What is the most electropositive metal

Name, symbol, atomic number Cesium, Cs, 55
seriesAlkali metals
Group, period, block1, 6, p
Look shiny silver-white
Mass fraction of the earth's envelope 6 · 10-4 %
Atomic mass 132,9054 u
Atomic radius (calculated) 265 (-) pm
Covalent radius 225 pm
Van der Waals radius pm
Electron configuration [Xe] 6s1
Electrons per energy level 2, 8, 18, 18, 8, 1
Work function 2.1 eV
1. Ionization energy 382 kJ / mol
Physical state firmly
Modifications 1
Crystal structure body-centered cubic
density 1.879 g / cm3
Mohs hardness ?
Melting point 301.60 K (28.45 ° C)
boiling point 944.15 K (671 ° C)
Molar volume 70,94 · 10-6 m3/ mol
Heat of evaporation 67.74 kJ / mol
Heat of fusion 2.092 kJ / mol
Vapor pressure

1,4 · 10-4Pa

Speed ​​of sound ? m / s
Specific heat capacity 240 J / (kg K)
Electric conductivity 4,89 · 106S / m
Thermal conductivity 36 W / (m K)
Oxidation states +1
Oxides (basicity) Cs2O (as CsOH strongly basic)
Normal potential -2.92 V (Cs+ + e- → Cs)
Electronegativity 0.7 (Pauling scale)
isotopeNHt1/2ZMZE MeVZP


32.06 hε1,195129Xe


29.21 minε2,983130Xe


9,689 dε0,352131Xe


6.479 dε2,120132Xe

100 %



2.0648 aβ-2,059134Ba


2 · 106aβ-0,2135Ba


13.16 d β-2,548136Ba


30.17 a β-0,5137Ba
safety instructions
Hazardous substance labeling
R and S phrases R: 11-14 / 15-34[1][2]
S: 8-16-26-36 / 37 / 39-43-45[1][2]
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions.

Cesium (Spelling according to IUPAC too Cesium), colloquially too Cesium or Cesium, is a chemical element in the periodic table of the elements with the symbol Cs and the atomic number 55. The extremely reactive, gold-colored alkali metal, shiny in a highly pure state, melts at body temperature.


Cesium was first detected in Dürkheim mineral water in 1860 together with rubidium by the German chemist Robert Wilhelm Bunsen and the German physicist Gustav Robert Kirchhoff, the inventors of spectral analysis. The name Cesium is from Latin caesius derived, which means "sky blue". The name refers to the typical spectral lines of the cesium, which are in the blue area, and was given by Bunsen / Kirchhoff in 1861.

In 1882, Carl Setterberg produced metallic cesium for the first time by melting electrolysis of the cyanide.


Only the mineral pollucite, which is also called Pollux and is mined in Bernic Lake in Manitoba, Canada, is commercially usable. The pure metal is produced by reducing cesium chloride with calcium.


Of all the stable elements in the periodic table, cesium has the largest atomic radius at 0.274 nm.

It is generally little known that high-purity cesium is silver-white in color. Even the slightest traces of oxygen, for example from the glass of an evacuated storage ampoule, are sufficient to produce the well-known gold-colored shine of the metal. The actual color of the metal is only visible for a short time after its distillation.

Cesium is the softest of all metals and extremely ductile. After mercury and the radioactive francium, which is practically non-existent in nature, it has the lowest melting point of all metals. This is 28.45 ° C, the boiling point is 671 ° C.

After fluorine, cesium is the most reactive stable element; it reacts with practically all other elements. The extremely violent, strongly exothermic reaction with water produces hydrogen and cesium hydroxide, the absolutely strongest metal hydroxide base.

In air, cesium burns with a blue-violet flame to form cesium hyperoxide.

Above 300 ° C it also attacks glass. Cesium is also the element with the greatest coefficient of thermal expansion (9.4 x 10-5 per ° C).



Of course just come 133Cs before, it is one of the anisotopic elements or pure elements.

The other, all radioactive cesium isotopes only occur as artificial fission products in nuclear reactions. The most important artificial isotope is the beta and gamma emitter137Cs with a half-life of 30 years. It became known primarily through the radioactive contamination after the Chernobyl reactor accident in 1986, although the anthropogenic input into nature (mainly soil) is mainly due to the above-ground nuclear weapons tests of the 1950s and 1960s. It has accumulated particularly in certain mushrooms (for example chestnut boletus in southern Germany and Austria) and in wild animals (mainly wild boar, in which up to 9000 Bq can still be measured today (southern Germany, Schluchsee)). The limit value for food in the EU is 600 Bq / kg.


  • Production of photo cells (especially for IR radiation) and photo multipliers
  • Infrared light sets for use with night vision devices.[3]
  • Camouflage ammunition [4]
  • Infrared permeable windows, lenses and prisms
  • Cesium vapor lamps for night vision devices
  • Production of cesium-doped catalysts
  • Atomic clocks, cesium clocks as time standards
  • Getter metal for vacuum tubes
  • 137Cs as a radiation source for medical applications in cancer therapy and in small radiation systems (such as Gammacell) in experiments on food irradiation
  • 137Cs heat source (thermionic batteries, isotope batteries)
  • Fuel for ion thrusters
  • 137Cs as a tracer for wind and water erosion
  • 137Cs as a radiation source for density measurements in geophysics

safety instructions

Cesium ignites spontaneously in air, which is why it must be stored under pure nitrogen or argon. In water it reacts exothermically with the development of gaseous hydrogen. Because of its high reactivity, it reacts explosively with water. The explosiveness can be increased by igniting the hydrogen produced in the process.


Cesium is one of the most electropositive elements. Due to the great difference in electronegativity from most other elements, it is found almost exclusively in ionic compounds, such as:

Other connections are e.g. B .:


In gaseous cesium the refractive index is less than 1. This means that the phase velocity of the electromagnetic wave