Editing Caesium (section)

==Characteristics==

===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.

=== Allotropes ===
Caesium exists in the form of different [[Allotropy|allotropes]], one of them a dimer called dicaesium.<ref>{{Cite journal |last=C. A. |first=Onate |date=18 March 2021 |title=Ro-vibrational energies of cesium dimer and lithium dimer with molecular attractive potential |journal=Scientific Reports |volume=11 |issue=1 |page=6198 |doi=10.1038/s41598-021-85761-x |pmid=33737625 |pmc=7973739 }}</ref><!-- More exist... See https://www.knowledgedoor.com/2/elements_handbook/allotropes.html#cesium -->

===Chemical properties===
[[File:Cesium water.theora.ogv|left|thumb|Addition of a small amount of caesium to cold water is explosive.|alt=A person adds a small amount of metal to a petri dish with cold water which produces a small explosion.]]
Caesium metal is highly reactive and [[pyrophoricity|pyrophoric]]. It ignites spontaneously in air, and reacts explosively with water even at low temperatures, more so than the other [[alkali metal]]s.<!--YES INCLUDING FRANCIUM--><ref name="USGS"/> It reacts with ice at temperatures as low as {{convert|−116|C}}.<ref name="KanerACS"/> Because of this high reactivity, caesium metal is classified as a [[hazardous material]]. It is stored and shipped in dry, saturated hydrocarbons such as [[mineral oil]]. It can be handled only under [[inert gas]], such as [[argon]]. However, a caesium-water explosion is often less powerful than a [[sodium]]-water explosion with a similar amount of sodium. This is because caesium explodes instantly upon contact with water, leaving little time for [[hydrogen]] to accumulate.<ref>Gray, Theodore (2012) ''The Elements'', Black Dog & Leventhal Publishers, p. 131, {{ISBN|1-57912-895-5}}.</ref> Caesium can be stored in vacuum-sealed [[borosilicate glass]] [[ampoule]]s. In quantities of more than about {{convert|100|g|oz}}, caesium is shipped in hermetically sealed, stainless steel containers.<ref name="USGS"/>

The chemistry of caesium is similar to that of other alkali metals, in particular [[rubidium]], the element above caesium in the periodic table.<ref name="greenwood"/> As expected for an alkali metal, the only common oxidation state is +1. It differs from this value in caesides, which contain the Cs<sup>−</sup> anion and thus have caesium in the −1 oxidation state.<ref name="caeside2"/> Under conditions of extreme pressure (greater than 30&nbsp;[[pascal (unit)|GPa]]), theoretical studies indicate that the inner 5p electrons could form chemical bonds, where caesium would behave as the seventh 5p element, suggesting that higher caesium fluorides with caesium in oxidation states from +2 to +6 could exist under such conditions.<ref>{{Cite journal |last1=Miao |first1=Maosheng |last2=Sun |first2=Yuanhui |last3=Zurek |first3=Eva |last4=Lin |first4=Haiqing |date=2020 |title=Chemistry under high pressure |url=https://www.nature.com/articles/s41570-020-0213-0 |journal=Nature Reviews Chemistry |language=en |volume=4 |issue=10 |pages=508–527 |doi=10.1038/s41570-020-0213-0 |issn=2397-3358}}</ref><ref>{{cite web |last=Moskowitz |first=Clara |title=A Basic Rule of Chemistry Can Be Broken, Calculations Show |url=http://www.scientificamerican.com/article.cfm?id=chemical-bonds-inner-shell-electrons |work=Scientific American |access-date=22 November 2013 |archive-date=22 November 2013 |archive-url=https://web.archive.org/web/20131122052856/http://www.scientificamerican.com/article.cfm?id=chemical-bonds-inner-shell-electrons |url-status=live }}</ref> Some slight differences arise from the fact that it has a higher [[atomic mass]] and is more [[electronegativity|electropositive]] than other (nonradioactive) alkali metals.<ref name="HollemanAF">{{cite book |publisher=Walter de Gruyter |date=1985 |edition=91–100 |pages=953–955 |isbn=978-3-11-007511-3 |title=Lehrbuch der Anorganischen Chemie |first1=Arnold F. |last1=Holleman |last2=Wiberg |first2=Egon |last3=Wiberg |first3=Nils |chapter=Vergleichende Übersicht über die Gruppe der Alkalimetalle |language=de}}</ref> Caesium is the most electropositive chemical element.<ref name="KanerACS"/> The caesium ion is also larger and [[HSAB theory|less "hard"]] than those of the lighter [[alkali metals]].

===Compounds===
[[File:CsCl polyhedra.png|thumb|left|upright|Ball-and-stick model of the cubic coordination of Cs and Cl in CsCl| alt=27 small grey spheres in 3 evenly spaced layers of nine. 8 spheres form a regular cube and 8 of those cubes form a larger cube. The grey spheres represent the caesium atoms. The center of each small cube is occupied by a small green sphere representing a chlorine atom. Thus, every chlorine is in the middle of a cube formed by caesium atoms and every caesium is in the middle of a cube formed by chlorine.]]

Most caesium compounds contain the element as the [[cation]] {{chem|Cs|+}}, which [[ionic bond|binds ionically]] to a wide variety of [[anion]]s. One noteworthy exception is the [[alkalide|caeside]] anion ({{chem|Cs|−}}),<ref name="caeside2"/> and others are the several suboxides (see section on oxides below). More recently, caesium is predicted to behave as a [[p-block]] element and capable of forming higher fluorides with higher [[oxidation state|oxidation states]] (i.e., CsF<sub>n</sub> with n&nbsp;>&nbsp;1) under high pressure.<ref>{{cite journal |last=Miao |first=Mao-sheng |date=2013 |title=Caesium in high oxidation states and as a p-block element |url=https://www.nature.com/articles/nchem.1754 |journal=Nature Chemistry |language=en |volume=5 |issue=10 |pages=846–852 |doi=10.1038/nchem.1754 |pmid=24056341 |arxiv=1212.6290 |bibcode=2013NatCh...5..846M |s2cid=38839337 |issn=1755-4349 |access-date=29 July 2022 |archive-date=9 July 2023 |archive-url=https://web.archive.org/web/20230709182954/https://www.nature.com/articles/nchem.1754 |url-status=live }}</ref> This prediction needs to be validated by further experiments.<ref>{{cite journal |last1=Sneed |first1=D. |last2=Pravica |first2=M. |last3=Kim |first3=E. |last4=Chen |first4=N. |last5=Park |first5=C. |last6=White |first6=M. |date=1 October 2017 |title=Forcing Cesium into Higher Oxidation States Using Useful hard x-ray Induced Chemistry under High Pressure |journal=Journal of Physics: Conference Series |language=ENGLISH |volume=950 |issue=11, 2017 |page=042055 |doi=10.1088/1742-6596/950/4/042055 |bibcode=2017JPhCS.950d2055S |osti=1409108 |s2cid=102912809 |issn=1742-6588|doi-access=free }}</ref>

Salts of Cs<sup>+</sup> are usually colourless unless the anion itself is coloured. Many of the simple salts are [[hygroscopic]], but less so than the corresponding salts of lighter alkali metals. The [[phosphate]],<ref>Hogan, C. M. (2011).{{cite web |url=http://www.eoearth.org/article/Phosphate?topic=49557 |title=Phosphate |access-date=17 June 2012 |archive-url=https://web.archive.org/web/20121025180158/http://www.eoearth.org/article/Phosphate?topic=49557 |archive-date=25 October 2012}} in ''Encyclopedia of Earth''. Jorgensen, A. and Cleveland, C.J. (eds.). National Council for Science and the Environment. Washington DC</ref> [[acetate]], [[carbonate]], [[halide]]s, [[oxide]], [[nitrate]], and [[sulfate]] salts are water-soluble. Its [[double salt]]s are often less soluble, and the low solubility of caesium aluminium sulfate is exploited in refining Cs from ores. The double salts with antimony (such as {{chem|CsSbCl|4}}), [[bismuth]], [[cadmium]], [[copper]], [[iron]], and [[lead]] are also poorly [[dissolution (chemistry)|soluble]].<ref name="USGS"/>

[[Caesium hydroxide]] (CsOH) is [[hygroscopic]] and strongly [[base (chemistry)|basic]].<ref name="greenwood"/> It rapidly [[etching|etches]] the surface of [[semiconductor]]s such as [[silicon]].<ref>{{cite book |url=https://books.google.com/books?id=F-8SltAKSF8C&pg=PA90 |title=Etching in microsystem technology |author=Köhler, Michael J. |page=90 |publisher=Wiley-VCH |isbn=978-3-527-29561-6 |date=1999 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> CsOH has been previously regarded by chemists as the "strongest base", reflecting the relatively weak attraction between the large Cs<sup>+</sup> ion and OH<sup>−</sup>;<ref name="CRC74"/> it is indeed the strongest [[Arrhenius base]]; however, a number of compounds such as [[n-butyllithium|''n''-butyllithium]], [[sodium amide]], [[sodium hydride]], [[caesium hydride]], etc., which cannot be dissolved in water as reacting violently with it but rather only used in some [[anhydrous]] [[polar aprotic solvents]], are far more basic on the basis of the [[Brønsted–Lowry acid–base theory]].<ref name="greenwood"/>

A [[stoichiometry|stoichiometric]] mixture of caesium and gold will react to form yellow [[caesium auride]] (Cs<sup>+</sup>Au<sup>−</sup>) upon heating. The auride anion here behaves as a [[pseudohalogen]]. The compound reacts violently with water, yielding [[caesium hydroxide]], metallic gold, and hydrogen gas; in liquid ammonia it can be reacted with a caesium-specific ion exchange resin to produce [[tetramethylammonium auride]]. The analogous [[platinum]] compound, red caesium platinide ({{chem2|Cs2Pt}}), contains the platinide ion that behaves as a {{chem name|pseudo[[chalcogen]]}}.<ref>{{cite journal |title=Effects of relativistic motion of electrons on the chemistry of gold and platinum |journal=Solid State Sciences |date=30 November 2005 |volume=7 |issue=12 |pages=1464–1474 |doi=10.1016/j.solidstatesciences.2005.06.015 |last=Jansen |first=Martin |bibcode=2005SSSci...7.1464J |doi-access=free}}</ref>

====Complexes====
Like all metal cations, Cs<sup>+</sup> forms complexes with [[Lewis base]]s in solution. Because of its large size, Cs<sup>+</sup> usually adopts [[coordination number]]s greater than 6, the number typical for the smaller alkali metal cations. This difference is apparent in the 8-coordination of CsCl. This high coordination number and [[HSAB|softness]] (tendency to form covalent bonds) are properties exploited in separating Cs<sup>+</sup> from other cations in the remediation of nuclear wastes, where <sup>137</sup>Cs<sup>+</sup> must be separated from large amounts of nonradioactive K<sup>+</sup>.<ref>{{cite book |last1=Moyer |first1=Bruce A. |last2=Birdwell |first2=Joseph F. |last3=Bonnesen |first3=Peter V. |last4=Delmau |first4=Laetitia H. |journal=Macrocyclic Chemistry |pages=383–405 |date=2005 |doi=10.1007/1-4020-3687-6_24 |isbn=978-1-4020-3364-3 |title=Use of Macrocycles in Nuclear-Waste Cleanup: A Realworld Application of a Calixcrown in Cesium Separation Technology}}.</ref>

====Halides====
[[File:CsX@DWNT.jpg|thumb|upright|Monatomic caesium halide wires grown inside double-wall [[carbon nanotube]]s ([[transmission electron microscopy|TEM image]]).<ref name="chains">{{cite journal |doi=10.1038/ncomms8943 |pmid=26228378 |pmc=4532884 |title=Single-atom electron energy loss spectroscopy of light elements |journal=Nature Communications |volume=6 |pages=7943 |year=2015 |last1=Senga |first1=Ryosuke |last2=Suenaga |first2=Kazu |bibcode=2015NatCo...6.7943S}}</ref>]]
[[Caesium fluoride]] (CsF) is a [[hygroscopic]] white solid that is widely used in [[organofluorine chemistry]] as a source of [[fluoride]] anions.<ref>{{cite journal |author=Evans, F. W. |author2=Litt, M. H. |author3=Weidler-Kubanek, A. M. |author4=Avonda, F. P. |title=Reactions Catalyzed by Potassium Fluoride. 111. The Knoevenagel Reaction |date=1968 |journal=Journal of Organic Chemistry |volume=33 |pages=1837–1839 |doi=10.1021/jo01269a028 |issue=5}}</ref> Caesium fluoride has the halite structure, which means that the Cs<sup>+</sup> and F<sup>−</sup> pack in a [[cubic closest packed]] array as do Na<sup>+</sup> and Cl<sup>−</sup> in [[sodium chloride]].<ref name="greenwood"/> Notably, caesium and fluorine have the lowest and highest [[electronegativity|electronegativities]], respectively, among all the known elements.

[[Caesium chloride]] (CsCl) crystallizes in the simple [[cubic crystal system]]. Also called the "caesium chloride structure",<ref name="HollemanAF"/> this structural motif is composed of a [[primitive cell|primitive]] cubic lattice with a two-atom basis, each with an eightfold [[coordination number|coordination]]; the chloride atoms lie upon the lattice points at the edges of the cube, while the caesium atoms lie in the holes in the centre of the cubes. This structure is shared with [[caesium bromide|CsBr]] and [[caesium iodide|CsI]], and many other compounds that do not contain Cs. In contrast, most other alkaline halides have the [[sodium chloride]] (NaCl) structure.<ref name="HollemanAF"/> The CsCl structure is preferred because Cs<sup>+</sup> has an [[ionic radius]] of 174&nbsp;[[picometer|pm]] and {{chem|Cl|−}} 181&nbsp;pm.<ref>{{cite book |last=Wells |first=A. F. |date=1984 |title=Structural Inorganic Chemistry |edition=5th |publisher=Oxford Science Publications |isbn=978-0-19-855370-0}}</ref>

====Oxides====
[[File:Cs11O3 cluster.png|thumb|left|upright=0.7|{{chem|Cs|11|O|3}} cluster|alt=The stick and ball diagram shows three regular octahedra, which are connected to the next one by one surface and the last one shares one surface with the first. All three have one edge in common. All eleven vertices are purple spheres representing caesium, and at the center of each octahedron is a small red sphere representing oxygen.]]

More so than the other alkali metals, caesium forms numerous binary compounds with [[oxygen]]. When caesium burns in air, the [[superoxide]] {{chem|CsO|2}} is the main product.<ref name="cotton">{{cite book |last=Cotton |first=F. Albert |author2=Wilkinson, G. |title=Advanced Inorganic Chemistry |date=1962 |publisher=John Wiley & Sons, Inc. |page=318 |isbn=978-0-471-84997-1}}</ref> The "normal" [[caesium oxide]] ({{chem|Cs|2|O}}) forms yellow-orange [[hexagonal crystal system|hexagonal]] crystals,<ref name="CRC">{{RubberBible87th|pages=451, 514}}</ref> and is the only oxide of the anti-[[cadmium chloride|{{chem|CdCl|2}}]] type.<ref name="ReferenceA">{{cite journal |doi=10.1021/j150537a022 |date=1956 |last1=Tsai |first1=Khi-Ruey |last2=Harris |first2=P. M. |last3=Lassettre |first3=E. N. |journal=Journal of Physical Chemistry |volume=60 |pages=338–344 |title=The Crystal Structure of Cesium Monoxide |issue=3 |url=http://www.dtic.mil/get-tr-doc/pdf?AD=AD0026963 |url-status=dead |archive-url=https://web.archive.org/web/20170924131429/http://www.dtic.mil/get-tr-doc/pdf?AD=AD0026963 |archive-date=24 September 2017}}</ref> It vaporizes at {{convert|250|°C}}, and decomposes to caesium metal and the [[peroxide]] [[caesium peroxide|{{chem|Cs|2|O|2}}]] at temperatures above {{convert|400|°C}}. In addition to the superoxide and the [[ozonide]] [[caesium ozonide|{{chem|CsO|3}}]],<ref>{{cite journal |doi=10.1007/BF00845494 |title=Synthesis of cesium ozonide through cesium superoxide |date=1963 |last1=Vol'nov |first1=I. I. |last2=Matveev |first2=V. V. |journal=Bulletin of the Academy of Sciences, USSR Division of Chemical Science |volume=12 |pages=1040–1043 |issue=6}}</ref><ref>{{cite journal |doi=10.1070/RC1971v040n02ABEH001903 |title=Alkali and Alkaline Earth Metal Ozonides |date=1971 |last1=Tokareva |first1=S. A. |journal=Russian Chemical Reviews |volume=40 |pages=165–174 |bibcode=1971RuCRv..40..165T |issue=2 |s2cid=250883291}}</ref> several brightly coloured [[suboxide]]s have also been studied.<ref name="Simon">{{cite journal |last=Simon |first=A. |title=Group 1 and 2 Suboxides and Subnitrides — Metals with Atomic Size Holes and Tunnels |journal=Coordination Chemistry Reviews |date=1997 |volume=163 |pages=253–270 |doi=10.1016/S0010-8545(97)00013-1}}</ref> These include {{chem|Cs|7|O}}, {{chem|Cs|4|O}}, {{chem|Cs|11|O|3}}, {{chem|Cs|3|O}} (dark-green<ref>{{cite journal |doi=10.1021/j150537a023 |date=1956 |last1=Tsai |first1=Khi-Ruey |last2=Harris |first2=P. M. |last3=Lassettre |first3=E. N. |journal=Journal of Physical Chemistry |volume=60 |pages=345–347 |title=The Crystal Structure of Tricesium Monoxide |issue=3}}</ref>), CsO, {{chem|Cs|3|O|2}},<ref>{{cite journal |doi=10.1007/s11669-009-9636-5 |title=Cs-O (Cesium-Oxygen) |date=2009 |last1=Okamoto |first1=H. |journal=Journal of Phase Equilibria and Diffusion |volume=31 |pages=86–87 |s2cid=96084147}}</ref> as well as {{chem|Cs|7|O|2}}.<ref>{{cite journal |doi=10.1021/jp036432o |title=Characterization of Oxides of Cesium |date=2004 |last1=Band |first1=A. |last2=Albu-Yaron |first2=A. |last3=Livneh |first3=T. |last4=Cohen |first4=H. |last5=Feldman |first5=Y. |last6=Shimon |first6=L. |last7=Popovitz-Biro |first7=R. |last8=Lyahovitskaya |first8=V. |last9=Tenne |first9=R. |journal=The Journal of Physical Chemistry B |volume=108 |pages=12360–12367 |issue=33}}</ref><ref>{{cite journal |doi=10.1002/zaac.19472550110 |title=Untersuchungen ber das System Csium-Sauerstoff |date=1947 |last1=Brauer |first1=G. |journal=Zeitschrift für Anorganische Chemie |volume=255 |issue=1–3 |pages=101–124}}</ref> The latter may be heated in a vacuum to generate {{chem|Cs|2|O}}.<ref name="ReferenceA"/> Binary compounds with [[sulfur]], [[selenium]], and [[tellurium]] also exist.<ref name="USGS"/>

===Isotopes===
{{Main|Isotopes of caesium}}
Caesium has 41 known [[isotope]]s, ranging in [[mass number]] (i.e. number of [[nucleon]]s in the nucleus) from 112 to 152. Several of these are synthesized from lighter elements by the slow neutron capture process ([[S-process]]) inside old stars<ref>{{cite journal |doi=10.1146/annurev.astro.37.1.239 |author=Busso, M. |author2=Gallino, R. |author3=Wasserburg, G. J. |title=Nucleosynthesis in Asymptotic Giant Branch Stars: Relevance for Galactic Enrichment and Solar System Formation |journal=Annual Review of Astronomy and Astrophysics |volume=37 |date=1999 |pages=239–309 |url=http://authors.library.caltech.edu/1194/1/BUSaraa99.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://authors.library.caltech.edu/1194/1/BUSaraa99.pdf |archive-date=10 October 2022 |url-status=live |access-date=20 February 2010 |bibcode=1999ARA&A..37..239B}}</ref> and by the [[R-process]] in [[supernova]] explosions.<ref>{{cite book |first=David |last=Arnett |date=1996 |title=Supernovae and Nucleosynthesis: An Investigation of the History of Matter, from the Big Bang to the Present |publisher=Princeton University Press |page=527 |isbn=978-0-691-01147-9}}</ref> The only [[stable nuclide|stable]] caesium isotope is <sup>133</sup>Cs, with 78 [[neutron]]s. Although it has a large [[nuclear spin]] ({{sfrac|7|2}}+), [[nuclear magnetic resonance]] studies can use this isotope.<ref name="NMR">{{cite journal |doi=10.1016/0277-5387(96)00018-6 |title=Complexation of caesium and rubidium cations with crown ethers in N,N-dimethylformamide |date=1996 |last1=Goff |first1=C. |journal=Polyhedron |volume=15 |pages=3897–3903 |last2=Matchette |first2=Michael A. |last3=Shabestary |first3=Nahid |last4=Khazaeli |first4=Sadegh |issue=21}}</ref>
[[File:Cs-137-decay.svg|thumb|Decay of caesium-137|alt=A graph showing the energetics of caesium-137 (nuclear spin: I={{sfrac|7|2}}+, half-life of about 30 years) decay. With a 94.6% probability, it decays by a 512&nbsp;keV beta emission into barium-137m (I=11/2-, t=2.55min); this further decays by a 662&nbsp;keV gamma emission with an 85.1% probability into barium-137 (I={{sfrac|3|2}}+). Alternatively, caesium-137 may decay directly into barium-137 by a 0.4% probability beta emission.]]
The radioactive [[caesium-135|<sup>135</sup>Cs]] has a very long half-life of about 2.3&nbsp;million years, the longest of all radioactive isotopes of caesium. [[caesium-137|<sup>137</sup>Cs]] and [[caesium-134|<sup>134</sup>Cs]] have half-lives of 30 and two years, respectively. <sup>137</sup>Cs decomposes to a short-lived [[barium-137m|<sup>137m</sup>Ba]] by [[beta decay]], and then to nonradioactive barium, while <sup>134</sup>Cs transforms into <sup>134</sup>Ba directly. The isotopes with mass numbers of 129, 131, 132 and 136, have half-lives between a day and two weeks, while most of the other isotopes have half-lives from a few seconds to fractions of a second. At least 21 metastable [[nuclear isomer]]s exist. Other than <sup>134m</sup>Cs (with a half-life of just under 3&nbsp;hours), all are very unstable and decay with half-lives of a few minutes or less.<ref>{{cite journal |doi=10.1016/0022-1902(55)80027-9 |title=The half-life of Cs137 |date=1955 |last1=Brown |first1=F. |last2=Hall |first2=G. R. |last3=Walter |first3=A. J. |journal=Journal of Inorganic and Nuclear Chemistry |volume=1 |pages=241–247 |issue=4–5 |bibcode=1955PhRv...99..188W}}</ref><ref name="nuclidetable">{{cite web |url=http://www.nndc.bnl.gov/chart/ |title=Interactive Chart of Nuclides |publisher=Brookhaven National Laboratory |author=Sonzogni, Alejandro |location=National Nuclear Data Center |access-date=6 June 2008 |archive-date=22 May 2008 |archive-url=https://web.archive.org/web/20080522125027/http://www.nndc.bnl.gov/chart |url-status=dead}}</ref>

The isotope <sup>135</sup>Cs is one of the [[long-lived fission product]]s of [[uranium]] produced in [[nuclear reactor technology|nuclear reactors]].<ref>{{cite conference |conference=Seventh Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation |date=14–16 October 2002 |place=Jeju, Korea |first1=Shigeo |last1=Ohki |first2=Naoyuki |last2=Takaki |title=Transmutation of Cesium-135 with Fast Reactors |url=http://www.oecd-nea.org/pt/docs/iem/jeju02/session6/SessionVI-08.pdf |access-date=26 September 2010 |archive-date=28 September 2011 |archive-url=https://web.archive.org/web/20110928005357/http://www.oecd-nea.org/pt/docs/iem/jeju02/session6/SessionVI-08.pdf |url-status=dead}}</ref> However, this [[fission product yield]] is reduced in most reactors because the predecessor, [[xenon-135|<sup>135</sup>Xe]], is a potent [[neutron poison]] and frequently transmutes to stable [[xenon-136|<sup>136</sup>Xe]] before it can decay to <sup>135</sup>Cs.<ref>{{cite report |chapter-url=http://canteach.candu.org/library/20040720.pdf |title=CANDU Fundamentals |publisher=[[CANDU Owners Group]] Inc. |chapter=20 Xenon: A Fission Product Poison |access-date=15 September 2010 |url-status=dead |archive-url=https://web.archive.org/web/20110723231319/http://canteach.candu.org/library/20040720.pdf |archive-date=23 July 2011}}</ref><ref>{{cite journal |journal=Journal of Environmental Radioactivity |title=Preliminary evaluation of <sup>135</sup>Cs/<sup>137</sup>Cs as a forensic tool for identifying source of radioactive contamination |first1=V. F. |last1=Taylor |first2=R. D. |last2=Evans |first3=R. J. |last3=Cornett |doi=10.1016/j.jenvrad.2007.07.006 |volume=99 |issue=1 |date=2008 |pages=109–118 |pmid=17869392}}</ref>

The [[beta decay]] from <sup>137</sup>Cs to <sup>137m</sup>Ba results in [[gamma ray|gamma radiation]] as the <sup>137m</sup>Ba relaxes to ground state <sup>137</sup>Ba, with the emitted photons having an energy of 0.6617 MeV.<ref>{{cite web |url=http://www.epa.gov/rpdweb00/radionuclides/cesium.html |title=Cesium {{pipe}} Radiation Protection |publisher=U.S. Environmental Protection Agency |date=28 June 2006 |access-date=15 February 2010 |url-status=dead |archive-url=https://web.archive.org/web/20110315034747/http://www.epa.gov/rpdweb00/radionuclides/cesium.html |archive-date=15 March 2011}}</ref> <sup>137</sup>Cs and [[strontium-90|<sup>90</sup>Sr]] are the principal [[medium-lived fission product|medium-lived]] products of [[nuclear fission]], and the prime sources of [[radioactive decay|radioactivity]] from [[spent nuclear fuel]] after several years of cooling, lasting several hundred years.<ref>{{cite report |url=http://www.ieer.org/reports/transm/hisham.html |title=IEER Report: Transmutation – Nuclear Alchemy Gamble |publisher=Institute for Energy and Environmental Research |date=24 May 2000 |access-date=15 February 2010 |first=Hisham |last=Zerriffi |archive-date=30 May 2011 |archive-url=https://web.archive.org/web/20110530074834/http://www.ieer.org/reports/transm/hisham.html |url-status=live }}</ref> Those two isotopes are the largest source of residual radioactivity in the area of the [[Chernobyl disaster]].<ref>{{cite report |url=http://www.iaea.org/Publications/Booklets/Chernobyl/chernobyl.pdf |title=Chernobyl's Legacy: Health, Environmental and Socia-Economic Impacts and Recommendations to the Governments of Belarus, Russian Federation and Ukraine |publisher=International Atomic Energy Agency |access-date=18 February 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100215212227/http://www.iaea.org/Publications/Booklets/Chernobyl/chernobyl.pdf |archive-date=15 February 2010}}</ref> Because of the low capture rate, disposing of <sup>137</sup>Cs through [[neutron capture]] is not feasible and the only current solution is to allow it to decay over time.<ref>{{cite journal |doi=10.3327/jnst.30.911 |title=Transmutation of Cesium-137 Using Proton Accelerator |first1=Takeshi |last1=Kase |first2=Kenji |last2=Konashi |first3=Hiroshi |last3=Takahashi |first4=Yasuo |last4=Hirao |volume=30 |issue=9 |date=1993 |pages=911–918 |journal=Journal of Nuclear Science and Technology |doi-access=free}}</ref>

Almost all caesium produced from nuclear fission comes from the [[beta decay]] of originally more neutron-rich fission products, passing through various [[isotopes of iodine]] and [[isotopes of xenon|xenon]].<ref>{{cite book |isbn=978-1-56032-088-3 |publisher=Taylor & Francis |date=1992 |first=Ronald Allen |last=Knief |chapter-url=https://books.google.com/books?id=EpuaUEQaeoUC&pg=PA43 |page=42 |chapter=Fission Fragments |title=Nuclear engineering: theory and technology of commercial nuclear power |access-date=8 May 2021 |archive-date=5 March 2024 |archive-url=https://web.archive.org/web/20240305132927/https://books.google.com/books?id=EpuaUEQaeoUC&pg=PA43#v=onepage&q&f=false |url-status=live }}</ref> Because iodine and xenon are volatile and can diffuse through nuclear fuel or air, radioactive caesium is often created far from the original site of fission.<ref>{{cite journal |title=Release of xenon-137 and iodine-137 from UO2 pellet by pulse neutron irradiation at NSRR |last1=Ishiwatari |first1=N. |last2=Nagai |first2=H. |pages=843–850 |volume=23 |issue=11 |journal=Nippon Genshiryoku Gakkaishi |osti=5714707}}</ref> With [[nuclear weapons testing]] in the 1950s through the 1980s, <sup>137</sup>Cs was released into the [[atmosphere of Earth|atmosphere]] and returned to the surface of the earth as a component of [[nuclear fallout|radioactive fallout]]. It is a ready marker of the movement of soil and sediment from those times.<ref name="USGS"/>

===Occurrence===
[[File:Pollucite-RoyalOntarioMuseum-Jan18-09.jpg|thumb|Pollucite, a caesium mineral|alt=A white mineral, from which white and pale pink crystals protrude]]
{{See also|:Category:Caesium minerals|l1=Caesium minerals}}
Caesium is a relatively rare element, estimated to average 3&nbsp;[[parts per million]] in the [[abundance of elements in Earth's crust|Earth's crust]].<ref>{{cite journal |last1=Turekian |first1=K. K. |last2=Wedepohl |first2=K. H. |title=Distribution of the elements in some major units of the Earth's crust |journal=Geological Society of America Bulletin |volume=72 |issue=2 |pages=175–192 |doi=10.1130/0016-7606(1961)72[175:DOTEIS]2.0.CO;2 |issn=0016-7606 |bibcode=1961GSAB...72..175T |year=1961 |doi-access=free}}</ref> It is the 45th most abundant element and 36th among the metals.<ref>{{Cite book |last1=Kloprogge |first1=J. Theo |url=https://books.google.com/books?id=hGa8DwAAQBAJ&dq=%2245th+most+abundant+element%22&pg=PA634 |title=The Periodic Table: Nature's Building Blocks: An Introduction to the Naturally Occurring Elements, Their Origins and Their Uses |last2=Ponce |first2=Concepcion P. |last3=Loomis |first3=Tom |date=2020-11-18 |publisher=Elsevier |isbn=978-0-12-821538-8 |language=en |access-date=16 May 2024 |archive-date=16 May 2024 |archive-url=https://web.archive.org/web/20240516231733/https://books.google.com/books?id=hGa8DwAAQBAJ&pg=PA634&dq=%2245th+most+abundant+element%22&hl=en&newbks=1&newbks_redir=0&source=gb_mobile_search&ovdme=1&sa=X&ved=2ahUKEwjc2fS_ppOGAxVP5MkDHTj0CTIQ6AF6BAgHEAM#v=onepage&q=%2245th%20most%20abundant%20element%22&f=false |url-status=live }}</ref>  Caesium is 30 times less abundant than [[rubidium]], with which it is closely associated, chemically.<ref name="USGS"/>

Due to its large [[ionic radius]], caesium is one of the "[[incompatible element]]s".<ref>{{cite web |url=http://www.asi.org/adb/02/13/02/cesium-occurrence-uses.html |title=Cesium as a Raw Material: Occurrence and Uses |first=Simon |last=Rowland |publisher=Artemis Society International |date=4 July 1998 |access-date=15 February 2010 |archive-date=8 July 2021 |archive-url=https://web.archive.org/web/20210708104437/http://www.asi.org/adb/02/13/02/cesium-occurrence-uses.html |url-status=dead }}</ref> During [[fractional crystallization (geology)|magma crystallization]], caesium is concentrated in the liquid phase and crystallizes last. Therefore, the largest deposits of caesium are zone [[pegmatite]] ore bodies formed by this enrichment process. Because caesium does not substitute for [[potassium]] as readily as rubidium does, the alkali evaporite minerals [[sylvite]] (KCl) and [[carnallite]] ({{chem|KMgCl|3|·6H|2|O}}) may contain only 0.002% caesium. Consequently, caesium is found in few minerals. Percentage amounts of caesium may be found in [[beryl]] ({{chem|Be|3|Al|2|(SiO|3|)|6}}) and [[avogadrite]] ({{chem|(K,Cs)BF|4}}), up to 15&nbsp;wt% Cs<sub>2</sub>O in the closely related mineral [[pezzottaite]] ({{chem|Cs|(Be|2|Li)|Al|2|Si|6|O|18}}), up to 8.4&nbsp;wt% Cs<sub>2</sub>O in the rare mineral [[londonite]] ({{chem|(Cs,K)Al|4|Be|4|(B,Be)|12|O|28}}), and less in the more widespread [[rhodizite]].<ref name="USGS"/> The only economically important ore for caesium is [[pollucite]] {{chem|Cs(AlSi|2|O|6|)}}, which is found in a few places around the world in zoned pegmatites, associated with the more commercially important [[lithium]] minerals, [[lepidolite]] and [[petalite]]. Within the pegmatites, the large grain size and the strong separation of the minerals results in high-grade ore for mining.<ref name="Cerny">{{cite journal |title=The Tanco Pegmatite at Bernic Lake, Manitoba: X. Pollucite |first1=Petr |last1=Černý |author-link1=Petr Černý |first2=F. M. |last2=Simpson |journal=Canadian Mineralogist |volume=16 |pages=325–333 |date=1978 |url=http://rruff.geo.arizona.edu/doclib/cm/vol38/CM38_877.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://rruff.geo.arizona.edu/doclib/cm/vol38/CM38_877.pdf |archive-date=10 October 2022 |url-status=live |access-date=26 September 2010}}</ref>

The world's most significant and richest known source of caesium is the [[Tanco Mine]] at [[Bernic Lake]] in [[Manitoba]], Canada, estimated to contain 350,000&nbsp;[[tonne|metric tons]] of pollucite ore, representing more than two-thirds of the world's reserve base.<ref name="Cerny"/><ref name="USGS-Cs2"/> Although the stoichiometric content of caesium in pollucite is 42.6%, pure pollucite samples from this deposit contain only about 34% caesium, while the average content is 24&nbsp;wt%.<ref name="USGS-Cs2">{{cite web |title=Cesium |last=Polyak |first=Désirée E. |url=http://minerals.usgs.gov/minerals/pubs/commodity/cesium/mcs-2009-cesiu.pdf |archive-url=https://web.archive.org/web/20090508233834/http://minerals.usgs.gov/minerals/pubs/commodity/cesium/mcs-2009-cesiu.pdf |archive-date=8 May 2009 |url-status=live |publisher=U.S. Geological Survey |access-date=17 October 2009}}</ref> Commercial pollucite contains more than 19% caesium.<ref>{{cite book |last=Norton |first=J. J. |date=1973 |chapter=Lithium, cesium, and rubidium—The rare alkali metals |editor=Brobst, D. A. |editor2=Pratt, W. P. |title=United States mineral resources |publisher=U.S. Geological Survey Professional |volume=Paper 820 |pages=365–378 |chapter-url=https://pubs.er.usgs.gov/usgspubs/pp/pp820 |access-date=26 September 2010 |archive-date=21 July 2010 |archive-url=https://web.archive.org/web/20100721060544/http://pubs.er.usgs.gov/usgspubs/pp/pp820 |url-status=dead}}</ref> The [[Bikita District|Bikita]] pegmatite deposit in [[Zimbabwe]] is mined for its petalite, but it also contains a significant amount of pollucite. Another notable source of pollucite is in the [[Erongo Region|Karibib Desert]], [[Namibia]].<ref name="USGS-Cs2"/> At the present rate of world mine production of 5 to 10&nbsp;metric tons per year, reserves will last for thousands of years.<ref name="USGS"/>