Editing Mendelevium (section)

==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" />