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Chemical element, symbol Cs and atomic number 55 / From Wikipedia, the free encyclopedia

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Caesium (IUPAC spelling;[8] cesium in American English)[note 1] is a chemical element; it has symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature.[note 2] Caesium has physical and chemical properties similar to those of rubidium and potassium. It is pyrophoric and reacts with water even at −116 °C (−177 °F). It is the least electronegative element, with a value of 0.79 on the Pauling scale. It has only one stable isotope, caesium-133. Caesium is mined mostly from pollucite. Caesium-137, a fission product, is extracted from waste produced by nuclear reactors. It has the largest atomic radius of all elements whose radii have been measured or calculated, at about 260 picometers.

Quick facts: Caesium, Pronunciation, Alternative name, App...
Caesium, 55Cs
Some pale gold metal, with a liquid-like texture and lustre, sealed in a glass ampoule
Pronunciation/ˈsziəm/ (SEE-zee-əm)
Alternative namecesium (US)
Appearancepale gold
Standard atomic weight Ar°(Cs)
Caesium in the periodic table


Atomic number (Z)55
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[3]
Heat of fusion2.09 kJ/mol
Heat of vaporization63.9 kJ/mol
Molar heat capacity32.210 J/(mol·K)
Vapour 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[4] (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 structure body-centred 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[5]
Young's modulus1.7 GPa
Bulk modulus1.6 GPa
Mohs hardness0.2
Brinell hardness0.14 MPa
CAS Number7440-46-2
Namingfrom Latin caesius 'bluish grey', for its spectral colours
DiscoveryRobert Bunsen and Gustav Kirchhoff (1860)
First isolationCarl Setterberg (1882)
Isotopes of caesium
Main isotopes[6] 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 1.33×106 y β 135Ba
137Cs synth 30.17 y[7] β 137Ba
Symbol_category_class.svg Category: Caesium
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The German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the newly developed method of flame spectroscopy. The first small-scale applications for caesium were as a "getter" in vacuum tubes and in photoelectric cells. In 1967, acting on Einstein's proof that the speed of light is the most-constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre. Since then, caesium has been widely used in highly accurate atomic clocks.

Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids, but it has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Nonradioactive caesium compounds are only mildly toxic, but the pure metal's tendency to react explosively with water means that caesium is considered a hazardous material, and the radioisotopes present a significant health and environmental hazard.

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