Editing Rubidium (section)

==History==
[[File:Kirchhoff Bunsen Roscoe.jpg|thumb|left|upright|[[Gustav Kirchhoff]] (left) and [[Robert Bunsen]] (center) discovered rubidium by spectroscopy. ''([[Henry Enfield Roscoe]] is on the right.)''| alt= Three middle-aged men, with the one in the middle sitting down. All wear long jackets, and the shorter man on the left has a beard.]]
Rubidium was discovered in 1861 by [[Robert Bunsen]] and [[Gustav Kirchhoff]], in Heidelberg, Germany, in the mineral [[lepidolite]] through [[flame spectroscopy]]. Because of the bright red lines in its [[emission spectrum]], they chose a name derived from the [[Latin]] word {{lang|la|rubidus}}, meaning "deep red".<ref name="BuKi1861">{{Cite journal |title = Chemische Analyse durch Spectralbeobachtungen |pages = 337–381 |first1 = G. |last1 = Kirchhoff |first2 = R. |last2 = Bunsen |author-link1 = Gustav Kirchhoff |author-link2 = Robert Bunsen |doi = 10.1002/andp.18611890702 |journal = [[Annalen der Physik|Annalen der Physik und Chemie]] |volume = 189 |issue = 7 |date = 1861 |bibcode=1861AnP...189..337K|hdl = 2027/hvd.32044080591324 |url = http://archiv.ub.uni-heidelberg.de/volltextserver/15657/1/spektral.pdf }}</ref><ref name="Weeks">{{Cite journal |title = The discovery of the elements. XIII. Some spectroscopic discoveries |pages = 1413–1434 |last = Weeks |first = Mary Elvira |author-link=Mary Elvira Weeks|doi=10.1021/ed009p1413 |journal = [[Journal of Chemical Education]] |volume =9 |issue =8 |date = 1932 |bibcode=1932JChEd...9.1413W}}</ref>

Rubidium is a minor component in [[lepidolite]]. Kirchhoff and Bunsen processed 150&nbsp;kg of a lepidolite containing only 0.24% rubidium monoxide (Rb<sub>2</sub>O). Both potassium and rubidium form insoluble salts with [[chloroplatinic acid]], but those salts show a slight difference in solubility in hot water. Therefore, the less soluble rubidium [[hexachloroplatinate]] (Rb<sub>2</sub>PtCl<sub>6</sub>) could be obtained by [[fractional crystallization (chemistry)|fractional crystallization]]. After reduction of the hexachloroplatinate with [[hydrogen]], the process yielded 0.51&nbsp;grams of [[rubidium chloride]] (RbCl) for further studies. Bunsen and Kirchhoff began their first large-scale isolation of caesium and rubidium compounds with {{Convert|44000|litre|USgal}} of mineral water, which yielded 7.3&nbsp;grams of [[caesium chloride]] and 9.2&nbsp;grams of [[rubidium chloride]].<ref name="BuKi1861" /><ref name="Weeks" /> Rubidium was the second element, shortly after caesium, to be discovered by spectroscopy, just one year after the invention of the [[spectroscope]] by Bunsen and Kirchhoff.<ref name="autogenerated1">{{cite web |url=http://pubs.acs.org/cen/80th/print/rubidium.html |title=C&EN: It's Elemental: The Periodic Table – Cesium |publisher=American Chemical Society |access-date=2010-02-25 |first=Stephen K. |last = Ritter |date = 2003}}</ref>

The two scientists used the rubidium chloride to estimate that the [[atomic weight]] of the new element was 85.36 (the currently accepted value is 85.47).<ref name="BuKi1861" /> They tried to generate elemental rubidium by electrolysis of molten rubidium chloride, but instead of a metal, they obtained a blue homogeneous substance, which "neither under the naked eye nor under the microscope showed the slightest trace of metallic substance". They presumed that it was a [[Non-stoichiometric compound|subchloride]] ({{chem|Rb|2|Cl}}); however, the product was probably a [[colloid]]al mixture of the metal and rubidium chloride.<ref>{{cite book |last=Zsigmondy |first=Richard |title=Colloids and the Ultra Microscope |publisher=Read books |date=2007 |isbn=978-1-4067-5938-9 |page=69 |url=https://books.google.com/books?id=Ac2mGhqjgUkC&pg=PAPA69 |access-date=2010-09-26}}</ref> In a second attempt to produce metallic rubidium, Bunsen was able to reduce rubidium by heating charred rubidium [[tartrate]]. Although the distilled rubidium was [[pyrophoric]], they were able to determine the density and the melting point. The quality of this research in the 1860s can be appraised by the fact that their determined density differs by less than 0.1&nbsp;g/cm<sup>3</sup><!--1.52--> and the melting point by less than 1&nbsp;°C <!--38.5&nbsp;°C--> from the presently accepted values.<ref>{{cite journal |last1=Bunsen |first1=R. |title=Ueber die Darstellung und die Eigenschaften des Rubidiums |journal = Annalen der Chemie und Pharmacie |volume = 125 |issue = 3 |pages = 367–368 |date = 1863 |doi = 10.1002/jlac.18631250314|url=https://zenodo.org/record/1427191 }}</ref>

The slight radioactivity of rubidium was discovered in 1908, but that was before the theory of isotopes was established in 1910, and the low level of activity (half-life greater than 10<sup>10</sup>&nbsp;years) made interpretation complicated. The now proven decay of <sup>87</sup>Rb to stable <sup>87</sup>Sr through [[beta decay]] was still under discussion in the late 1940s.<ref>{{cite journal |doi = 10.1080/14786441008520248 | journal = Philosophical Magazine |series=Series 7| volume = 43 | issue = 345 | date = 1952 | first = G. M. | last = Lewis |pages = 1070–1074 | title =The natural radioactivity of rubidium}}</ref><ref>{{cite journal | last1= Campbell| first1 = N. R.| last2= Wood | first2= A. | date = 1908 | volume = 14 | page = 15 | title=The Radioactivity of Rubidium |journal=Proceedings of the Cambridge Philosophical Society| url=https://archive.org/stream/proceedingsofcam15190810camb/proceedingsofcam15190810camb_djvu.txt}}</ref>

Rubidium had minimal industrial value before the 1920s.<ref name="USGS"/> Since then, the most important use of rubidium is research and development, primarily in chemical and electronic applications. In 1995, rubidium-87 was used to produce a [[Bose–Einstein condensate]],<ref>{{cite press release |title = The 2001 Nobel Prize in Physics |year = 2001 |website = [[Nobel Institute]] nobelprize.org |url = http://nobelprize.org/nobel_prizes/physics/laureates/2001/press.html |access-date = 2010-02-01}}</ref> for which the discoverers, [[Eric Allin Cornell]], [[Carl Edwin Wieman]] and [[Wolfgang Ketterle]], won the 2001 [[Nobel Prize in Physics]].<ref>{{cite journal |last = Levi |first = Barbara Goss |author-link=Barbara Goss Levi |year=2001 |title = Cornell, Ketterle, and Wieman share Nobel Prize for Bose-Einstein condensates |journal = [[Physics Today]] |volume = 54 |issue = 12 |pages = 14–16|bibcode = 2001PhT....54l..14L  |doi = 10.1063/1.1445529|doi-access = free }}</ref>