Editing Mendelevium

{{infobox mendelevium}}
'''Mendelevium''' is a [[synthetic element|synthetic chemical element]]; it has [[Chemical symbol|symbol]] '''Md''' ([[#rename|formerly '''Mv''']]) and [[atomic number]] 101. A metallic [[radioactive]] [[transuranium element]] in the [[actinide]] series, it is the first element by atomic number that currently cannot be produced in [[Macroscopic scale|macroscopic]] quantities by [[neutron]] bombardment of [[Light element|lighter elements]]. It is the third-to-last actinide and the ninth [[transuranic element]] and the first transfermium. It can only be produced in [[particle accelerator]]s by bombarding lighter elements with charged particles. Seventeen [[isotopes of mendelevium|isotopes]] are known; the most stable is <sup>258</sup>Md with [[half-life]] 51.59&nbsp;days; however, the shorter-lived <sup>256</sup>Md (half-life 77.7&nbsp;[[minute]]s) is most commonly used in chemistry because it can be produced on a larger scale.

Mendelevium was discovered by bombarding [[einsteinium]] with [[alpha particle]]s in 1955, the method still used to produce it today. It is named after [[Dmitri Mendeleev]], the father of the [[periodic table]]. Using available [[microgram]] quantities of einsteinium-253, over a million mendelevium atoms may be made each hour. The chemistry of mendelevium is typical for the late actinides, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state. All known isotopes of mendelevium have short half-lives; there are currently no uses for it outside basic [[scientific research]], and only small amounts are produced.

==Discovery==
[[File:Berkeley 60-inch cyclotron.jpg|thumb|upright=0.7|left|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]]
Mendelevium was the ninth [[transuranic element]] to be synthesized. It was first [[discovery of the chemical elements|synthesized]] by [[Albert Ghiorso]], [[Glenn T. Seaborg]], [[Gregory Robert Choppin]], Bernard G. Harvey, and team leader [[Stanley G. Thompson]] in early 1955 at the University of California, Berkeley. The team produced <sup>256</sup>Md ([[half-life]] of 77.7 minutes{{NUBASE2020|ref}}) when they bombarded an <sup>253</sup>[[einsteinium|Es]] target consisting of only a [[1,000,000,000|billion]] (10<sup>9</sup>) einsteinium atoms with [[alpha particle]]s ([[helium]] nuclei) in the [[Berkeley Radiation Laboratory]]'s 60-inch [[cyclotron]], thus increasing the target's atomic number by two. <sup>256</sup>Md thus became the first isotope of any element to be synthesized one atom at a time. In total, seventeen mendelevium atoms were produced.<ref name="discovery">{{cite book|doi=10.1103/PhysRev.98.1518|url=https://books.google.com/books?id=e53sNAOXrdMC&pg=PA101|title=New Element Mendelevium, Atomic Number 101|date=1955|last1=Ghiorso|first1=A.|last2=Harvey|first2=B.|last3=Choppin|first3=G.|last4=Thompson|first4=S.|last5=Seaborg|first5=Glenn T.|journal=Physical Review|volume=98|pages=1518–1519|bibcode = 1955PhRv...98.1518G|isbn=9789810214401|issue=5 }}</ref> This discovery was part of a program, begun in 1952, that irradiated [[plutonium]] with neutrons to transmute it into heavier actinides.<ref name="Choppin">{{cite journal|first = Gregory R.|last = Choppin|date = 2003 |title = Mendelevium|journal = Chemical and Engineering News|url = http://pubs.acs.org/cen/80th/mendelevium.html|volume = 81|issue = 36}}</ref> This method was necessary as the previous method used to synthesize transuranic elements, [[neutron capture]], could not work because of a lack of known [[beta decay]]ing [[isotopes of fermium]] that would produce isotopes of the next element, mendelevium, and also due to the very short half-life to [[spontaneous fission]] of <sup>258</sup>[[fermium|Fm]] that thus constituted a hard limit to the success of the neutron capture process.{{NUBASE2020|ref}}

{{External media
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| video1= [https://www.youtube.com/watch?v=DrssJRb301k Reenactment of the discovery of mendelevium] at Berkeley 
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To predict if the production of mendelevium would be possible, the team made use of a rough calculation. The number of atoms that would be produced would be approximately equal to the product of the number of atoms of target material, the target's cross section, the ion beam intensity, and the time of bombardment; this last factor was related to the half-life of the product when bombarding for a time on the order of its half-life. This gave one atom per experiment. Thus under optimum conditions, the preparation of only one atom of element 101 per experiment could be expected. This calculation demonstrated that it was feasible to go ahead with the experiment.<ref name="discovery" /> The target material, einsteinium-253, could be produced readily from irradiating [[plutonium]]: one year of irradiation would give a billion atoms, and its three-week [[half-life]] meant that the element 101 experiments could be conducted in one week after the produced einsteinium was separated and purified to make the target. However, it was necessary to upgrade the cyclotron to obtain the needed intensity of 10<sup>14</sup> alpha particles per second; Seaborg applied for the necessary funds.<ref name="Choppin" />

[[File:Md datasheet.jpg|thumb|left|The data sheet, showing stylus tracing and notes, that proved the discovery of mendelevium.]]
While Seaborg applied for funding, Harvey worked on the einsteinium target, while Thomson and Choppin focused on methods for chemical isolation. Choppin suggested using [[α-hydroxyisobutyric acid]] to separate the mendelevium atoms from those of the lighter actinides.<ref name="Choppin" /> The actual synthesis was done by a recoil technique, introduced by Albert Ghiorso. In this technique, the einsteinium was placed on the opposite side of the target from the beam, so that the recoiling mendelevium atoms would get enough [[momentum]] to leave the target and be caught on a catcher foil made of gold. This recoil target was made by an electroplating technique, developed by Alfred Chetham-Strode. This technique gave a very high yield, which was absolutely necessary when working with such a rare and valuable product as the einsteinium target material.<ref name="discovery" /> The recoil target consisted of 10<sup>9</sup> atoms of <sup>253</sup>Es which were deposited electrolytically on a thin gold foil. It was bombarded by 41&nbsp;[[MeV]] [[alpha particle]]s in the [[Berkeley cyclotron]] with a very high beam density of 6×10<sup>13</sup> particles per second over an area of 0.05&nbsp;cm<sup>2</sup>. The target was cooled by water or [[liquid helium]], and the foil could be replaced.<ref name="discovery" /><ref name="book1">{{cite book|url=https://books.google.com/books?id=4KcVj3xqsrAC&pg=PA40|pages=40–42|title=On beyond uranium: journey to the end of the periodic table|author=Hofmann, Sigurd|publisher=CRC Press|isbn=978-0-415-28496-7|date=2002}}</ref>

Initial experiments were carried out in September 1954. No alpha decay was seen from mendelevium atoms; thus, Ghiorso suggested that the mendelevium had all decayed by [[electron capture]] to [[fermium]] and that the experiment should be repeated to search instead for [[spontaneous fission]] events.<ref name="Choppin" /> The repetition of the experiment happened in February 1955.<ref name="Choppin" />

[[File:DIMendeleevCab.jpg|thumb|right|The element was named after [[Dmitri Mendeleev]].]]
On the day of discovery, 19 February, alpha irradiation of the einsteinium target occurred in three three-hour sessions. The cyclotron was in the [[University of California]] campus, while the Radiation Laboratory was on the next hill. To deal with this situation, a complex procedure was used: Ghiorso took the catcher foils (there were three targets and three foils) from the cyclotron to Harvey, who would use [[aqua regia]] to dissolve it and pass it through an [[anion]]-exchange [[resin]] column to separate out the [[transuranium element]]s from the gold and other products.<ref name="Choppin" /><ref name="book2">{{cite book|url=https://archive.org/details/newchemistry00hall|url-access=registration|pages=[https://archive.org/details/newchemistry00hall/page/9 9]–11|title=The new chemistry|author=Hall, Nina|publisher=Cambridge University Press|date=2000|isbn=978-0-521-45224-3}}</ref> The resultant drops entered a [[test tube]], which Choppin and Ghiorso took in a car to get to the Radiation Laboratory as soon as possible. There Thompson and Choppin used a [[cation]]-exchange resin column and the α-hydroxyisobutyric acid. The solution drops were collected on [[platinum]] disks and dried under heat lamps. The three disks were expected to contain respectively the fermium, no new elements, and the mendelevium. Finally, they were placed in their own counters, which were connected to recorders such that spontaneous fission events would be recorded as huge deflections in a graph showing the number and time of the decays. There thus was no direct detection, but by observation of spontaneous fission events arising from its electron-capture daughter <sup>256</sup>Fm. The first one was identified with a "hooray" followed by a "double hooray" and a "triple hooray". The fourth one eventually officially proved the chemical identification of the 101st element, mendelevium. In total, five decays were reported up until 4&nbsp;a.m. Seaborg was notified and the team left to sleep.<ref name="Choppin" /> Additional analysis and further experimentation showed the produced mendelevium isotope to have mass 256 and to decay by electron capture to fermium-256 with a half-life of 157.6&nbsp;minutes.{{NUBASE2020|ref}}

{{quotation|We thought it fitting that there be an element named for the Russian chemist Dmitri Mendeleev, who had developed the periodic table. In nearly all our experiments discovering transuranium elements, we'd depended on his method of predicting chemical properties based on the element's position in the table. But in the middle of the Cold War, naming an element for a Russian was a somewhat bold gesture that did not sit well with some American critics.<ref>[http://www.vanderkrogt.net/elements/element.php?sym=Md 101. Mendelevium – Elementymology & Elements Multidict]. Peter van der Krogt.</ref>|Glenn T. Seaborg}}

Being the first of the second hundred of the chemical elements, it was decided that the element would be named "mendelevium" after the Russian chemist [[Dmitri Mendeleev]], father of the [[periodic table]]. Because this discovery came during the [[Cold War]], Seaborg had to request permission of the government of the [[United States]] to propose that the element be named for a Russian, but it was granted.<ref name="Choppin" /> {{anchor|rename}}The name "mendelevium" was accepted by the [[International Union of Pure and Applied Chemistry]] (IUPAC) in 1955 with symbol "Mv",<ref>{{cite book |title=Comptes rendus de la confèrence IUPAC|date=1955|url=https://books.google.com/books?id=WJhYAAAAYAAJ |last1=Chemistry |first1=International Union of Pure and Applied}}</ref> which was changed to "Md" in the next IUPAC General Assembly (Paris, 1957).<ref>{{cite book |title=Comptes rendus de la confèrence IUPAC|date=1957|url=https://books.google.com/books?id=f5hYAAAAYAAJ |last1=Chemistry |first1=International Union of Pure and Applied}}</ref>

==Characteristics==

===Physical===
[[File:Fblock fd promotion energy.png|thumb|upright=1.6|right|Energy required to promote an f electron to the d subshell for the f-block [[lanthanides]] and [[actinides]]. Above around 210&nbsp;kJ/mol, this energy is too high to be provided for by the greater [[crystal energy]] of the trivalent state and thus [[einsteinium]], [[fermium]], and mendelevium form divalent metals like the lanthanides [[europium]] and [[ytterbium]]. ([[Nobelium]] is also expected to form a divalent metal, but this has not yet been confirmed.)<ref>{{cite book|first = Richard G.|last = Haire|ref = 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|isbn = 978-1-4020-3555-5|access-date = 2014-08-04|archive-date = 2010-07-17|archive-url = https://web.archive.org/web/20100717154427/http://radchem.nevada.edu/classes/rdch710/files/einsteinium.pdf|url-status = dead}}</ref>]]
In the [[periodic table]], mendelevium is located to the right of the actinide [[fermium]], to the left of the actinide [[nobelium]], and below the lanthanide [[thulium]]. Mendelevium metal has not yet been prepared in bulk quantities, and bulk preparation is currently impossible.<ref name="Silva16345">Silva, pp. 1634–5</ref> Nevertheless, a number of predictions and some preliminary experimental results have been done regarding its properties.<ref name="Silva16345" />

The lanthanides and actinides, in the metallic state, can exist as either divalent (such as [[europium]] and [[ytterbium]]) or trivalent (most other lanthanides) metals. The former have f<sup>''n''</sup>s<sup>2</sup> configurations, whereas the latter have f<sup>''n''−1</sup>d<sup>1</sup>s<sup>2</sup> configurations. In 1975, Johansson and Rosengren examined the measured and predicted values for the [[cohesive energy|cohesive energies]] ([[enthalpy|enthalpies]] of crystallization) of the metallic [[lanthanide]]s and [[actinide]]s, both as divalent and trivalent metals.<ref name="Silva16268">Silva, pp. 1626–8</ref><ref>{{cite journal|doi=10.1103/PhysRevB.11.2836|title=Generalized phase diagram for the rare-earth elements: Calculations and correlations of bulk properties|date=1975|last1=Johansson|first1=Börje|last2=Rosengren|first2=Anders|journal=Physical Review B|volume=11|pages=2836–2857|bibcode = 1975PhRvB..11.2836J|issue=8 }}</ref> The conclusion was that the increased binding energy of the [Rn]5f<sup>12</sup>6d<sup>1</sup>7s<sup>2</sup> configuration over the [Rn]5f<sup>13</sup>7s<sup>2</sup> configuration for mendelevium was not enough to compensate for the energy needed to promote one 5f electron to 6d, as is true also for the very late actinides: thus [[einsteinium]], [[fermium]], mendelevium, and [[nobelium]] were expected to be divalent metals.<ref name="Silva16268" /> The increasing predominance of the divalent state well before the actinide series concludes is attributed to the [[relativistic quantum chemistry|relativistic]] stabilization of the 5f electrons, which increases with increasing atomic number.<ref>{{cite book|doi=10.1021/bk-1980-0131.ch012|title=Lanthanide and Actinide Chemistry and Spectroscopy|volume=131|pages=[https://archive.org/details/lanthanideactini0000unse/page/239 239–263]|date=1980|isbn=9780841205680|author=Hulet, E. K.|editor=Edelstein, Norman M.|chapter=Chapter 12. Chemistry of the Heaviest Actinides: Fermium, Mendelevium, Nobelium, and Lawrencium|series=ACS Symposium Series|chapter-url=https://archive.org/details/lanthanideactini0000unse|url=https://archive.org/details/lanthanideactini0000unse/page/239}}</ref> [[Thermochromatography|Thermochromatographic]] studies with trace quantities of mendelevium by Zvara and Hübener from 1976 to 1982 confirmed this prediction.<ref name="Silva16345" /> In 1990, Haire and Gibson estimated mendelevium metal to have an [[enthalpy of sublimation]] between 134 and 142&nbsp;kJ/mol.<ref name="Silva16345" /> Divalent mendelevium metal should have a [[metallic radius]] of around {{val|194|10|u=[[picometer|pm]]}}.<ref name="Silva16345" /> Like the other divalent late actinides (except the once again trivalent [[lawrencium]]), metallic mendelevium should assume a [[face-centered cubic]] crystal structure.<ref name="density" /> Mendelevium's melting point has been estimated at 800&nbsp;°C, the same value as that predicted for the neighboring element nobelium.<ref>{{cite book|ref=Haynes|editor=Haynes, William M.|date=2011|title= CRC Handbook of Chemistry and Physics |edition=92nd|publisher= CRC Press|isbn=978-1439855119|pages=4.121–4.123}}</ref> Its density is predicted to be around {{val|10.3|0.7|u=g/cm<sup>3</sup>}}.<ref name="density" />

===Chemical===
The chemistry of mendelevium is mostly known only in solution, in which it can take on the +3 or +2 [[oxidation state]]s. The +1 state has also been reported, but has not yet been confirmed.<ref name="Silva16356">Silva, pp. 1635–6</ref>

Before mendelevium's discovery, [[Glenn T. Seaborg|Seaborg]] and Katz predicted that it should be predominantly trivalent in aqueous solution and hence should behave similarly to other tripositive lanthanides and actinides. After the synthesis of mendelevium in 1955, these predictions were confirmed, first in the observation at its discovery that it [[elution|eluted]] just after fermium in the trivalent actinide elution sequence from a cation-exchange column of resin, and later the 1967 observation that mendelevium could form insoluble [[hydroxide]]s and [[fluoride]]s that coprecipitated with trivalent lanthanide salts.<ref name="Silva16356" /> Cation-exchange and solvent extraction studies led to the conclusion that mendelevium was a trivalent actinide with an ionic radius somewhat smaller than that of the previous actinide, fermium.<ref name="Silva16356" /> Mendelevium can form [[coordination complex]]es with 1,2-cyclohexanedinitrilotetraacetic acid (DCTA).<ref name="Silva16356" />

In [[redox|reducing]] conditions, mendelevium(III) can be easily reduced to mendelevium(II), which is stable in aqueous solution.<ref name="Silva16356" /> The [[standard reduction potential]] of the ''E''°(Md<sup>3+</sup>→Md<sup>2+</sup>) couple was variously estimated in 1967 as −0.10&nbsp;V or −0.20&nbsp;V:<ref name="Silva16356" /> later 2013 experiments established the value as {{val|−0.16|0.05|u=V}}.<ref>
{{cite journal |last1=Toyoshima |first1=Atsushi |last2=Li |first2=Zijie |first3=Masato |last3=Asai |first4=Nozomi |last4=Sato |first5=Tetsuya K. |last5=Sato |first6=Takahiro |last6=Kikuchi |first7=Yusuke |last7=Kaneya |first8=Yoshihiro |last8=Kitatsuji |first9=Kazuaki |last9=Tsukada |first10=Yuichiro |last10=Nagame |first11=Matthias |last11=Schädel |first12=Kazuhiro |last12=Ooe |first13=Yoshitaka |last13=Kasamatsu |first14=Atsushi |last14=Shinohara |first15=Hiromitsu |last15=Haba |first16=Julia |last16=Even |date=11 October 2013 |title=Measurement of the Md<sup>3+</sup>/Md<sup>2+</sup> Reduction Potential Studied with Flow Electrolytic Chromatography |journal=Inorganic Chemistry |volume=52 |issue=21 |pages=12311–3 |doi=10.1021/ic401571h|pmid=24116851 }}</ref> In comparison, ''E''°(Md<sup>3+</sup>→Md<sup>0</sup>) should be around −1.74&nbsp;V, and ''E''°(Md<sup>2+</sup>→Md<sup>0</sup>) should be around −2.5&nbsp;V.<ref name="Silva16356" /> Mendelevium(II)'s elution behavior has been compared with that of [[strontium]](II) and [[europium]](II).<ref name="Silva16356" />

In 1973, mendelevium(I) was reported to have been produced by Russian scientists, who obtained it by reducing higher oxidation states of mendelevium with [[samarium]](II). It was found to be stable in neutral water–[[ethanol]] solution and be [[Homologous series|homologous]] to [[caesium]](I). However, later experiments found no evidence for mendelevium(I) and found that mendelevium behaved like divalent elements when reduced, not like the monovalent [[alkali metal]]s.<ref name="Silva16356" /> Nevertheless, the Russian team conducted further studies on the [[thermodynamics]] of cocrystallizing mendelevium with alkali metal [[chloride]]s, and concluded that mendelevium(I) had formed and could form mixed crystals with divalent elements, thus cocrystallizing with them. The status of the +1 oxidation state is still tentative.<ref name="Silva16356" />

The electrode potential ''E''°(Md<sup>4+</sup>→Md<sup>3+</sup>) was predicted in 1975 to be +5.4&nbsp;V; 1967 experiments with the strong oxidizing agent [[sodium bismuthate]] were unable to oxidize mendelevium(III) to mendelevium(IV).<ref name="Silva16356" />

===Atomic===
A mendelevium atom has 101 electrons. They are expected to be arranged in the configuration [Rn]5f<sup>13</sup>7s<sup>2</sup> (ground state [[term symbol]] <sup>2</sup>F<sub>7/2</sub>), although experimental verification of this electron configuration had not yet been made as of 2006. The fifteen electrons in the 5f and 7s subshells are [[valence electron]]s.<ref name="Silva16334">Silva, pp. 1633–4</ref> In forming compounds, three valence electrons may be lost, leaving behind a [Rn]5f<sup>12</sup> core: this conforms to the trend set by the other actinides with their [Rn]&nbsp;5f<sup>''n''</sup> electron configurations in the tripositive state. The first [[ionization potential]] of mendelevium was measured to be at most (6.58&nbsp;±&nbsp;0.07)&nbsp;[[electronvolt|eV]] in 1974, based on the assumption that the 7s electrons would ionize before the 5f ones;<ref name="NIST">{{cite journal|first1=W. C. |last1=Martin |first2=Lucy |last2=Hagan |first3=Joseph |last3=Reader |first4=Jack |last4=Sugan |date=1974 |title=Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions |url=https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |journal=J. Phys. Chem. Ref. Data |volume=3 |issue=3 |pages=771–9 |access-date=2013-10-19 |doi=10.1063/1.3253147 |url-status=dead |archive-url=https://web.archive.org/web/20140211144635/https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |archive-date=2014-02-11 |bibcode=1974JPCRD...3..771M }}</ref> this value has since not yet been refined further due to mendelevium's scarcity and high radioactivity.<ref>David R. Lide (ed), ''CRC Handbook of Chemistry and Physics, 84th Edition''. CRC Press. Boca Raton, Florida, 2003; Section 10, Atomic, Molecular, and Optical Physics; Ionization Potentials of Atoms and Atomic Ions</ref> The ionic radius of [[coordination number|hexacoordinate]] Md<sup>3+</sup> had been preliminarily estimated in 1978 to be around 91.2&nbsp;pm;<ref name="Silva16356" /> 1988 calculations based on the logarithmic trend between [[distribution coefficient]]s and ionic radius produced a value of 89.6&nbsp;pm, as well as an [[enthalpy of hydration]] of {{val|−3654|12|u=kJ/mol}}.<ref name="Silva16356" /> Md<sup>2+</sup> should have an ionic radius of 115&nbsp;pm and hydration enthalpy −1413&nbsp;kJ/mol; Md<sup>+</sup> should have ionic radius 117&nbsp;pm.<ref name="Silva16356" />

===Isotopes===
{{main|Isotopes of mendelevium}}

Seventeen isotopes of mendelevium are known, with mass numbers from 244 to 260; all are radioactive.<ref name="Silva16301">Silva, pp. 1630–1</ref> Additionally, 14 [[nuclear isomer]]s are known.<!--244m,245m,246m,247m,249m,250m,251m,252m,253m,254m,255m,256m1,256m2,258m, too many to list-->{{NUBASE2020|ref}} Of these, the longest-lived isotope is <sup>258</sup>Md with a half-life of 51.59&nbsp;days, and the longest-lived isomer is <sup>258m</sup>Md with a half-life of 57.0&nbsp;minutes.{{NUBASE2020|ref}} Nevertheless, the shorter-lived <sup>256</sup>Md (half-life 1.295&nbsp;hours<!--77.7 minutes-->) is more often used in chemical experimentation because it can be produced in larger quantities from [[alpha particle]] irradiation of einsteinium.<ref name="Silva16301" /> After <sup>258</sup>Md, the next most stable mendelevium isotopes are <sup>260</sup>Md with a half-life of 27.8&nbsp;days, <sup>257</sup>Md with a half-life of 5.52&nbsp;hours, <sup>259</sup>Md with a half-life of 1.60&nbsp;hours, and <sup>256</sup>Md with a half-life of 1.295&nbsp;hours. All of the remaining mendelevium isotopes have half-lives that are less than an hour, and the majority of these have half-lives that are less than 5&nbsp;minutes.{{NUBASE2020|ref}}<ref name="Silva16301" />

The half-lives of mendelevium isotopes mostly increase smoothly from <sup>244</sup>Md onwards, reaching a maximum at <sup>258</sup>Md.{{NUBASE2020|ref}}<ref name="Silva16301" /> Experiments and predictions suggest that the half-lives will then decrease, apart from <sup>260</sup>Md with a half-life of 27.8&nbsp;days,{{NUBASE2020|ref}}<ref name="Silva16301" /> as [[spontaneous fission]] becomes the dominant decay mode{{NUBASE2020|ref}} due to the mutual repulsion of the protons posing a limit to the island of relative stability of long-lived nuclei in the [[actinide]] series.<ref name="Nurmia">{{cite journal|first = Matti|last = Nurmia|date = 2003 |title = Nobelium|journal = Chemical and Engineering News|url = http://pubs.acs.org/cen/80th/nobelium.html|volume = 81|issue = 36|page = 178|doi = 10.1021/cen-v081n036.p178}}</ref> In addition, mendelevium is the element with the highest atomic number that has a known isotope with a half-life longer than one day.{{NUBASE2020|ref}}

Mendelevium-256, the chemically most important isotope of mendelevium, decays through [[electron capture]] 90% of the time and [[alpha decay]] 10% of the time.<ref name="Silva16301" /> It is most easily detected through the [[spontaneous fission]] of its electron capture daughter [[fermium-256]], but in the presence of other nuclides that undergo spontaneous fission, alpha decays at the characteristic energies for mendelevium-256 (7.205 and 7.139&nbsp;[[electronvolt|MeV]]) can provide more useful identification.<ref name="Silva16313" />

==Production and isolation==
The lightest isotopes (<sup>244</sup>Md to <sup>247</sup>Md) are mostly produced through bombardment of [[bismuth]] targets with [[argon]] ions, while slightly heavier ones (<sup>248</sup>Md to <sup>253</sup>Md) are produced by bombarding [[plutonium]] and [[americium]] targets with ions of [[carbon]] and [[nitrogen]]. The most important and most stable isotopes are in the range from <sup>254</sup>Md to <sup>258</sup>Md and are produced through bombardment of [[einsteinium]] with alpha particles: einsteinium-253, −254, and −255 can all be used. <sup>259</sup>Md is produced as a [[decay product|daughter]] of <sup>259</sup>[[nobelium|No]], and <sup>260</sup>Md can be produced in a [[Nuclear reaction#Transfer reactions|transfer reaction]] between einsteinium-254 and [[oxygen-18]].<ref name="Silva16301" /> Typically, the most commonly used isotope <sup>256</sup>Md is produced by bombarding either einsteinium-253 or −254 with alpha particles: einsteinium-254 is preferred when available because it has a longer half-life and therefore can be used as a target for longer.<ref name="Silva16301" /> Using available microgram quantities of einsteinium, [[femtogram]] quantities of mendelevium-256 may be produced.<ref name="Silva16301" />

The recoil [[momentum]] of the produced mendelevium-256 atoms is used to bring them physically far away from the einsteinium target from which they are produced, bringing them onto a thin foil of metal (usually [[beryllium]], [[aluminium]], [[platinum]], or [[gold]]) just behind the target in a vacuum.<ref name="Silva16313">Silva, pp. 1631–3</ref> This eliminates the need for immediate chemical separation, which is both costly and prevents reusing of the expensive einsteinium target.<ref name="Silva16313" /> The mendelevium atoms are then trapped in a gas atmosphere (frequently [[helium]]), and a gas jet from a small opening in the reaction chamber carries the mendelevium along.<ref name="Silva16313" /> Using a long [[capillary tube]], and including [[potassium chloride]] aerosols in the helium gas, the mendelevium atoms can be transported over tens of [[meter]]s to be chemically analyzed and have their quantity determined.<ref name="book2" /><ref name="Silva16313" /> The mendelevium can then be separated from the foil material and other [[fission product]]s by applying acid to the foil and then [[coprecipitation|coprecipitating]] the mendelevium with [[lanthanum fluoride]], then using a [[cation-exchange]] resin column with a 10% [[ethanol]] solution saturated with [[hydrochloric acid]], acting as an [[eluant]]. However, if the foil is made of gold and thin enough, it is enough to simply dissolve the gold in [[aqua regia]] before separating the trivalent actinides from the gold using [[anion-exchange]] [[chromatography]], the eluant being 6&nbsp;M&nbsp;hydrochloric acid.<ref name="Silva16313" />

Mendelevium can finally be separated from the other trivalent actinides using selective elution from a cation-exchange resin column, the eluant being ammonia α-HIB.<ref name="Silva16313" /> Using the gas-jet method often renders the first two steps unnecessary.<ref name="Silva16313" /> The above procedure is the most commonly used one for the separation of transeinsteinium elements.<ref name="Silva16313" />

Another possible way to separate the trivalent actinides is via solvent extraction chromatography using bis-(2-ethylhexyl) phosphoric acid (abbreviated as HDEHP) as the stationary organic phase and [[nitric acid]] as the mobile aqueous phase. The actinide elution sequence is reversed from that of the cation-exchange resin column, so that the heavier actinides elute later. The mendelevium separated by this method has the advantage of being free of organic complexing agent compared to the resin column; the disadvantage is that mendelevium then elutes very late in the elution sequence, after fermium.<ref name="book2" /><ref name="Silva16313" />

Another method to isolate mendelevium exploits the distinct elution properties of Md<sup>2+</sup> from those of Es<sup>3+</sup> and Fm<sup>3+</sup>. The initial steps are the same as above, and employs HDEHP for extraction chromatography, but coprecipitates the mendelevium with terbium fluoride instead of lanthanum fluoride. Then, 50&nbsp;mg of [[chromium]] is added to the mendelevium to reduce it to the +2 state in 0.1&nbsp;M&nbsp;hydrochloric acid with [[zinc]] or [[mercury (element)|mercury]].<ref name="Silva16313" /> The solvent extraction then proceeds, and while the trivalent and tetravalent lanthanides and actinides remain on the column, mendelevium(II) does not and stays in the hydrochloric acid. It is then reoxidized to the +3 state using [[hydrogen peroxide]] and then isolated by selective elution with 2&nbsp;M&nbsp;hydrochloric acid (to remove impurities, including chromium) and finally 6&nbsp;M&nbsp;hydrochloric acid (to remove the mendelevium).<ref name="Silva16313" /> It is also possible to use a column of cationite and zinc amalgam, using 1&nbsp;M&nbsp;hydrochloric acid as an eluant, reducing Md(III) to Md(II) where it behaves like the [[alkaline earth metals]].<ref name="Silva16313" /> Thermochromatographic chemical isolation could be achieved using the volatile mendelevium [[hexafluoroacetylacetonate]]: the analogous fermium compound is also known and is also volatile.<ref name="Silva16313" />

==Toxicity==
Though few people come in contact with mendelevium, the [[International Commission on Radiological Protection]] has set annual exposure limits for the most stable isotope. For mendelevium-258, the ingestion limit was set at 9×10<sup>5</sup> [[becquerel]]s (1 Bq = 1 decay per second). Given the half-life of this isotope, this is only 2.48&nbsp;ng (nanograms). The inhalation limit is at 6000 Bq or 16.5 pg (picogram).<ref>{{cite book|last1=Koch|first1=Lothar|title=Transuranium Elements, in Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley|date=2000|doi=10.1002/14356007.a27_167|chapter=Transuranium Elements|isbn=978-3527306732}}</ref>

==Notes==
{{Notelist}}

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

==Bibliography==
*{{cite book|last=Silva |first=Robert J. |chapter=Fermium, Mendelevium, Nobelium, and Lawrencium |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 |pages=1621–1651 |chapter-url=http://radchem.nevada.edu/classes/rdch710/files/Fm%20to%20Lr.pdf |doi=10.1007/1-4020-3598-5_13 |isbn=978-1-4020-3555-5 |url-status=dead |archive-url=https://web.archive.org/web/20100717155410/http://radchem.nevada.edu/classes/rdch710/files/Fm%20to%20Lr.pdf |archive-date=2010-07-17 }}

==Further reading==
* Hoffman, D.C., Ghiorso, A., Seaborg, G. T. The transuranium people: the inside story, (2000), 201–229
* Morss, L. R., Edelstein, N. M., Fuger, J., The chemistry of the actinide and transactinide element, 3, (2006), 1630–1636
* ''A Guide to the Elements – Revised Edition'', Albert Stwertka, (Oxford University Press; 1998) {{ISBN|0-19-508083-1}}

==External links==
{{Commons|Mendelevium}}
{{Wiktionary|mendelevium}}
*[https://periodic.lanl.gov/101.shtml Los Alamos National Laboratory – Mendelevium]
*[https://education.jlab.org/itselemental/ele101.html It's Elemental – Mendelevium]
* [http://www.periodicvideos.com/videos/101.htm Mendelevium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
*[https://environmentalchemistry.com/yogi/periodic/Md.html Environmental Chemistry – Md info]
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[[Category:Mendelevium| ]]
[[Category:Chemical elements]]
[[Category:Chemical elements with face-centered cubic structure]]
[[Category:Actinides]]
[[Category:Synthetic elements]]
[[Category:Dmitri Mendeleev]]