Editing Lanthanum (section)

==Characteristics==

===Physical===
Lanthanum is the first element and prototype of the lanthanide series. In the periodic table, it appears to the right of the [[alkaline earth metal]] [[barium]] and to the left of the lanthanide cerium. Lanthanum is generally considered the first of the f-block elements by authors writing on the subject.<ref name=Fluck>{{cite journal |last=Fluck |first=E. |year=1988 |title=New notations in the periodic table |journal=[[Pure and Applied Chemistry]] |volume=60 |issue=3 |pages=431–436 |s2cid=96704008 |doi=10.1351/pac198860030431 |url=http://www.iupac.org/publications/pac/1988/pdf/6003x0431.pdf |url-status=live |access-date=24 March 2012 |archive-url=https://web.archive.org/web/20120325152951/http://www.iupac.org/publications/pac/1988/pdf/6003x0431.pdf |archive-date=25 March 2012 }}</ref><ref>{{cite book |author1-link=Lev Landau |first1=L.D. |last1=Landau |author2-link=Evgeny Lifshitz |first2=E.M. |last2=Lifshitz |year=1958 |title=Quantum Mechanics: Non-relativistic theory |edition=1st |volume=3 |pages=256–257 |publisher=[[Pergamon Press]] }}</ref><ref name=Jensen1982>{{cite journal |first=W.B. |last=Jensen |author-link=William B. Jensen |year=1982 |title=The positions of lanthanum (actinium) and lutetium (lawrencium) in the periodic table |journal=Journal of Chemical Education |volume=59 |issue=8 |pages=634–636 |bibcode=1982JChEd..59..634J |doi=10.1021/ed059p634}}</ref><ref name=Jensen2015>{{cite journal |last=Jensen |first=William B. |year=2015 |title=The positions of lanthanum (actinium) and lutetium (lawrencium) in the periodic table: An update |journal=Foundations of Chemistry |volume=17 |pages=23–31 |doi=10.1007/s10698-015-9216-1 |s2cid=98624395 |url=https://link.springer.com/article/10.1007/s10698-015-9216-1 |access-date=28 January 2021 }}</ref><ref>{{cite journal |last=Scerri |first=Eric |date=18 January 2021 |title=Provisional report on discussions on group&nbsp;3 of the periodic table |journal=Chemistry International |volume=43 |issue=1 |pages=31–34 |doi=10.1515/ci-2021-0115 |doi-access=free |s2cid=231694898 }}</ref> The 57&nbsp;electrons of a lanthanum atom are arranged in the [[electron configuration|configuration]] [Xe]5d{{sup|1}}6s{{sup|2}}, with three valence electrons outside the noble gas core. In chemical reactions, lanthanum almost always gives up these three valence electrons from the 5d and 6s [[electron shell#Subshells|subshells]] to form the +3 oxidation state, achieving the stable configuration of the preceding noble gas [[xenon]].<ref name=Greenwood1106>{{harvp|Greenwood|Earnshaw|1984|p=1106}}</ref> Some lanthanum(II) compounds are also known, but they are usually much less stable.<ref name=patnaik>{{cite book |last=Patnaik |first=Pradyot |year=2003 |title=Handbook of Inorganic Chemical Compounds |publisher=McGraw-Hill |isbn=978-0-07-049439-8 |pages=444–446 |url={{Google books|plainurl=yes|id=Xqj-TTzkvTEC|page=243 }} |access-date=2009-06-06}}</ref><ref>{{cite journal |bibcode=2008AngCh.120.1510H |title=Lanthanum Does Form Stable Molecular Compounds in the +2 Oxidation State |last1=Hitchcock |first1=Peter B. |last2=Lappert |first2=Michael F. |last3=Maron |first3=Laurent |last4=Protchenko |first4=Andrey V. |journal=Angewandte Chemie |year=2008 |volume=120 |issue=8 |page=1510 |doi=10.1002/ange.200704887 }}</ref> Lanthanum monoxide (LaO) produces strong absorption bands in some [[stellar spectra]].<ref>{{cite journal |last1=Jevons |first1=W. |year=1928 |title=The band spectrum of lanthanum monoxide |journal=Proceedings of the Physical Society |volume=41 |issue=1 |page=520 |bibcode=1928PPS....41..520J |doi=10.1088/0959-5309/41/1/355 }}</ref>

Among the lanthanides, lanthanum is exceptional as it has no 4f electrons as a single gas-phase atom. Thus it is only very weakly [[paramagnetic]], unlike the strongly paramagnetic later lanthanides (with the exceptions of the last two, [[ytterbium]] and [[lutetium]], where the 4f shell is completely full).<ref>{{cite book |last1 = Cullity |first1 = B.D. |last2 = Graham |first2 = C.D. |year = 2011 |title = Introduction to Magnetic Materials |publisher = John Wiley & Sons |place = New York, NY |isbn = 9781118211496 }}</ref> However, the 4f shell of lanthanum can become partially occupied in chemical environments and participate in chemical bonding.<ref name=Wittig>{{cite conference |last = Wittig |first = Jörg |date = 19–24 March 1973 |title=Festkörper Probleme (plenary lecture) |lang=de |trans-title=Solid state problems (plenary lecture) |editor-last=Queisser |editor-first=H.J. |conference=The Divisions Semiconductor Physics, Surface Physics, Low Temperature Physics, High Polymers, Thermodynamics and Statistical Mechanics, of the German Physical Society |place=Münster, DE |publisher=Springer |isbn=978-3-528-08019-8 |series=Advances in Solid State Physics |volume=13 |publication-place=Berlin, DE / Heidelberg, DE |pages=375–396 |chapter=The pressure variable in solid state physics: What about 4f-band superconductors? |doi=10.1007/BFb0108579 }}</ref><ref>{{cite journal | last1=Krinsky | first1=Jamin L. | last2=Minasian | first2=Stefan G. | last3=Arnold | first3=John | date=2010-12-08 | title=Covalent lanthanide chemistry near the limit of weak bonding: Observation of {{chem|(Cp|Si|Me|3|)|3|Ce−E|Cp*}} and a comprehensive density functional theory analysis of {{chem|Cp|3|Ln−E|Cp}} (E = Al, Ga) | journal=Inorganic Chemistry | publisher=American Chemical Society (ACS) | volume=50 | issue=1 | pages=345–357 | issn=0020-1669 | doi=10.1021/ic102028d | pmid=21141834 }}</ref> For example, the melting points of the trivalent lanthanides (all but [[europium]] and ytterbium) are related to the extent of hybridisation of the 6s, 5d, and 4f electrons (lowering with increasing 4f involvement),<ref>{{cite book |last=Gschneidner |first=Karl A., Jr. |year=2016 |title=Handbook on the Physics and Chemistry of Rare Earths |isbn=978-0-444-63851-9 |editor-first=Jean-Claude G.  |editor-last=Bünzli |volume=50 |pages=12–16 |chapter=282&nbsp;Systematics |editor-last2=Vitalij K. Pecharsky}}</ref> and lanthanum has the second-lowest melting point among them: 920&nbsp;°C. (Europium and ytterbium have lower melting points because they delocalise about two electrons per atom rather than three.)<ref>{{cite book |last1 = Krishnamurthy |first1 = Nagaiyar |last2 = Gupta |first2 = Chiranjib Kumar |year = 2004 |title = Extractive Metallurgy of Rare Earths |publisher = CRC Press |isbn = 0-415-33340-7 }}</ref> This chemical availability of f orbitals justifies lanthanum's placement in the f-block despite its anomalous ground-state configuration<ref name=Hamilton>{{cite journal |last=Hamilton |first=David C. |date=1965 |title=Position of lanthanum in the periodic table |journal=American Journal of Physics |volume=33 |issue=8 |pages=637–640 |bibcode=1965AmJPh..33..637H |doi=10.1119/1.1972042}}</ref><ref name=JensenLr>{{cite report |last=Jensen |first=W.B. |year=2015 |title=Some comments on the position of lawrencium in the periodic table |url=http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/251.%20Lawrencium.pdf |url-status=dead |archive-url=https://web.archive.org/web/20151223091325/http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/251.%20Lawrencium.pdf |archive-date=23 December 2015 |access-date=20 September 2015}}</ref> (which is merely the result of strong interelectronic repulsion making it less profitable to occupy the 4f shell, as it is small and close to the core electrons).<ref>{{Cite journal |last=Jørgensen |first=Christian |date=1973 |title=The Loose Connection between Electron Configuration and the Chemical Behavior of the Heavy Elements (Transuranics) |journal=Angewandte Chemie International Edition |volume=12 |issue=1 |pages=12–19 |doi=10.1002/anie.197300121}}</ref>

The lanthanides become harder as the series is traversed: as expected, lanthanum is a soft metal. Lanthanum has a relatively high [[resistivity]] of 615&nbsp;nΩm at room temperature; in comparison, the value for the good conductor aluminium is only 26.50&nbsp;nΩm.<ref name=Greenwood1429>{{harvp|Greenwood|Earnshaw|1984|p=1429}}</ref><ref name=CRC>{{RubberBible86th}}</ref> Lanthanum is the least volatile of the lanthanides.<ref name=radio>{{cite report |title=The Radiochemistry of the Rare Earths, Scandium, Yttrium, and Actinium |publisher=Los Alamos National Laboratory |place = Los Alamos, NM |url=http://library.lanl.gov/cgi-bin/getfile?rc000021.pdf |url-status=live |via=lanl.gov |access-date=2016-06-23 |archive-url=https://web.archive.org/web/20210831203424/https://library.lanl.gov/cgi-bin/getfile?rc000021.pdf |archive-date=2021-08-31 }}</ref> Like most of the lanthanides, lanthanum has a [[Hexagonal crystal system|hexagonal crystal structure]] at room temperature ({{mvar|α}}-La). At 310&nbsp;°C, lanthanum changes to a [[face-centered cubic]] structure ({{mvar|β}}-La), and at 865&nbsp;°C, it changes to a [[body-centered cubic]] structure ({{mvar|γ}}-La).<ref name=CRC/>

===Chemical===
As expected from [[periodic trend]]s, lanthanum has the largest [[atomic radius]] of the lanthanides. Hence, it is the most reactive among them, tarnishing quite rapidly in air, turning completely dark after several hours and can readily burn to form [[lanthanum(III) oxide]], {{chem|La|2|O|3}}, which is almost as [[Basic (chemistry)|basic]] as [[calcium oxide]].<ref>{{harvp|Greenwood|Earnshaw|1984|pp=1105–1107}}</ref> A centimeter-sized sample of lanthanum will corrode completely in a year as its oxide [[spallation|spalls]] off like iron [[rust]], instead of forming a protective oxide coating like [[aluminium]], scandium, yttrium, and lutetium.<ref>{{Cite web |title=Rare-Earth Metal Long Term Air Exposure Test |url=http://www.elementsales.com/re_exp/index.htm |access-date=2009-08-08}}</ref> Lanthanum reacts with the [[halogen]]s at room temperature to form the trihalides, and upon warming will form [[binary compound]]s with the nonmetals nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic.<ref name=Greenwood1106/><ref name=patnaik/> Lanthanum reacts slowly with water to form [[lanthanum hydroxide|lanthanum(III) hydroxide]], {{chem|La|(OH)|3}}.<ref name=webelements>{{Cite web |title=Chemical reactions of lanthanum |url=https://www.webelements.com/lanthanum/chemistry.html |access-date=2009-06-06 |publisher=Webelements}}</ref> In dilute [[sulfuric acid]], lanthanum readily forms the aquated tripositive ion {{chem|[La|(H|2|O)|9|]|3+}}: This is colorless in aqueous solution since {{chem|La|3+}} has no d or f electrons.<ref name=webelements/> Lanthanum is the strongest and [[HSAB theory|hardest]] base among the [[rare earth element]]s, which is again expected from its being the largest of them.<ref name=Greenwood1434>{{harvp|Greenwood|Earnshaw|1984|p=1434}}</ref>

Some lanthanum(II) compounds are also known, but they are much less stable.<ref name=patnaik/><!-- Although mentioned in the section on physical properties, someone looking for chemical characteristics easily misses it their --> Therefore, in officially naming compounds of lanthanum its oxidation number always is to be mentioned.

===Isotopes===
[[File:Lanthanum stable nucleus.png|thumb|left|280px|Excerpt from the [[chart of nuclides]] showing stable isotopes (black) from barium ({{nobr| {{mvar|Z}} {{=}} 56 }}) to neodymium ({{nobr| {{mvar|Z}} {{=}} 60 }})]]
{{Main|Isotopes of lanthanum}}
Naturally occurring lanthanum is made up of two isotopes, the stable {{chem|139|La}} and the [[primordial nuclide|primordial long-lived radioisotope]] {{chem|138|La}}. {{chem|139|La}} is by far the most abundant, making up 99.910% of natural lanthanum: it is produced in the [[s-process]] (slow [[neutron]] capture, which occurs in low- to medium-mass stars) and the [[r-process]] (rapid neutron capture, which occurs in core-collapse [[supernova]]e). It is the only stable isotope of lanthanum.<ref name=Audi>{{NUBASE 2003}}</ref> The very rare isotope {{chem|138|La}} is one of the few primordial [[odd–odd nuclei]], with a long half-life of {{nobr|1.05×{{10^|11}} years.}} It is one of the proton-rich [[p-nuclei]] which cannot be produced in the [[s-process|s-]] or [[r-process]]es. {{chem|138|La}}, along with the even rarer [[tantalum-180m|{{chem|180m|Ta}}]], is produced in the ν-process, where [[neutrino]]s interact with stable nuclei.<ref name=nu-process>{{cite journal |last1=Woosley |first1=S.E. |last2=Hartmann |first2=D.H. |last3=Hoffman |first3=R.D. |last4=Haxton |first4=W.C. |year=1990 |title=The ν-process |journal=The Astrophysical Journal |volume=356 |pages=272–301 |bibcode=1990ApJ...356..272W |doi=10.1086/168839}}</ref> All other lanthanum isotopes are [[synthetic radioisotope|synthetic]]: With the exception of {{chem|137|La}} with a half-life of about 60,000&nbsp;years, all of them have half-lives less than two days, and most have half-lives less than a minute. The isotopes {{chem|139|La}} and {{chem|140|La}} occur as [[fission product]]s of uranium.<ref name=Audi/>