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Caesium is an [[alkali]] [[metal]]. Its [[melt]]ing point is low (28 °C). It is extremely [[Reaction|react]]ive. Because of its high reactivity, it is a [[danger]]ous chemical. It may set itself on fire (ignite) in air. It [[Explosion|explode]]s on contact with water. It reacts more violently than the other [[alkali metal]]s with water. Because of this, caesium is stored in [[mineral oil]].<ref name=USGS>William C. Butterman ''et al'' 2004. {{cite web|url = http://pubs.usgs.gov/of/2004/1432/2004-1432.pdf|publisher = United States Geological Survey|accessdate = 2009-12-27|title = Mineral Commodity Profile: Cesium}}</ref>
Caesium is an [[alkali]] [[metal]]. Its [[melt]]ing point is low (28 °C). It is extremely [[Reaction|react]]ive. Because of its high reactivity, it is a [[danger]]ous chemical. It may set itself on fire (ignite) in air. It [[Explosion|explode]]s on contact with water. It reacts more violently than the other [[alkali metal]]s with water. Because of this, caesium is stored in [[mineral oil]].<ref name=USGS>William C. Butterman ''et al'' 2004. {{cite web|url = http://pubs.usgs.gov/of/2004/1432/2004-1432.pdf|publisher = United States Geological Survey|accessdate = 2009-12-27|title = Mineral Commodity Profile: Cesium}}</ref>
Caesium is a rare element. Since there is little caesium on the [[Earth]], it is rather expensive. The human body does not need caesium. In large amounts, its [[chemical compound]]<nowiki/>s are mildly poisonous because it is close to [[potassium]], which the body does need.
Caesium is a rare element. Since there is little caesium on the [[Earth]], it is rather expensive. The human body does not need caesium. In large amounts, its [[chemical compound]]s are mildly poisonous because it is close to [[potassium]], which the body does need.
== History ==
== History ==
Caesium was first described in 1860, by [[Gustav Robert Kirchhoff]] and [[Robert Wilhelm Bunsen]]. They were testing [[mineral water]], from [[Bad Dürkheim]] in [[Rhineland-Palatinate]]. After they separated [[calcium]], [[strontium]], [[magnesium]] and [[lithium]], they saw two lines in the "blue" range of the [[spectrum]]. Because of these lines, they concluded that in addition to the elements already found, there must be another unknown substance in the mineral water. They named this substance caesium, after the color [[blue]].<ref name="bunsen">G. Kirchhoff, R. Bunsen: ''Chemische Analyse durch Spectralbeobachtungen''. In: ''Annalen der Physik und Chemie''. 1861, 189, 7, S. 337–381 ({{doi|10.1002/andp.18611890702}}).</ref>
Caesium was first described in 1860, by [[Gustav Robert Kirchhoff]] and [[Robert Wilhelm Bunsen]]. They were testing [[mineral water]], from [[Bad Dürkheim]] in [[Rhineland-Palatinate]]. After they separated [[calcium]], [[strontium]], [[magnesium]] and [[lithium]], they saw two lines in the "blue" range of the [[spectrum]]. Because of these lines, they concluded that in addition to the elements already found, there must be another unknown substance in the mineral water. They named this substance caesium, after the color [[blue]].<ref name="bunsen">G. Kirchhoff, R. Bunsen: ''Chemische Analyse durch Spectralbeobachtungen''. In: ''Annalen der Physik und Chemie''. 1861, 189, 7, S. 337–381 ({{doi|10.1002/andp.18611890702}}).</ref>
== Caesium Media ==
== Caesium Media ==
<gallery widths='160px' heights='100%' mode='traditional' caption=''>
<gallery widths='160px' heights='100%' mode='traditional' caption=''>
File:CsCrystals.JPG|High-purity caesium stored in [[argon]].
File:CsCrystals.JPG|High-purity caesium stored in [[argon]]
File:Rb&Cs crystals.jpg|Caesium crystals (golden) compared to [[rubidium]] crystals (silvery)
File:Rb&Cs crystals.jpg|Caesium crystals (golden) compared to [[rubidium]] crystals (silvery)
File:Cesium water.theora.ogv|Addition of a small amount of caesium to cold water is explosive.
File:Cesium water.theora.webm|Addition of a small amount of caesium to cold water is explosive.
File:CsX@DWNT.jpg|Monatomic caesium halide wires grown inside double-wall [[carbon nanotube]]s ([[transmission electron microscopy|TEM image]]).
File:CsX@DWNT.jpg|Monatomic caesium halide wires grown inside double-wall [[carbon nanotube]]s ([[transmission electron microscopy|TEM image]]).
File:Cs-137-decay.svg|Cs-137 decay diagram
File:Cs-137-decay.svg|Cs-137 decay diagram
File:Pollucite-RoyalOntarioMuseum-Jan18-09.jpg|Pollucite, a caesium mineral
File:Pollucite-RoyalOntarioMuseum-Jan18-09.jpg|[[Pollucite]], a caesium mineral
File:Usno-mc.jpg|Atomic clock ensemble at the U.S. Naval Observatory
File:Usno-mc.jpg|Atomic clock ensemble at the U.S. Naval Observatory