55CsCaesium

Caesium (IUPAC spelling; cesium in American English) is a chemical element with the 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. 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.

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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Atomic properties

Standard atomic weight132.905 ±5.99999·10-8
Atomic mass132.905 u

Atomic radii

Radius (empirical)265 pm
Radius (calculated)298 pm
Covalent radius244 ±11 pm
Van der Waals radius343 pm

Atomic shell

Electron configurationXe 6s1
Ionization energy(1st) 3.8939056 eV
(2nd) 23.15745 eV
(3rd) 33.195 eV
(4th) 43 eV
(5th) 56 eV
Shell model
Electrons
k-shell:2
l-shell:8
m-shell:18
n-shell:18
o-shell:8
p-shell:1

Physical properties

Phasesolid
Density1.9 g·cm−3 (293.1 K)
Molar volume7.094·10-5 m3·mol−1

Temperatures

Melting point301.7 K
Boiling point944 K
Liquid range642.3 K
Critical point1,938 K @ 9.4 MPa
Transition temperature1.66 K

Enthalpies

Melting enthalpy2.09 kJ·mol-1
Enthalpy of vaporization65 kJ·mol-1
Binding energy76 kJ·mol-1

Heat and conductivity

Thermal conductivity36 W·m-1·K-1
Work function2.14 eV

Hardnesses

Mohs hardness0.2
Brinell hardness0.14 NM·m-2

Elastic properties

Young’s modulus1.7 GPa
Bulk modulus1.6 GPa

Electrical properties

Electrical conductivity4.76·106 S·m-1
Resistance2.1·10-7 Ωm

Magnetism

Magnetismparamagnetic

Chemical properties

Basicitystrongly basic
Oxidation state+1
Standard potential-2.923 V (Cs+ + e- → Cs)

Electronegativity

Pauling scale0.79
Sanderson scale0.22
Allred-Rochow scale0.86
Mulliken scale0.62
Nagle scale0.79
Pearson absolute negativity2.18 eV

Other properties

Natural occurrenceprimordial
Crystal structureBody-centered cubic
Goldschmidt Classificationlithophile
Superconductorwith transition tempperature under special conditions
Price/kg61,800 USD

Natural abundances

satellite_alt
Universe
0.8 ppb ≈ 8.04·109 M☉
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Sun
8 ppb ≈ 1.59·1010 Mt
destruction
Meteorite
140 ppb ≈ 140 mg
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Earth’s crust
1,900 ppb ≈ 52,600 Mt
water_drop
Oceans
0.5 ppb ≈ 685 kt
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Flowing water
0.05 ppb ≈ 800 t
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Human body
20 ppb ≈ 1.4 mg