Editing Caesium (section)

===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"/>