Editing Caesium (section)

===Physical properties===
[[File:CsCrystals.JPG|left|thumb|High-purity caesium-133 stored in [[argon]].|alt=Y-shaped yellowish crystal in glass ampoule, looking like the branch of a pine tree]]
Of all elements that are solid at room temperature, caesium is the softest: it has a hardness of 0.2 [[Mohs scale|Mohs]]. It is a very [[ductility|ductile]], pale metal, which darkens in the presence of trace amounts of [[oxygen]].<ref name="USGS">{{cite web |url=http://pubs.usgs.gov/of/2004/1432/2004-1432.pdf |publisher=United States Geological Survey |access-date=27 December 2009 |title=Mineral Commodity Profile: Cesium |first1=William C. |last1=Butterman |first2=William E. |last2=Brooks |first3=Robert G. Jr. |last3=Reese |date=2004 |url-status=dead |archive-url=https://web.archive.org/web/20070207015229/http://pubs.usgs.gov/of/2004/1432/2004-1432.pdf |archive-date=7 February 2007}}</ref><ref>{{cite book |title=Exploring Chemical Elements and their Compounds |author=Heiserman, David L. |publisher=McGraw-Hill |date=1992 |isbn=978-0-8306-3015-8 |pages=[https://archive.org/details/exploringchemica00heis/page/201 201]–203 |url-access=registration |url=https://archive.org/details/exploringchemica00heis}}</ref><ref>{{cite book |title=The Chemistry of the Liquid Alkali Metals |last=Addison |first=C. C. |date=1984 |publisher=Wiley |isbn=978-0-471-90508-0 |access-date=28 September 2012 |url=http://www.cs.rochester.edu/users/faculty/nelson/cesium/cesium_color.html |archive-date=8 September 2021 |archive-url=https://web.archive.org/web/20210908125520/https://www.cs.rochester.edu/users/faculty/nelson/cesium/cesium_color.html |url-status=live }}</ref> When in the presence of [[mineral oil]] (where it is best kept during transport), it loses its metallic [[lustre (mineralogy)|lustre]] and takes on a duller, grey appearance. It has a [[melting point]] of {{convert|28.5|C}}, making it one of the few elemental metals that are liquid near [[room temperature]]. The others are [[rubidium]] ({{convert|39|C|F|disp=sqbr}}), [[francium]] (estimated at {{convert|27|C|F|disp=sqbr}}), [[mercury (element)|mercury]] ({{convert|−39|C|F|disp=sqbr}}), and [[gallium]] ({{convert|30|C|F|disp=sqbr}}); bromine is also liquid at room temperature (melting at {{convert|−7.2|C|F|disp=sqbr}}), but it is a [[halogen]] and not a metal.  [[Mercury (element)|Mercury]] is the only stable elemental metal with a known melting point lower than caesium.<ref name="KanerACS">{{cite web |url=http://pubs.acs.org/cen/80th/print/cesium.html |title=C&EN: It's Elemental: The Periodic Table – Cesium |publisher=American Chemical Society |access-date=25 February 2010 |author=Kaner, Richard |date=2003 |archive-date=18 June 2015 |archive-url=https://web.archive.org/web/20150618061523/http://pubs.acs.org/cen/80th/print/cesium.html |url-status=live }}</ref> In addition, the metal has a rather low [[boiling point]], {{convert|641|C}}, the [[list of elements by boiling point|lowest]] of all stable metals other than mercury.<ref name="RSC"/> [[Copernicium]] and [[flerovium]] have been predicted to have lower boiling points than mercury and caesium, but they are extremely radioactive and it is not certain if they are metals.<ref name=CRNL>{{cite journal |last1=Mewes |first1=J.-M. |last2=Smits |first2=O. R. |last3=Kresse |first3=G. |last4=Schwerdtfeger |first4=P. |title=Copernicium is a Relativistic Noble Liquid |journal=Angewandte Chemie International Edition  |date=2019 |volume=58 |issue=50 |pages=17964–17968 |doi=10.1002/anie.201906966 |pmid=31596013 |url=https://www.researchgate.net/publication/336389017|pmc=6916354 }}</ref><ref name=liquid>{{cite journal|last1=Mewes|first1=Jan-Michael|last2=Schwerdtfeger|first2=Peter|date=11 February 2021|title=Exclusively Relativistic: Periodic Trends in the Melting and Boiling Points of Group 12|journal=Angewandte Chemie|volume= 60|issue= 14|pages= 7703–7709|doi=10.1002/anie.202100486|pmid=33576164|pmc=8048430}}</ref> 

[[File:Rb&Cs crystals.jpg|left|thumb|Caesium crystals (golden) compared to [[rubidium]] crystals (silvery)]]
Caesium forms [[alloy]]s with the other alkali metals, [[gold]], and mercury ([[amalgam (chemistry)|amalgams]]). At temperatures below {{convert|650|°C}}, it does not alloy with [[cobalt]], [[iron]], [[molybdenum]], [[nickel]], [[platinum]], [[tantalum]], or [[tungsten]]. It forms well-defined [[intermetallics|intermetallic compounds]] with [[antimony]], [[gallium]], [[indium]], and [[thorium]], which are [[photosensitive]].<ref name="USGS"/> It mixes with all the other alkali metals (except lithium); the alloy with a [[molar concentration|molar]] distribution of 41% caesium, 47% [[potassium]], and 12% [[sodium]] has the lowest melting point of any known metal alloy, at {{convert|−78|C}}.<ref name="KanerACS"/><ref>{{cite conference |url=http://symp15.nist.gov/pdf/p564.pdf |title=Density of melts of alkali metals and their Na-K-Cs and Na-K-Rb ternary systems |author=Taova, T. M. |conference=Fifteenth symposium on thermophysical properties, Boulder, Colorado, United States |date=22 June 2003 |access-date=26 September 2010 |display-authors=etal |url-status=dead |archive-url=https://web.archive.org/web/20061009133313/http://symp15.nist.gov/pdf/p564.pdf |archive-date=9 October 2006}}</ref> A few amalgams have been studied: {{chem|CsHg|2}} is black with a purple metallic [[lustre (mineralogy)|lustre]], while CsHg is golden-coloured, also with a metallic lustre.<ref>{{cite journal |doi=10.1016/S0079-6786(97)81004-7 |journal=Progress in Solid State Chemistry |volume=25 |date=1997 |pages=73–123 |title=Alkali metal amalgams, a group of unusual alloys |first=H. J. |last=Deiseroth |issue=1–2}}</ref>

The golden colour of caesium comes from the decreasing frequency of light required to excite electrons of the alkali metals as the group is descended. For lithium through rubidium this frequency is in the ultraviolet, but for caesium it enters the blue–violet end of the spectrum; in other words, the [[plasma oscillation|plasmonic frequency]] of the alkali metals becomes lower from lithium to caesium. Thus caesium transmits and partially absorbs violet light preferentially while other colours (having lower frequency) are reflected; hence it appears yellowish.<ref>{{cite book |last=Addison |first=C. C. |date=1984 |title=The chemistry of the liquid alkali metals |publisher=Wiley |page=7 |isbn=9780471905080}}</ref> Its compounds burn with a blue<ref name="CRC74">{{cite book |url=https://books.google.com/books?id=QdU-lRMjOsgC&pg=PA13 |page=13 |first=Charles T. |last=Lynch |publisher=CRC Press |date=1974 |title=CRC Handbook of Materials Science |isbn=978-0-8493-2321-8 |access-date=8 May 2021 |archive-date=5 March 2024 |archive-url=https://web.archive.org/web/20240305132944/https://books.google.com/books?id=QdU-lRMjOsgC&pg=PA13 |url-status=live }}</ref><ref name="flametests"/> or violet<ref name="flametests">{{cite web |url=http://www.chemguide.co.uk/inorganic/group1/flametests.html |title=Flame Tests |author=Clark, Jim |date=2005 |work=chemguide |access-date=29 January 2012 |archive-date=4 December 2017 |archive-url=https://web.archive.org/web/20171204162315/http://chemguide.co.uk/inorganic/group1/flametests.html |url-status=live }}</ref> colour.