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Caesium or cesium[note 1] is a chemical element wi seembol Cs an atomic nummer 55. It is a saft, sillery-gowd alkali metal wi a meltin pynt o 28 °C (82 °F), which maks it ane o anly five elemental metals that are liquid at or near ruim temperatur.[note 2] Caesium is an alkali metal an haes pheesical an chemical properties seemilar tae those o rubidium an potassium. The metal is extremely reactive an pyrophoric, reactin wi watter even at −116 °C (−177 °F). It is the least electronegative element havin a stable isotope, caesium-133. Caesium is mined maistly frae pollucite, while the radioisotopes, especially caesium-137, a fission product, are extractit frae waste produced bi nuclear reactors.

Caesium,  55Cs
Some pale gowd metal, wi a liquid-lik texture an lustre, sealed in a gless ampoule
General properties
Appearance sillery gowd
Staundart atomic wecht (Ar, staundart) 132.90545196(6)[1]
Caesium in the periodic cairt
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 Airn Cobalt Nickel Capper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Siller (element) 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 Gowd Mercur (element) Thallium Leid (element) 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 Ununtrium Flerovium Ununpentium Livermorium Ununseptium Ununoctium


Atomic nummer (Z) 55
Group group 1 (alkali metals)
Period period 6
Element category   alkali metal
Block s-block
Electron confeeguration [Xe] 6s1
Electrons per shell
2, 8, 18, 18, 8, 1
Pheesical properties
Phase (at STP) solit
Meltin pynt 301.59 K ​(28.44 °C, ​83.19 °F)
Bylin pynt 944 K ​(671 °C, ​1240 °F)
Density (near r.t.) 1.93 g/cm3
when liquid (at m.p.) 1.843 g/cm3
Creetical pynt 1938 K, 9.4 MPa
Heat o fusion 2.09 kJ/mol
Heat o vapourisation 63.9 kJ/mol
Molar heat capacity 32.210 J/(mol·K)
Vapour pressur
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
(strangly basic oxide)
Electronegativity Pauling scale: 0.79
Atomic radius empirical: 265 pm
Covalent radius 244±11 pm
Van der Waals radius 343 pm
Coloyr lines in a spectral range
Spectral lines o Caesium
Creestal structurbody-centred cubic (bcc)
Body-centered cubic creestal structur for caesium
Thermal expansion 97 µm/(m·K) (at 25 °C)
Thermal conductivity 35.9 W/(m·K)
Electrical resistivity 205 n Ω·m (at 20 °C)
Magnetic orderin paramagnetic[2]
Young's modulus 1.7 GPa
Bouk modulus 1.6 GPa
Mohs haurdness 0.2
Brinell haurdness 0.14 MPa
CAS Nummer 7440-46-2
Diskivery Robert Bunsen an Gustav Kirchhoff (1860)
First isolation Carl Setterberg (1882)
Main isotopes o caesium
Iso­tope Abun­dance Hauf-life (t1/2) Decay mode Pro­duct
133Cs 100% stable
134Cs syn 2.0648 y ε 134Xe
β 134Ba
135Cs trace 2.3×106 y β 135Ba
137Cs trace 30.17 y[3] β 137Ba
| references | in Wikidata

Twa German chemists, Robert Bunsen an Gustav Kirchhoff, discovered caesium in 1860 bi the newly developed method o flame spectroscopy. The first smaa-scale applications for caesium wur as a "getter" in vacuum tubes an in photoelectric cells. In 1967, a specific frequency frae the emission spectrum o caesium-133 wis chosen tae be uised in the definition o the seicont bi the Internaitional Seestem o Units. Syne then, caesium haes been widely uised in atomic clocks.

Syne the 1990s, the lairgest application o the element haes been as caesium formate for drilling fluids. It haes a range o applications in the production o electricity, in electronics, an in chemistry. The radioactive isotope caesium-137 haes a hauf-life o aboot 30 years an is uised in medical applications, industrial gauges, an hydrology. Altho the element is anly mildly toxic, it is a hazardous material as a metal an its radioisotopes present a heich heal risk if released intae the environment.


  1. Caesium is the spellin recommendit bi the Internaitional Union o Pure an Applee'd Chemistry (IUPAC).[4] The American Chemical Society (ACS) haes uised the spellin cesium syne 1921,[5][6] follaein Webster's New International Dictionary. The element wis named efter the Latin word cæsius, meanin "bluish gray".[7] Hence, an alternative orthography is cæsium. Mair spellin explanation at ae/oe vs e.
  2. Alang wi rubidium (39 °C [102 °F]), francium (estimatit at 27 °C [81 °F]), mercur (−39 °C [−38 °F]), an gallium (30 °C [86 °F]); bromine is an aa liquid at ruim temperatur (meltin at −7.2 °C, 19 °F) but it is a halogen, nae a metal.


  1. Meija, J.; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91. doi:10.1515/pac-2015-0305. 
  2. "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. 
  3. "NIST Radionuclide Half-Life Measurements". NIST. Retrieved 2011-03-13. 
  4. Internaitional Union o Pure an Applee'd Chemistry (2005). Nomenclature o Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSCIUPAC. ISBN 0-85404-438-8. pp. 248–49. Electronic version..
  5. Coghill, Anne M.; Garson, Lorrin R., eds. (2006). The ACS Style Guide: Effective Communication of Scientific Information (3rd ed.). Washington, D.C.: American Chemical Society. p. 127. ISBN 0-8412-3999-1. 
  6. Coplen, T. B.; Peiser, H. S. (1998). "History of the recommended atomic-weight values from 1882 to 1997: a comparison of differences from current values to the estimated uncertainties of earlier values" (PDF). Pure Appl. Chem. 70 (1): 237–257. doi:10.1351/pac199870010237. 
  7. OED entry for "caesium". Second edition, 1989; online version June 2012; accessed 07 September 2012. Earlier version first published in New English Dictionary, 1888.

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