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Caesium, 00Cs
Some pale gold metal, with a liquid-like texture and lustre, sealed in a glass ampoule
Pronunciation/ˈsziəm/ (SEE-zee-əm)
Alternative namecesium (US, informal)
Appearancepale gold
Standard atomic weight Ar°(Cs)
Caesium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


Groupgroup 1: hydrogen and alkali metals
Periodperiod 6
Block  s-block
Electron configuration[Xe] 6s1
Electrons per shell2, 8, 18, 18, 8, 1
Physical properties
Phase at STPsolid
Melting point301.7 K ​(28.5 °C, ​83.3 °F)
Boiling point944 K ​(671 °C, ​1240 °F)
Density (near r.t.)1.93 g/cm3
when liquid (at m.p.)1.843 g/cm3
Critical point1938 K, 9.4 MPa[2]
Heat of fusion2.09 kJ/mol
Heat of vaporization63.9 kJ/mol
Molar heat capacity32.210 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 418 469 534 623 750 940
Atomic properties
Oxidation states−1, +1[3] (a strongly basic oxide)
ElectronegativityPauling scale: 0.79
Ionization energies
  • 1st: 375.7 kJ/mol
  • 2nd: 2234.3 kJ/mol
  • 3rd: 3400 kJ/mol
Atomic radiusempirical: 265 pm
Covalent radius244±11 pm
Van der Waals radius343 pm
Color lines in a spectral range
Spectral lines of caesium
Other properties
Natural occurrenceprimordial
Crystal structurebody-centered cubic (bcc)
Bodycentredcubic crystal structure for caesium
Thermal expansion97 µm/(m⋅K) (at 25 °C)
Thermal conductivity35.9 W/(m⋅K)
Electrical resistivity205 nΩ⋅m (at 20 °C)
Magnetic orderingparamagnetic[4]
Young's modulus1.7 GPa
Bulk modulus1.6 GPa
Mohs hardness0.2
Brinell hardness0.14 MPa
CAS Number7440-46-2
Namingfrom Latin caesius, sky blue, for its spectral colours
DiscoveryRobert Bunsen and Gustav Kirchhoff (1860)
First isolationCarl Setterberg (1882)
Isotopes of caesium
Main isotopes[5] Decay
abun­dance half-life (t1/2) mode pro­duct
131Cs synth 9.7 d ε 131Xe
133Cs 100% stable
134Cs synth 2.0648 y ε 134Xe
β 134Ba
135Cs trace 2.3×106 y β 135Ba
137Cs synth 30.17 y[6] β 137Ba
 Category: Caesium
| references

Caesium (or cesium) is the chemical element with the atomic number 55 on the periodic table. Its symbol is Cs.

Caesium is an alkali metal. Its melting point is low (28 °C). It is extremely reactive. Because of its high reactivity, it is a dangerous chemical. It may set itself on fire (ignite) in air. It explodes on contact with water. It reacts more violently than the other alkali metals with water. Because of this, caesium is stored in mineral oil.[7]

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 compounds are mildly poisonous because it is close to potassium, which the body does need.

History[change | change source]

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.[8]

Isotopes and compounds[change | change source]

Caesium has at least 39 known isotopes ranging in atomic mass from 112 to 151. Only one of these, 133Cs, is stable. Therefore, the naturally-occurring isotope of caesium is 133Cs, which is not radioactive. 133Cs is used in atomic clocks, its vibration frequency used to define the length of the second. Another isotope, 137Cs is not made naturally but is made after nuclear fission has been done. It is very radioactive and used as an industrial gamma ray source.

Caesium forms compounds with many other chemical elements. Caesium formate is used in oil drilling because of its high density.

Reactivity[change | change source]

Caesium is extremely reactive in air and water. Caesium rapidly oxidizes in air and can spontaneously combust[9] (randomly catch on fire) at any moment. For this reason, it must be stored in kerosene or a mineral oil, like other group one elements (lithium, sodium, rubidium, and francium.) In water, caesium violently reacts to make caesium hydroxide (2CsOH).[10] The Caesium sinks for about one second, then explodes. The explosion is over 50 times the size of the element dropped in the water, and the explosion is enough to break a common Pyrex beaker, flask, or test tube.

References[change | change source]

  1. "Standard Atomic Weights: Caesium". CIAAW. 2013.
  2. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. p. 4.121. ISBN 1439855110.
  3. 3.0 3.1 Dye, J. L. (1979). "Compounds of Alkali Metal Anions". Angewandte Chemie International Edition. 18 (8): 587–598. doi:10.1002/anie.197905871.
  4. "Magnetic susceptibility of the elements and inorganic compounds". Handbook of Chemistry and Physics (PDF) (87th ed.). CRC press. ISBN 0-8493-0487-3. Retrieved 2010-09-26.
  5. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  6. 6.0 6.1 "NIST Radionuclide Half-Life Measurements". NIST. Retrieved 2011-03-13. Cite error: Invalid <ref> tag; name "isotope" defined multiple times with different content
  7. William C. Butterman et al 2004. "Mineral Commodity Profile: Cesium" (PDF). United States Geological Survey. Retrieved 2009-12-27.
  8. 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).
  9. Interest, Compound (2019-08-06). "IYPT 2019 Elements 055: Caesium: Atomic clocks and explosive reactions". Compound Interest. Retrieved 2019-11-13.
  10. "WebElements Periodic Table » Caesium » reactions of elements". www.webelements.com. Retrieved 2019-11-13.

Other websites[change | change source]