|last7=Yang|first7=Ping|date=2018-05-07|title=Bond Covalency and Oxidation State of Actinide Ions Complexed with Therapeutic Chelating Agent 3,4,3-LI(1,2-HOPO)|journal=Inorganic Chemistry|volume=57|issue=9|pages=5352–5363|doi=10.1021/acs.inorgchem.8b00345|pmid=29624372|osti=1458511|issn=0020-1669|url=http://www.escholarship.org/uc/item/4tc1b0xz}}</ref> but [[Valence (chemistry)#tetravalent|tetravalent]] (+4),<ref name=":0" /> [[Valence (chemistry)#pentavalent|pentavalent]] (+5),<ref name="5+">{{cite journal |author1=Attila Kovács |author2=Phuong D. Dau |author3=Joaquim Marçalo |author4=John K. Gibson |title=Pentavalent Curium, Berkelium, and Californium in Nitrate Complexes: Extending Actinide Chemistry and Oxidation States |journal=Inorganic Chemistry |date=2018 |volume=57 |issue=15 |pages=9453–9467 |doi=10.1021/acs.inorgchem.8b01450 |publisher=ACS Publications |pmid=30040397 |osti=1631597 |s2cid=51717837 |language=en|url=https://escholarship.org/content/qt9195b5hh/qt9195b5hh.pdf }}</ref> and possibly [[Valence (chemistry)#divalent|divalent]] (+2) berkelium compounds are also known. The existence of divalent berkelium salts is uncertain and has only been reported in mixed [[lanthanum(III) chloride]]-[[strontium chloride]] melts.{{sfn|Peterson|Hobart|1984|p=55}}<ref>{{cite journal|last1=Sullivan|first1=Jim C.|last2=Schmidt|first2=K. H.|last3=Morss|first3=L. R.|last4=Pippin|first4=C. G.|last5=Williams|first5=C.|title=Pulse radiolysis studies of berkelium(III): preparation and identification of berkelium(II) in aqueous perchlorate media|journal=Inorganic Chemistry|volume=27|pages=597|date=1988|doi=10.1021/ic00277a005|issue=4}}</ref> A similar behavior is observed for the lanthanide analogue of berkelium, [[terbium]].<ref name="Thompson1950" /> Aqueous solutions of {{chem2|Bk(3+)}} ions are green in most acids. The color of {{chem2|Bk(4+)}} ions is yellow in [[hydrochloric acid]] and orange-yellow in [[sulfuric acid]].{{sfn|Peterson|Hobart|1984|p=55}}{{sfn|Holleman|Wiberg|2007|p=1956}}{{sfn|Greenwood|Earnshaw|1997|p=1265}} 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]], [[halogen]]s, [[chalcogen]]s and [[pnictogen]]s to form various binary compounds.{{sfn|Peterson|Hobart|1984|p=45}}<ref name="H&P" />

===Isotopes===
{{main|Isotopes of berkelium}}
Nineteen isotopes and six [[nuclear isomer]]s (excited states of an isotope) of berkelium have been characterized, with mass numbers ranging from 233 to 253 (except 235 and 237).<ref name="nubase">{{NUBASE2016}}</ref> All of them are radioactive. The longest [[half-life|half-lives]] are observed for <sup>247</sup>Bk (1,380&nbsp;years), <sup>248</sup>Bk (over&nbsp;300&nbsp;years), and <sup>249</sup>Bk (330&nbsp;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 [[Beta decay|β-particles]] which are inconvenient for detection. Its [[alpha decay|alpha radiation]] is rather weak (1.45{{e|-3}}%) 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.<ref name="nubase" /><ref name="TBE">{{cite book|author = B. Myasoedov|title = Analytical chemistry of transplutonium elements| place =Moscow|publisher = Nauka|date = 1972|isbn = 978-0-470-62715-0|display-authors=etal}}</ref>

===Occurrence===
All berkelium isotopes have a half-life<!-- of up to 1,380 years---> far too short to be [[primordial nuclide|primordial]].<!---over ~50 My---> 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 testing|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 [[United States]]' first [[thermonuclear weapon]], [[Ivy Mike]], (1 November 1952, [[Enewetak Atoll]]), revealed high concentrations of various actinides, including berkelium. For reasons of military secrecy, this result was not published until 1956.<ref>{{cite journal|last1=Fields|first1=P. R.|last2=Studier|first2=M. H.|last3=Diamond|first3=H.|last4=Mech|first4=J. F.|last5=Inghram|first5=M. G.|last6=Pyle|first6=G. L.|last7=Stevens|first7=C. M.|last8=Fried|first8=S.|last9=Manning|first9=W. M.|last10=Ghiorso|first10=A.|last11=Thompson|first11=S. G.|last12=Higgins|first12=G. H.|last13=Seaborg|first13=G. T.|display-authors=3|title=Transplutonium Elements in Thermonuclear Test Debris|date=1956|journal=Physical Review|volume=102|issue=1|pages=180–182|doi=10.1103/PhysRev.102.180|bibcode=1956PhRv..102..180F}}</ref>

Nuclear reactors produce mostly, among the berkelium isotopes, berkelium-249. During the storage and before the fuel disposal, most of it [[beta decay]]s to californium-249. The latter has a half-life of 351 years, which is relatively long compared to the half-lives of other isotopes produced in the reactor,<ref>{{cite web|url = http://www.nndc.bnl.gov/chart/ |editor = Alejandro A. Sonzogni (Database Manager)|title = Chart of Nuclides|publisher = National Nuclear Data Center, [[Brookhaven National Laboratory]]|access-date = 2010-03-01|year = 2008|location = Upton, New York|ref = CITEREFNNDC2008|archive-date = 10 October 2018|archive-url = https://web.archive.org/web/20181010070007/http://www.nndc.bnl.gov/chart/|url-status = dead}}</ref> and is therefore undesirable in the disposal products.

The [[transuranium element]]s from [[americium]] to [[fermium]], including berkelium, occurred naturally in the [[natural nuclear fission reactor]] at [[Oklo]], but no longer do so.<ref name="emsley">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks: An A-Z Guide to the Elements|edition=New|date=2011|publisher=Oxford University Press|location=New York, NY|isbn=978-0-19-960563-7}}</ref>

Berkelium is also one of the elements that have theoretically been detected in [[Przybylski's Star]].<ref name=gopka08>{{cite journal |last1=Gopka |first1=V. F. |last2=Yushchenko |first2=A. V. |last3=Yushchenko |first3=V. A. |last4=Panov |first4=I. V. |last5=Kim |first5=Ch. |title=Identification of absorption lines of short half-life actinides in the spectrum of Przybylski's star (HD 101065) |journal=Kinematics and Physics of Celestial Bodies |date=15 May 2008 |volume=24 |issue=2 |pages=89–98 |doi=10.3103/S0884591308020049 |bibcode = 2008KPCB...24...89G |s2cid=120526363 }}</ref>

==History==
[[File:Glenn Seaborg - 1964.jpg|thumb|left|upright|[[Glenn T. Seaborg]]]]
[[File:Berkeley 60-inch cyclotron.jpg|thumb|right|upright|The 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley, in August 1939|alt=Black-and-white picture of heavy machinery with two operators sitting aside]]
[[File:The University of California Berkeley 1868.svg|thumb|right|upright|Berkelium is named after UC Berkeley.|alt=The Seal of the University of California, Berkeley (UC Berkeley)]]

Although very small amounts of berkelium were possibly produced in previous nuclear experiments, it was [[timeline of chemical element discoveries|first intentionally synthesized]], isolated and identified in December 1949 by [[Glenn T. Seaborg]], [[Albert Ghiorso]], [[Stanley Gerald 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.<ref name="Thompson1950">{{cite journal |last1=Thompson |first1=Stanley G. |author-link1=Stanley Gerald Thompson |last2=Seaborg |first2=Glenn T. |author-link2=Glenn T. Seaborg |date=1950 |title=Chemical properties of berkelium |url=https://www.osti.gov/biblio/932812 |journal=Lawrence Berkeley National Lab |doi=10.2172/932812|hdl=2027/mdp.39015086479683 |osti=932812 |hdl-access=free }}</ref><ref>{{cite journal|last1=Thompson|first1=S.|last2=Ghiorso|first2=A.|last3=Seaborg|first3=G.|title=Element 97|journal=Physical Review|volume=77|pages=838|date=1950|doi=10.1103/PhysRev.77.838.2|issue=6|bibcode = 1950PhRv...77..838T |doi-access=free}}</ref><ref name="E97">{{cite journal|last1=Thompson|first1=S.|last2=Ghiorso|first2=A.|last3=Seaborg|first3=G.|title=The New Element Berkelium (Atomic Number 97)|doi=10.1103/PhysRev.80.781|date=1950|pages=781|volume=80|journal=Physical Review|url=http://www.osti.gov/accomplishments/documents/fullText/ACC0045.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.osti.gov/accomplishments/documents/fullText/ACC0045.pdf |archive-date=2022-10-09 |url-status=live|issue=5|bibcode = 1950PhRv...80..781T }} [http://www.osti.gov/cgi-bin/rd_accomplishments/display_biblio.cgi?id=ACC0045&numPages=38&fp=N Abstract]</ref><ref>{{cite journal|title=Chemical Properties of Berkelium|last1=Thompson|first1=S. G.|last2=Cunningham|first2=B. B.|last3=Seaborg|first3=G. T.|journal=Journal of the American Chemical Society|volume=72|pages=2798|date=1950|doi=10.1021/ja01162a538|issue=6|url=https://digital.library.unt.edu/ark:/67531/metadc896202/|hdl=2027/mdp.39015086479683|hdl-access=free}}</ref><ref>{{cite magazine|url=http://www.newyorker.com/magazine/1950/04/08/comment-3910|title=Comment|date=April 1950|magazine=The New Yorker|access-date=4 June 2017}}</ref>

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 Curie|Marie]] and [[Pierre Curie]] as the lanthanide above it, [[gadolinium]], was named after the explorer of the [[rare-earth element]]s [[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."<ref name="E97" /> This tradition ended with berkelium, though, as the naming of the next discovered actinide, [[californium]], was not related to its lanthanide analogue [[dysprosium]], but after the discovery place.<ref>{{cite book|last = Heiserman|first = David L.|year = 1992|title = Exploring Chemical Elements and their Compounds|publisher = TAB Books|isbn = 978-0-8306-3018-9|chapter = Element 98: Californium|chapter-url = https://books.google.com/books?id=24l-Cpal9oIC|page = [https://archive.org/details/exploringchemica01heis/page/347 347]|url-access = registration|url = https://archive.org/details/exploringchemica01heis}}</ref>

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 ([[americium-241|<sup>241</sup>Am]]) [[nitrate]] solution was coated on a [[platinum]] foil, the solution was evaporated and the residue converted by annealing to [[americium dioxide]] ({{chem2|AmO2}}). This target was irradiated with 35&nbsp;MeV [[alpha particle]]s 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 <sup>243</sup>Bk isotope and two free [[neutron]]s:<ref name="E97" />

:{{nuclide|Am|241}} + {{nuclide|He|4}} → {{nuclide|Bk|243}} + 2{{nuclide|neutronium|1}}

After the irradiation, the coating was dissolved with [[nitric acid]] and then precipitated as the [[hydroxide]] using concentrated aqueous [[ammonia solution]]. The product was [[centrifugation|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]] ({{chem2|AmF3}}). 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]].<ref name="E97" />

[[File:Elutionskurven Tb Gd Eu und Bk Cm Am.png|thumb|[[Chromatography|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)<ref name = "E97" />|alt=Graphs showing similar elution curves (metal amount vs. drops) for (top vs. bottom) terbium vs. berkelium, gadolinium vs. curium, europium vs. americium]]
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 [[chromatography|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 [[K-alpha|characteristic X-rays]] and [[Internal conversion|conversion electron]] signals, a berkelium isotope was eventually detected. Its [[mass number]] was uncertain between 243 and 244 in the initial report,<ref name="Thompson1950" /> but was later established as 243.<ref name = "E97" />

==Synthesis and extraction==

===Preparation of isotopes===
Berkelium is produced by bombarding lighter actinides [[uranium]] (<sup>238</sup>U) or [[plutonium]] (<sup>239</sup>Pu) with [[neutron]]s 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:<ref>{{cite journal|last1=Thompson|first1=S.|last2=Ghiorso|first2=A.|last3=Harvey|first3=B.|last4=Choppin|first4=G.|title=Transcurium Isotopes Produced in the Neutron Irradiation of Plutonium|journal=Physical Review|volume=93|pages=908|date=1954|doi=10.1103/PhysRev.93.908|issue=4|bibcode = 1954PhRv...93..908T |url=http://www.escholarship.org/uc/item/2wj6c5kh|doi-access=free}}</ref>

:<chem>^{238}_{92}U ->[\ce{(n,\gamma)}] ^{239}_{92}U ->[\beta^-][23.5 \ \ce{min}] ^{239}_{93}Np ->[\beta^-][2.3565 \ \ce{d}] ^{239}_{94}Pu</chem> <small>(the times are [[half-life|half-lives]])</small>

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, US. The higher flux promotes fusion reactions involving not one but several neutrons, converting <sup>239</sup>Pu to <sup>244</sup>Cm and then to <sup>249</sup>Cm:
:<math chem>\begin{align}
\ce{^{239}_{94}Pu ->[\ce{4(n,\gamma)}] ^{243}_{94}Pu ->[\beta^-][4.956 \ \ce{h}] ^{243}_{95}Am ->[\ce{(n,\gamma)}] ^{244}_{95}Am ->[\beta^-][10.1 \ \ce{h}]} & \ce{^{244}_{96}Cm}\\
& \ce{^{244}_{96}Cm ->[\ce{5(n,\gamma)}] ^{249}_{96}Cm}
\end{align}</math>

Curium-249 has a short half-life of 64 minutes, and thus its further conversion to <sup>250</sup>Cm has a low probability. Instead, it transforms by beta-decay into <sup>249</sup>Bk:<ref name="nubase" />
:<chem>^{249}_{96}Cm ->[{\beta^-}][64.15 \ \ce{min}] ^{249}_{97}Bk ->[\beta^-][330 \ \ce{d}] ^{249}_{98}Cf</chem>

The thus-produced <sup>249</sup>Bk has a long half-life of 330 days and thus can capture another neutron. However, the product, <sup>250</sup>Bk, 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 <sup>250</sup>Cf:<ref>{{cite journal|last1=Magnusson|first1=L.|last2=Studier|first2=M.|last3=Fields|first3=P.|last4=Stevens|first4=C.|last5=Mech|first5=J.|last6=Friedman|first6=A.|last7=Diamond|first7=H.|last8=Huizenga|first8=J.|title=Berkelium and Californium Isotopes Produced in Neutron Irradiation of Plutonium|journal=Physical Review|volume=96|pages=1576|date=1954|doi=10.1103/PhysRev.96.1576|issue=6|bibcode = 1954PhRv...96.1576M }}</ref><ref>{{cite journal|last1=Eastwood|first1=T.|last2=Butler|first2=J.|last3=Cabell|first3=M.|last4=Jackson|first4=H.|last5=Schuman|first5=R.|last6=Rourke|first6=F.|last7=Collins|first7=T.|title=Isotopes of Berkelium and Californium Produced by Neutron Irradiation of Plutonium|journal=Physical Review|volume=107|pages=1635|date=1957|doi=10.1103/PhysRev.107.1635|issue=6|bibcode = 1957PhRv..107.1635E }}</ref>
:<chem>^{249}_{97}Bk ->[\ce{(n,\gamma)}] ^{250}_{97}Bk ->[\beta^-][3.212 \ \ce{h}] ^{250}_{98}Cf</chem>

Although <sup>247</sup>Bk is the most stable isotope of berkelium, its production in nuclear reactors is very difficult because its potential progenitor <sup>247</sup>Cm has never been observed to undergo beta decay.{{NUBASE2016|ref}} Thus, <sup>249</sup>Bk is the most accessible isotope of berkelium, which still is available only in small quantities (only 0.66&nbsp;grams have been produced in the US over the period 1967–1983{{sfn|Peterson|Hobart|1984|p=30}}) at a high price of the order 185 [[United States dollar|USD]] per microgram.<ref name="CRC">Hammond C. R. "The elements" in {{RubberBible86th}}</ref> It is the only berkelium isotope available in bulk quantities, and thus the only berkelium isotope whose properties can be extensively studied.<ref name="bk">{{cite journal |last=Trabesinger |first=A. |title=Peaceful berkelium |date=2017 |journal=[[Nature Chemistry]] |volume=9 |issue=9 |page=924 |doi=10.1038/nchem.2845 |pmid=28837169 |bibcode=2017NatCh...9..924T |doi-access=free }}</ref>

The isotope <sup>248</sup>Bk was first obtained in 1956 by bombarding a mixture of curium isotopes with 25&nbsp;MeV α-particles. Although its direct detection was hindered by strong signal interference with <sup>245</sup>Bk, the existence of a new isotope was proven by the growth of the decay product <sup>248</sup>Cf which had been previously characterized. The half-life of <sup>248</sup>Bk was estimated as {{val|23|5}} hours,<ref>{{cite journal|last1=Hulet|first1=E.|title=New Isotope of Berkelium|journal=Physical Review|volume=102|issue=1|pages=182|date=1956|doi=10.1103/PhysRev.102.182|bibcode = 1956PhRv..102..182H }}</ref> though later 1965 work gave a half-life in excess of 300&nbsp;years (which may be due to an isomeric state).<ref>{{Cite journal | last1 = Milsted | first1 = J. | last2 = Friedman | first2 = A. M. | last3 = Stevens | first3 = C. M. | doi = 10.1016/0029-5582(65)90719-4 | title = The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248 | journal = Nuclear Physics | volume = 71 | issue = 2 | pages = 299 | year = 1965 | bibcode = 1965NucPh..71..299M }}</ref> Berkelium-247 was produced during the same year by irradiating <sup>244</sup>Cm with alpha-particles:<ref>{{cite journal|last1=Milsted|first1=J.|title=The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248|journal=Nuclear Physics|volume=71|pages=299|date=1965|doi=10.1016/0029-5582(65)90719-4|issue=2|bibcode = 1965NucPh..71..299M|last2=Friedman|first2=A. M.|last3=Stevens|first3=C. M. }}</ref>
:<math chem>\begin{cases}
\ce{^{244}_{96}Cm ->[\ce{(\alpha,n)}] ^{247}_{98}Cf ->[\epsilon][3.11 \ \ce{h}] ^{247}_{97}Bk}\\
\ce{^{244}_{96}Cm ->[\ce{(\alpha,p)}] ^{247}_{97}Bk}
\end{cases}</math>

Berkelium-242 was synthesized in 1979 by bombarding <sup>235</sup>U with <sup>11</sup>B, <sup>238</sup>U with <sup>10</sup>B, <sup>232</sup>Th with <sup>14</sup>N or <sup>232</sup>Th with <sup>15</sup>N. It converts by [[electron capture]] to <sup>242</sup>Cm with a half-life of {{val|7.0|1.3}} minutes. A search for an initially suspected isotope <sup>241</sup>Bk was then unsuccessful;<ref>{{cite journal|last1=Williams|first1=Kimberly|last2=Seaborg|first2=Glenn|title=New isotope <sup>242</sup>Bk|journal=Physical Review C|volume=19|pages=1794|date=1979|doi=10.1103/PhysRevC.19.1794|bibcode = 1979PhRvC..19.1794W|issue=5 }}</ref> <sup>241</sup>Bk has since been synthesized.<ref name="nucleonica">{{cite web |url=http://www.nucleonica.net/unc.aspx |title=Nucleonica: Universal Nuclide Chart |author=Nucleonica |date=2007–2011 |publisher=Nucleonica |access-date=22 July 2011}}</ref>
:<math chem>\begin{cases}
\ce{^{235}_{92}U + ^{11}_{5}B -> ^{242}_{97}Bk + 4^{1}_{0}n} & \ce{^{232}_{90}Th + ^{14}_{7}N -> ^{242}_{97}Bk + 4^{1}_{0}n}\\
\ce{^{238}_{92}U + ^{10}_{5}B -> ^{242}_{97}Bk + 6^{1}_{0}n} & \ce{^{232}_{90}Th + ^{15}_{7}N -> ^{242}_{97}Bk + 5^{1}_{0}n}
\end{cases}</math>

===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 {{Not a typo|berkelium(III)}} solutions to convert it to the +4 state, such as [[bromate]]s ({{chem2|BrO3(-)}}), [[bismuthate]]s ({{chem2|BiO3(-)}}), [[Chromate and dichromate|chromate]]s ({{chem2|CrO4(2-)}} and {{chem2|Cr2O7(2−)}}), silver(I) thiolate ({{chem2|Ag2S2O8}}), lead(IV) oxide ({{chem2|PbO2}}), [[ozone]] ({{chem2|O3}}), 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.<ref name=":0">{{cite journal |last1=Deblonde |first1=Gauthier |last2=Sturzbecher-Hoehne |first2=Manuel |last3=Rupert |first3=Peter |last4=An |first4=Dahlia |last5=Illy |first5=Marie-Claire |last6=Ralston |first6=Corie |last7=brabec |first7=Jiri |last8=de Jong |first8=Wibe |last9=Strong |first9=Roland |last10=Abergel |first10=Rebecca |title=Chelation and stabilization of berkelium in oxidation state +IV |journal=Nature Chemistry |date=2017 |volume=9 |issue=9 |pages=843–849 |doi=10.1038/nchem.2759 |pmid=28837177 |bibcode=2017NatCh...9..843D |osti=1436161 |url=http://www.escholarship.org/uc/item/9zn3q96n }}</ref> {{Not a typo|Berkelium(IV)}} is then extracted with [[ion exchange]], extraction [[chromatography]] or liquid-liquid extraction using HDEHP (bis-(2-ethylhexyl) phosphoric acid), [[amine]]s, [[tributyl phosphate]] or various other reagents. These procedures separate berkelium from most trivalent actinides and [[lanthanide]]s, except for the lanthanide [[cerium]] (lanthanides are absent in the irradiation target but are created in various [[nuclear fission]] decay chains).{{sfn|Peterson|Hobart|1984|p=32}}

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 [[hydroxide]]s, filtered and dissolved in nitric acid. It is then treated with high-pressure [[elution]] from [[Ion exchange|cation exchange]] resins, and the berkelium phase is oxidized and extracted using one of the procedures described above.{{sfn|Peterson|Hobart|1984|p=32}} Reduction of the thus-obtained {{Not a typo|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.{{sfn|Peterson|Hobart|1984|pp=33–34}}

===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 [[Idaho National Laboratory|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&nbsp;micrograms) of berkelium by Burris B. Cunningham and [[Stanley Gerald Thompson]] in 1958, after a continuous reactor irradiation of this target for six years.{{sfn|Peterson|Hobart|1984|p=30}}<ref>S. G. Thompson, BB Cunningham: "First Macroscopic Observations of the Chemical Properties of Berkelium and californium," supplement to Paper P/825 presented at the Second International Conference on Peaceful Uses of Atomic Energy, Geneva, 1958</ref> 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.<ref name="H&P">{{cite book|first1 = David E.|last1 = Hobart|first2 = Joseph R.|last2 = Peterson|contribution = Berkelium|title = The Chemistry of the Actinide and Transactinide Elements|editor1-first = Lester R.|editor1-last = Morss|editor2-first = Norman M.|editor2-last = Edelstein|editor3-first = Jean|editor3-last = Fuger|edition = 3rd|date = 2006|volume = 3|publisher = Springer|location = Dordrecht, the Netherlands|pages = 1444–98|url = http://radchem.nevada.edu/classes/rdch710/files/berkelium.pdf|doi = 10.1007/1-4020-3598-5_10|isbn = 978-1-4020-3555-5|access-date = 30 September 2010|archive-date = 17 July 2010|archive-url = https://web.archive.org/web/20100717153832/http://radchem.nevada.edu/classes/rdch710/files/berkelium.pdf|url-status = dead}}</ref>{{sfn|Peterson|Hobart|1984|p=38}}

The world's major irradiation sources are the 85-megawatt High Flux Isotope Reactor at the [[Oak Ridge National Laboratory]] in Tennessee, USA,<ref>{{cite web|title = High Flux Isotope Reactor|url = http://neutrons.ornl.gov/facilities/HFIR/|publisher = Oak Ridge National Laboratory|access-date = 2010-09-23}}</ref> and the SM-2 loop reactor at the [[Research Institute of Atomic Reactors]] (NIIAR) in [[Dimitrovgrad, Russia]],<ref>{{cite web|title = Радионуклидные источники и препараты|url = http://www.niiar.ru/?q=radioisotope_application|publisher = Research Institute of Atomic Reactors|access-date = 2010-09-26}}</ref> 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,<ref name="Es">{{cite book|first = Richard G.|last = Haire|contribution = Einsteinium|title = The Chemistry of the Actinide and Transactinide Elements|editor1-first = Lester R.|editor1-last = Morss|editor2-first = Norman M.|editor2-last = Edelstein|editor3-first = Jean|editor3-last = Fuger|edition = 3rd|date = 2006|volume = 3|publisher = Springer|location = Dordrecht, the Netherlands|pages = 1577–1620|url = http://radchem.nevada.edu/classes/rdch710/files/einsteinium.pdf|doi = 10.1007/1-4020-3598-5_12|url-status = dead|archive-url = https://web.archive.org/web/20100717154427/http://radchem.nevada.edu/classes/rdch710/files/einsteinium.pdf|archive-date = 17 July 2010|isbn = 978-1-4020-3555-5}}</ref> 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 [[Kilogram#SI multiples|decigram]] quantities of [[californium]], [[Kilogram#SI multiples|milligram]] quantities of berkelium-249 and [[einsteinium]], and [[Kilogram#SI multiples|picogram]] quantities of [[fermium]].{{sfn|Greenwood|Earnshaw|1997|p=1262}}<ref>{{cite journal|first1 = C. E.|last1 = Porter|first2 = F. D. Jr. |last2 = Riley|first3 = R. D.|last3 = Vandergrift|first4 = L. K.|last4 = Felker|title = Fermium Purification Using Teva Resin Extraction Chromatography|journal = Sep. Sci. Technol.|volume = 32|issue = 1–4|date = 1997|pages = 83–92|doi = 10.1080/01496399708003188|url = https://zenodo.org/record/1234415}}</ref> In total, just over one gram of berkelium-249 has been produced at Oak Ridge since 1967.<ref name="H&P" />

The first berkelium metal sample weighing 1.7&nbsp;micrograms was prepared in 1971 by the reduction of {{Not a typo|berkelium(III)}} fluoride with [[lithium]] vapor at 1000&nbsp;°C; the fluoride was suspended on a tungsten wire above a [[tantalum]] crucible containing molten lithium. Later, metal samples weighing up to 0.5&nbsp;milligrams were obtained with this method.<ref name="Peterson">{{cite journal|last1 = Peterson|first1 = J. R.|last2 = Fahey|first2 = J. A.|last3 = Baybarz|first3 = R. D.|date = 1971|title = The crystal structures and lattice parameters of berkelium metal|journal = J. Inorg. Nucl. Chem.|volume = 33|issue = 10|pages = 3345–51|doi = 10.1016/0022-1902(71)80656-5}}</ref>{{sfn|Peterson|Hobart|1984|p=41}}
:{{chem2|BkF3 + 3 Li → Bk + 3 LiF}}

Similar results are obtained with {{Not a typo|berkelium(IV)}} fluoride.<ref name="pressure2">{{cite journal|last1=Itie|first1=J. P.|last2=Peterson|first2=J. R.|last3=Haire|first3=R. G.|last4=Dufour|first4=C.|last5=Benedict|first5=U.|journal=Journal of Physics F: Metal Physics|volume=15|pages=L213|date=1985|doi=10.1088/0305-4608/15/9/001|title=Delocalisation of 5f electrons in berkelium-californium alloys under pressure|issue=9|bibcode = 1985JPhF...15L.213I |url=https://zenodo.org/record/1235718}}</ref> Berkelium metal can also be produced by the reduction of {{Not a typo|berkelium(IV)}} oxide with [[thorium]] or [[lanthanum]].{{sfn|Peterson|Hobart|1984|p=41}}<ref>{{cite book|last1 = Spirlet|first1 = J. C.|last2 = Peterson|first2 = J. R.|last3 = Asprey|first3 = L. B.|date = 1987|title = Preparation and Purification of Actinide Metals|journal = Adv. Inorg. Chem.|volume = 31|pages = [https://archive.org/details/trent_0116400835595_31/page/1 1–41]|doi = 10.1016/S0898-8838(08)60220-2|series = Advances in Inorganic Chemistry|isbn = 9780120236312|url = https://archive.org/details/trent_0116400835595_31/page/1}}</ref>

==Compounds==
{{main|Berkelium compounds}}

===Oxides===
Two oxides of berkelium are known, with the berkelium [[redox|oxidation]] state of +3 ({{chem2|Bk2O3}}) and +4 ([[Berkelium(IV) oxide|{{chem2|BkO2}}]]).<ref>{{cite journal|last1=Peterson|first1=J.|title=Crystal structures and lattice parameters of the compounds of berkelium I. Berkelium dioxide and cubic berkelium sesquioxide|journal=Inorganic and Nuclear Chemistry Letters|volume=3|pages=327|date=1967|doi=10.1016/0020-1650(67)80037-0|issue=9|last2=Cunningham|first2=B. B.|url=https://escholarship.org/uc/item/5mv047vq}}</ref> {{Not a typo|Berkelium(IV)}} oxide is a brown solid,<ref name="BK_OX">{{cite journal|last1=Baybarz|first1=R. D.|title=The berkelium oxide system|journal=Journal of Inorganic and Nuclear Chemistry|volume=30|pages=1769–1773|date=1968|doi=10.1016/0022-1902(68)80352-5|issue=7}}</ref> while {{Not a typo|berkelium(III)}} oxide is a yellow-green solid with a melting point of 1920&nbsp;°C{{sfn|Holleman|Wiberg|2007|p=1972}}<ref name="BK_OX" /> and is formed from BkO<sub>2</sub> by [[redox|reduction]] with molecular [[hydrogen]]:
:{{chem2|2 BkO2 + H2 → Bk2O3 + H2O}}

Upon heating to 1200&nbsp;°C, the oxide {{chem2|Bk2O3}} undergoes a phase change; it undergoes another phase change at 1750&nbsp;°C. Such three-phase behavior is typical for the actinide [[sesquioxide]]s. {{Not a typo|Berkelium(II)}} oxide, BkO, has been reported as a brittle gray solid but its exact chemical composition remains uncertain.{{sfn|Peterson|Hobart|1984|p=51}}

===Halides===
In [[halide]]s, berkelium assumes the oxidation states +3 and +4.{{sfn|Holleman|Wiberg|2007|p=1969}} The +3 state is the most stable, especially in solutions, while the tetravalent halides {{chem2|BkF4}} and {{chem2|Cs2BkCl6}} are only known in the solid phase.{{sfn|Peterson|Hobart|1984|p=47}} The coordination of berkelium atom in its trivalent fluoride and chloride is tricapped [[Octahedral molecular geometry#Trigonal prismatic geometry|trigonal prismatic]], with the [[coordination number]] of 9. In trivalent bromide, it is bicapped trigonal prismatic (coordination 8) or [[Octahedral molecular geometry|octahedral]] (coordination 6),<ref name="conv" /> and in the iodide it is octahedral.{{sfn|Greenwood|Earnshaw|1997|p=1270}}

{| Class = "wikitable" style ="float:right; text-align: center; text-size:90%"
|-
! Oxidation <br>number
! F
! Cl
! Br
! I
|-
! +4
| {{chem2|BkF4}}<br /> (yellow{{sfn|Greenwood|Earnshaw|1997|p=1270}})
| {{chem2|Cs2BkCl6}}<br />(orange{{sfn|Peterson|Hobart|1984|p=51}})
|
|
|-
! +3
| {{chem2|BkF3}}<br /> (yellow{{sfn|Greenwood|Earnshaw|1997|p=1270}})
| {{chem2|BkCl3}}<br /> (green{{sfn|Greenwood|Earnshaw|1997|p=1270}})<br />{{chem2|Cs2NaBkCl6}}{{sfn|Peterson|Hobart|1984|p=48}}
| {{chem2|BkBr3}}<ref name="conv">{{cite journal|last1=Young|first1=J. P.|last2=Haire|first2=R. G.|last3=Peterson|first3=J. R.|last4=Ensor|first4=D. D.|last5=Fellows|first5=R. L.|title=Chemical consequences of radioactive decay. 1. Study of californium-249 ingrowth into crystalline berkelium-249 tribromide: a new crystalline phase of californium tribromide|journal=Inorganic Chemistry|volume=19|pages=2209|date=1980|doi=10.1021/ic50210a003|issue=8}}</ref><ref>{{cite journal|last1=Burns|first1=J.|title=Crystallographic studies of some transuranic trihalides: 239PuCl3, 244CmBr3, 249BkBr3 and 249CfBr3|journal=Journal of Inorganic and Nuclear Chemistry|volume=37|pages=743|date=1975|doi=10.1016/0022-1902(75)80532-X|issue=3|last2=Peterson|first2=J. R.|last3=Stevenson|first3=J. N.}}</ref><br />(yellow-green{{sfn|Greenwood|Earnshaw|1997|p=1270}})
| {{chem2|BkI3}}<br /> (yellow{{sfn|Greenwood|Earnshaw|1997|p=1270}})
|}

{{Not a typo|Berkelium(IV)}} fluoride ({{chem2|BkF4}}) is a yellow-green ionic solid and is isotypic with [[uranium tetrafluoride]] or [[zirconium tetrafluoride]].{{sfn|Peterson|Hobart|1984|p=48}}<ref name="BKF_3_4" /><ref name="f1">{{cite journal|last1=Keenan|first1=Thomas K.|last2=Asprey|first2=Larned B.|title=Lattice constants of actinide tetrafluorides including berkelium|journal=Inorganic Chemistry|volume=8|pages=235|date=1969|doi=10.1021/ic50072a011|issue=2}}</ref> {{Not a typo|Berkelium(III)}} fluoride ({{chem2|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&nbsp;°C, it transforms to the structure found in [[lanthanum trifluoride]].{{sfn|Peterson|Hobart|1984|p=48}}<ref name="BKF_3_4">{{cite journal|last1=Ensor|first1=D.|title=Absorption spectrophotometric study of {{Not a typo|berkelium(III)}} and (IV) fluorides in the solid state|journal=Journal of Inorganic and Nuclear Chemistry|volume=43|pages=1001|date=1981|doi=10.1016/0022-1902(81)80164-9|issue=5|last2=Peterson|first2=J. R.|last3=Haire|first3=R. G.|last4=Young|first4=J. P.}}</ref><ref>{{cite journal|last1=Peterson|first1=J. R.|last2=Cunningham|first2=B. B.|title=Crystal structures and lattice parameters of the compounds of berkelium—IV berkelium trifluoride☆|journal=Journal of Inorganic and Nuclear Chemistry|volume=30|pages=1775|date=1968|doi=10.1016/0022-1902(68)80353-7|issue=7|url=https://escholarship.org/uc/item/1mn5c30t}}</ref>

Visible amounts of [[Berkelium(III) chloride|{{Not a typo|berkelium(III)}} chloride]] ({{chem2|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&nbsp;°C.<ref name="o1" /> This green solid has a melting point of 600&nbsp;°C,{{sfn|Holleman|Wiberg|2007|p=1969}} and is isotypic with [[uranium(III) chloride]].<ref>{{cite journal|last1=Peterson|first1=J. R.|last2=Cunningham|first2=B. B.|title=Crystal structures and lattice parameters of the compounds of berkelium—IIBerkelium trichloride|journal=Journal of Inorganic and Nuclear Chemistry|volume=30|pages=823|date=1968|doi=10.1016/0022-1902(68)80443-9|issue=3}}</ref><ref>{{cite journal|last1=Peterson|first1=J. R.|last2=Young|first2=J. P.|last3=Ensor|first3=D. D.|last4=Haire|first4=R. G.|title=Absorption spectrophotometric and x-ray diffraction studies of the trichlorides of berkelium-249 and californium-249|journal=Inorganic Chemistry|volume=25|pages=3779|date=1986|doi=10.1021/ic00241a015|issue=21}}</ref> Upon heating to nearly melting point, {{chem2|BkCl3}} converts into an orthorhombic phase.{{sfn|Peterson|Hobart|1984|p=52}}

Two forms of {{Not a typo|berkelium(III)}} bromide are known: one with berkelium having coordination 6, and one with coordination 8.{{sfn|Peterson|Hobart|1984|p=38}} The latter is less stable and transforms to the former phase upon heating to about 350&nbsp;°C. An important phenomenon for radioactive solids has been studied on these two crystal forms: the structure of fresh and aged <sup>249</sup>BkBr<sub>3</sub> samples was probed by [[X-ray crystallography|X-ray diffraction]] over a period longer than 3 years, so that various fractions of berkelium-249 had [[beta decay]]ed to californium-249. No change in structure was observed upon the <sup>249</sup>BkBr<sub>3</sub>—<sup>249</sup>CfBr<sub>3</sub> transformation. However, other differences were noted for <sup>249</sup>BkBr<sub>3</sub> and <sup>249</sup>CfBr<sub>3</sub>. For example, the latter could be reduced with hydrogen to <sup>249</sup>CfBr<sub>2</sub>, but the former could not – this result was reproduced on individual <sup>249</sup>BkBr<sub>3</sub> and <sup>249</sup>CfBr<sub>3</sub> samples, as well on the samples containing both bromides.<ref name="conv" /> 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, <sup>249</sup>Cf, 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.{{sfn|Peterson|Hobart|1984|p=47}}

===Other inorganic compounds===
The [[Pnictogen#Compounds|pnictides]] of berkelium-249 of the type BkX are known for the elements [[nitrogen]],<ref name="n1">{{cite journal|last1=Stevenson|first1=J.|last2=Peterson|first2=J.|title=Preparation and structural studies of elemental curium-248 and the nitrides of curium-248 and berkelium-249|journal=Journal of the Less Common Metals|volume=66|pages=201|date=1979|doi=10.1016/0022-5088(79)90229-7|issue=2}}</ref> [[phosphorus]], [[arsenic]] and [[antimony]]. They crystallize in the [[Cubic crystal system|rock-salt structure]] and are prepared by the reaction of either {{Not a typo|berkelium(III)}} hydride ({{chem2|BkH3}}) or metallic berkelium with these elements at elevated temperature (about 600&nbsp;°C) under high vacuum.<ref>{{cite journal|last1=Damien|first1=D.|last2=Haire|first2=R. G.|last3=Peterson|first3=J. R.|title=Preparation and lattice parameters of <sup>249</sup>Bk monopnictides|journal=Journal of Inorganic and Nuclear Chemistry|volume=42|pages=995|date=1980|doi=10.1016/0022-1902(80)80390-3|issue=7}}</ref>

{{Not a typo|Berkelium(III)}} sulfide, {{chem2|Bk2S3}}, is prepared by either treating berkelium oxide with a mixture of [[hydrogen sulfide]] and [[carbon disulfide]] vapors at 1130&nbsp;°C, or by directly reacting metallic berkelium with elemental sulfur. These procedures yield brownish-black crystals.{{sfn|Peterson|Hobart|1984|p=53}}

{{Not a typo|Berkelium(III)}} and {{Not a typo|berkelium(IV)}} hydroxides are both stable in 1 [[Molar concentration|molar]] solutions of [[sodium hydroxide]]. {{Not a typo|Berkelium(III)}} [[phosphate]] ({{chem2|BkPO4}}) has been prepared as a solid, which shows strong [[fluorescence]] under excitation with a green light.{{sfn|Peterson|Hobart|1984|pp=39–40}} Berkelium hydrides are produced by reacting metal with hydrogen gas at temperatures about 250&nbsp;°C.<ref name="n1" /> They are non-stoichiometric with the nominal formula {{chem|BkH|2+''x''}} (0 < ''x'' < 1).{{sfn|Peterson|Hobart|1984|p=53}} Several other salts of berkelium are known, including an oxysulfide ({{chem2|Bk2O2S}}), and hydrated [[Berkelium(III) nitrate|nitrate]] ({{chem|Bk(NO|3|)|3|·4H|2|O}}), chloride ({{chem|BkCl|3|·6H|2|O}}), [[sulfate]] ({{chem|Bk|2|(SO|4|)|3|·12H|2|O}}) and [[oxalate]] ({{chem|Bk|2|(C|2|O|4|)|3|·4H|2|O}}).{{sfn|Peterson|Hobart|1984|p=47}} Thermal decomposition at about 600&nbsp;°C in an [[argon]] atmosphere (to avoid oxidation to {{chem2|BkO2}}) of {{chem|Bk|2|(SO|4|)|3|·12H|2|O}} yields the crystals of {{Not a typo|berkelium(III)}} oxysulfate ({{chem2|Bk2O2SO4}}). This compound is thermally stable to at least 1000&nbsp;°C in inert atmosphere.{{sfn|Peterson|Hobart|1984|p=54}}

===Organoberkelium compounds===
Berkelium forms a trigonal (η<sup>5</sup>–C<sub>5</sub>H<sub>5</sub>)<sub>3</sub>Bk [[metallocene]] complex with three [[Cyclopentadienyl complex|cyclopentadienyl]] rings, which can be synthesized by reacting {{Not a typo|berkelium(III)}} chloride with the molten [[beryllocene]] ({{chem2|Be(C5H5)2}}) at about 70&nbsp;°C. It has an amber color and a density of 2.47&nbsp;g/cm<sup>3</sup>. The complex is stable to heating to at least 250&nbsp;°C, and sublimates without melting at about 350&nbsp;°C. The high radioactivity of berkelium gradually destroys the compound (within a period of weeks).<ref name="o1">{{cite journal|last1=Laubereau|first1=Peter G.|last2=Burns|first2=John H.|title=Microchemical preparation of tricyclopentadienyl compounds of berkelium, californium, and some lanthanide elements|journal=Inorganic Chemistry|volume=9|pages=1091|date=1970|doi=10.1021/ic50087a018|issue=5}}</ref><ref>Christoph Elschenbroich ''Organometallic Chemistry'', 6th Edition, Wiesbaden 2008, {{ISBN|978-3-8351-0167-8}}, pp. 583–584</ref> One cyclopentadienyl ring in (η<sup>5</sup>–C<sub>5</sub>H<sub>5</sub>)<sub>3</sub>Bk can be substituted by chlorine to yield {{chem2|[Bk(C5H5)2Cl]2}}. The optical absorption spectra of this compound are very similar to those of (η<sup>5</sup>–C<sub>5</sub>H<sub>5</sub>)<sub>3</sub>Bk.{{sfn|Peterson|Hobart|1984|pp=41, 54}}

==Applications==
[[File:Berkelium.jpg|thumb|22 [[Kilogram#SI multiples|milligrams]] of berkelium (as [[berkelium(III) nitrate|nitrate]]) prepared at [[High Flux Isotope Reactor|HFIR]] in 2009 at a cost of approximately one million dollars, used for the synthesis of [[tennessine]] in [[Joint Institute for Nuclear Research|JINR]]<ref>[http://news.sciencemag.org/sciencenow/2010/04/finally-element-117-is-here.html Finally, Element 117 Is Here!] {{webarchive |url=https://web.archive.org/web/20101030102347/http://news.sciencemag.org/sciencenow/2010/04/finally-element-117-is-here.html |date=30 October 2010 }}, Science Now, 7 April 2010</ref>|alt=A very small sample of a blue liquid in a plastic pipette held by a hand wearing heavy protection equipment]]
There is currently no use for any isotope of berkelium outside basic scientific research.<ref name="H&P" /> Berkelium-249 is a common target nuclide to prepare still heavier [[transuranium element]]s and [[superheavy element]]s,<ref name="Stwertka">Stwertka, Albert. ''A Guide to the Elements'', Oxford University Press, 1996, p. 211. {{ISBN|0-19-508083-1}}</ref> such as [[lawrencium]], [[rutherfordium]] and [[bohrium]].<ref name="H&P" /> 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.<ref name="H&P" /><ref>{{cite book|first = Richard G.|last = Haire|contribution = Californium|title = The Chemistry of the Actinide and Transactinide Elements|editor1-first = Lester R.|editor1-last = Morss|editor2-first = Norman M.|editor2-last = Edelstein|editor3-first = Jean|editor3-last = Fuger|edition = 3rd|date = 2006|volume = 3|publisher = Springer|location = Dordrecht, the Netherlands|pages = 1499–1576|url = http://radchem.nevada.edu/classes/rdch710/files/californium.pdf|doi = 10.1007/1-4020-3598-5_11|url-status = dead|archive-url = https://web.archive.org/web/20100717154045/http://radchem.nevada.edu/classes/rdch710/files/californium.pdf|archive-date = 17 July 2010|isbn = 978-1-4020-3555-5}}</ref>

A 22&nbsp;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.<ref>[https://str.llnl.gov/OctNov10/shaughnessy.html Collaboration Expands the Periodic Table, One Element at a Time] {{Webarchive|url=https://web.archive.org/web/20110718033621/https://str.llnl.gov/OctNov10/shaughnessy.html |date=18 July 2011 }}, Science and Technology Review, Lawrence Livermore National Laboratory, October/November 2010</ref><ref>[https://www.sciencedaily.com/releases/2010/04/100406181611.htm Nuclear Missing Link Created at Last: Superheavy Element 117], Science daily, 7 April 2010</ref>

==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 [[Neutron cross section|cross section]] of 710 [[barn (unit)|barns]] for [[Neutron temperature#Thermal|thermal neutrons]], 1200 barns [[neutron capture|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.<ref>G. Pfennig, H. Klewe-Nebenius, W. Seelmann Eggebert (Eds.): Karlsruhe [[nuclide]], 7 Edition, 2006</ref><ref>{{cite journal|last1=Chadwick|first1=M. B.|last2=Obložinský|first2=P.|last3=Herman|first3=M.|last4=Greene|first4=N. M.|last5=McKnight|first5=R. D.|last6=Smith|first6=D. L.|display-authors=3|title=ENDF/B-VII.0: Next Generation Evaluated Nuclear Data Library for Nuclear Science and Technology|date=2006|journal=Nuclear Data Sheets|volume=107|issue=12|pages=2931–3060|doi=10.1016/j.nds.2006.11.001|bibcode=2006NDS...107.2931C|url=https://digital.library.unt.edu/ark:/67531/metadc888955/}}</ref><ref>{{cite journal|last1=Koning|first1=A. J.|last2=Avrigeanu|first2=M.|last3=Avrigeanu|first3=V.|last4=Batistoni|first4=P.|last5=Bauge|first5=E.|last6=Bé|first6=M.-M. |title=The JEFF evaluated nuclear data project |display-authors=3|date=2007|journal=International Conference on Nuclear Data for Science and Technology|volume=ND2007|issue=194|doi=10.1051/ndata:07476|doi-access=free}}</ref> In principle, berkelium-249 can sustain a [[nuclear chain reaction]] in a [[Breeder reactor#Fast breeder reactor|fast breeder reactor]]. Its [[critical mass]] is relatively high at 192&nbsp;kg, which can be reduced with a water or steel reflector but would still exceed the world production of this isotope.<ref name="irsn">Institut de Radioprotection et de Sûreté Nucléaire: [http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf "Evaluation of nuclear criticality safety. data and limits for actinides in transport"] {{webarchive|url=https://web.archive.org/web/20110519171204/http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf |date=19 May 2011 }}, p. 16</ref>

Berkelium-247 can maintain a 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&nbsp;kg for a bare sphere, 41.2&nbsp;kg with a water reflector and 35.2&nbsp;kg with a steel reflector (30&nbsp;cm thickness).<ref name="irsn" />

==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 ([[electron]]s for berkelium and [[alpha particle]]s, [[neutron]]s, or both for most other actinides). The low energy of electrons emitted from berkelium-249 (less than 126&nbsp;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 [[glovebox]] in a dedicated laboratory.<ref>Emeleus, H. J. [https://books.google.com/books?id=K5_LSQqeZ_IC&pg=PA32 Advances in inorganic chemistry], Academic Press, 1987, {{ISBN|0-12-023631-1}} p. 32</ref>

Most available berkelium toxicity data originate from research on animals. Upon ingestion by rats, only about 0.01% of berkelium ends in the blood stream. From there, about 65% goes to the bones, where it remains for about 50&nbsp;years, 25% to the lungs (biological half-life about 20&nbsp;years), 0.035% to the testicles or 0.01% to the ovaries where berkelium stays indefinitely. The balance of about 10% is excreted.<ref>International Commission on Radiological Protection [https://books.google.com/books?id=WTxcCV4w0VEC&pg=PA14 Limits for intakes of radionuclides by workers, Part 4, Volume 19, Issue 4], Elsevier Health Sciences, ISBN, 0080368867 p. 14</ref> In all these organs berkelium might promote cancer, and in the [[skeleton]], its radiation can damage red blood cells. The maximum permissible amount of berkelium-249 in the human skeleton is 0.4&nbsp;[[Kilogram#SI multiples|nanogram]]s.<ref name="CRC" /><ref>Pradyot Patnaik. ''Handbook of Inorganic Chemicals'' McGraw-Hill, 2002, {{ISBN|0-07-049439-8}}</ref>

==References==
{{Reflist|30em}}

==Bibliography==
* {{cite book|last1=Greenwood|first1= Norman N.|last2=Earnshaw| first2=Alan|date=1997|title=Chemistry of the Elements |edition=2nd|place=Oxford|publisher= Butterworth-Heinemann|isbn=978-0-08-037941-8}}
* {{cite book|last1=Holleman|first1=Arnold F. |last2=Wiberg|first2=Nils |title=Textbook of Inorganic Chemistry|edition=102nd |publisher=de Gruyter|place=Berlin |date=2007|isbn=978-3-11-017770-1}}
* {{cite book|last1=Peterson|first1=J. R.|last2=Hobart|first2=D. E.|chapter-url=https://books.google.com/books?id=U-YOlLVuV1YC&pg=PA29|chapter=The Chemistry of Berkelium|editor-last=Emeléus|editor-first=Harry Julius|title=Advances in inorganic chemistry and radiochemistry|volume=28|publisher=Academic Press|date=1984|isbn=978-0-12-023628-2|pages=[https://archive.org/details/advancesininorga0000unse_p1d0/page/29 29–64]|doi=10.1016/S0898-8838(08)60204-4|url=https://archive.org/details/advancesininorga0000unse_p1d0/page/29}}

==External links==
* [http://www.periodicvideos.com/videos/097.htm Berkelium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)

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[[Category:Berkelium| ]]
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