Caesium (Cs) (Cesium)

Caesium is a chemical element with an atomic number of 55 in the periodic table of elements. It’s the 45th most abundant element in Earth’s crust. Being a member of the alkali metals family of periodic table elements, caesium has been classified as one of the incompatible elements.

Caesium has eight valence electrons that make this element the most reactive metal of all the other alkali metals. 

Fact Box

Chemical and Physical Properties of Caesium

The symbol in the periodic table of elements: (Cs)

Atomic number: 55

Atomic weight (mass): 132.9054 g.mol -1

Group number: 1 (Alkali metals)

Period: 6

Color: Silvery-golden alkali metal

Physical state: Liquid at or near room temperature

Half-life: From 500(100) µs to 2.3 million years

Electronegativity according to Pauling: 0.79

Density: 1.9 at 20°C

Melting point: 28.5 °C (83.3 °F)

Boiling point: 669 °C

Van der Waals radius: 0.267 nm

Ionic radius: 0.167

Isotopes: 40

Most characteristic isotope: 133Cs, 137Cs

Electronic shell: [Xe] 6s1

The energy of the first ionization: 375.6 kJ.mol -1

The energy of the second ionization: N/A

Discovery date: In 1860 by Robert Bunsen, Gustav Kirchhoff, and Carl Setterberg

Caesium has the periodic table symbol Cs, atomic number 55, and an atomic mass of 132.9054 g.mol -1. The ground state electronic configuration of neutral caesium is [Xe] 6s1

Caesium is a soft and ductile metal in a liquid aggregate state at room temperature. It reaches its boiling point at 669 °C, while the melting point of caesium is achieved at 28.5 °C (83.3 °F). This member of the alkali metals family of elements has an electronegativity of 0.79 according to Pauling which makes it the least electronegative chemical element of the periodic table. The atomic radius of caesium according to van der Waals is 0.267 nm. It’s the most alkaline element of the periodic table. 

Both the physical and the chemical properties of caesium resemble those of rubidium and potassium. The crystal structure of caesium is a body-centered cubic, resembling two interpenetrating simple cubic lattices.

This alkali metal is pyrophoric by nature, i.e. it spontaneously ignites after being exposed to air for only 5 minutes. In contact with cold water, caesium creates volatile, explosive reactions due to the formation of hydrogen gas and a solution of caesium ions and hydroxide ions:

2Cs (s) + 2H₂O (l) → H₂ (g) + 2Cs⁺ (aq) + 2OH⁻ (aq)                        

The most volatile caesium reactions occur in the presence of fluorine, chlorine, bromine, and iodine, while sulfur and phosphorus trigger its incandescence (emission of light) after exposure to high temperatures. 

How Was Caesium Discovered?

The story of this chemical element begins in 1846 when the German metallurgical chemist and a son of a miner, Karl Friedrich Plattner (1800 – 1858), conducted research on the mineral pollucite (caesium aluminum silicate).

Despite having an ore containing a chemical element that was waiting to be discovered, Plattner could not provide scientific proof for 7% of the elements that made up the mineral in his hands. Unfortunately, he also mistakenly thought that caesium is, in fact, a sodium and potassium compound. 

Robert Bunsen and Gustav Kirchhoff

In 1860, the two German scientists Robert Bunsen and Gustav Kirchhoff applied the new spectroscopic method in their scientific research for the first time. By attempting an experiment on the spectral emissions of a mineral water sample rich in lithium compounds, the chemist and the physicist teamed up and observed the newly occurring sky-blue lines in the spectrum which were incomparable to any known chemical element. This gave them a sign that they are on the way to the discovery of a new chemical element, the caesium. 

In their further experimental attempts, Bunsen and Kirchoff managed to produce approximately 7 grams of caesium chloride, but never succeeded in isolating the pure elemental form of the new alkali metal. Whatever technique they applied, the result was always a type of blue molten mass.

Carl Theodor Setterberg

Twenty years later, the Swedish chemist Carl Theodor Setterberg (1853 – 1941) was the scientist who, in the end, earned the credits for the discovery of caesium. Namely, by conducting electrolysis of molten caesium cyanide (CsCN) at the University of Bonn, Setterberg succeeded in isolating the pure elemental form of caesium.

First, he extracted lithium from lepidolite ore, Setterberg added a mixture of cesium-based salts to the chemical. This reaction led him to the first isolated quantity of caesium.  

The Revolutionary Invention of the Spectroscopy

The year was 1822. John Herschel heated a compound in one of his chemical attempts, and among the different colors of flame that passed through the prism, Herschel also observed unseen lines of the spectrum. This prismatic spectrum of a flame was a point of interest for this English polymath (mathematician, astronomer, chemist, experimental photographer, and inventor of the blueprint). 

Herschel believed that this occurrence is due to the existence of substances that must be analyzed with a more powerful technique and equipment needed to measure the interactions between light and matter, rather than the usual technique of electrolysis.

Fast forward to 1989, Robert Bunsen entertained the idea of using filters to block the intense colors emitted by the substances (such as chromium yellow) so that the less intensive colors (potassium’s ultraviolet, for instance) could be more easily detected. For this, he came up with the idea for spectroscopy – the technique he used to identify the chemical substances that emit light under applied heat. 

Despite the failure to introduce a new chemical to the world, Bunsen and Kirchoff received the Davy Medal for their scientific efforts and discoveries in the realm of spectrum analysis. 

How Did Caesium Get Its Name?

The Latin word caesius, meaning ‘heavenly blue’, was used to name the element caesium (also spelled cesium in American English) as a result of the unique sky-blue lines it emits. 

Where Can You Find Caesium?

The minerals pollucite (CsAlSi2O6 ) and lepidolite (K(Li,Al)₃(Al,Si,Rb)₄O₁₀(F,OH)₂) are the main sources of caesium. In nature, caesium often occurs along with rubidium and other alkali metals. Ores of these minerals can be found on locations in Bernic Lake, Manitoba, Canada, Zimbabwe, South-West Africa, and in the United States. 

Caesium traces can be also found in the human body. The quantities of this trace element in the human body can be measured by analysis of blood, urine, and feces. 

Caesium in Everyday Life

The Use of Caesium and the Atomic Clocks

One of the most widespread everyday uses of caesium is its application in atomic clocks. Caesium is used in atomic clocks because of the vibration frequency of the caesium atom. The electron resonance frequency of the caesium atom is 9,192,631,770 cycles per second, which gives caesium clocks an incredible accuracy. Atomic clocks enable the function of GPS, cell phones, and the Internet. 

The Use of Caesium in Mineral Water Production

Naturally occurring in springs of mineral water, caesium salts are also applied in the process of commercial production of mineral water. The caesium isotopes are also used for the purification of the water.  

Other Uses of Caesium 

This rare element is also used as one of the vital components in:

  • Drilling fluids (caesium formate (HCOO−Cs+); 
  • Nuclear fuel (together with xenon);
  • Vacuum tubes;
  • Photoelectric cells;
  • DNA separation techniques;
  • Traffic controls;
  • Special optical glass manufacture (cesium nitrate and caesium salts);
  • Radiation monitoring equipment;
  • Infrared detectors;
  • Night vision devices;
  • Radiography. 

How Dangerous Is Caesium?

Exposure to caesium can occur after emission of the radioactive cesium isotopes from the radioactive waste of a nuclear plant after a nuclear catastrophe (such as the ones in Chernobyl, in 1986 and Fukushima in 2011), nuclear bomb tests and accidents, or as an occupational hazard, directly affecting the workers in nuclear plants. 

This radioactive form of caesium can be inhaled from the contaminated air, consumed via contaminated food and water, or contaminated foods grown in caesium contaminated soil. Caesium is most easily absorbed in the soft tissues, muscles, and bones, but it remains for a relatively short time in the body. Exposure to high levels of caesium can result in severe burns on the skin, acute radiation sickness, and cancer which may lead to fatal consequences upon human health.  

Environmental Effects of Caesium

Caesium-137 is both a beta and gamma emitter. Caesium isotopes can be easily dispersed in nature after atmospheric nuclear weapons tests, nuclear plant spills of radioactive waste material, etc. 

According to the United States Agency for Toxic Substances and Disease Registry, the high mobility of 137Cs isotopes makes the cleaning up of caesium particles from the environment difficult. Released as CsCl, this chemical element becomes liquid when exposed to the humidity of air or rain. This speeds up its absorption in the soil and porous surfaces, thus contaminating larger areas in a short time. 

Isotopes of Caesium

Caesium-133 is the only stable, naturally occurring, and non-radioactive isotope of this chemical element. The caesium-137 radioisotope has a half-life of 30.2 years and is used in radiotherapy. 

The following is a tabular representation of the main isotopes of caesium:

 abun­dancehalf-life (t1/2)modepro­duct 
131Cssyn9.7 dε131Xe 
134Cssyn2.0648 yε134Xe 
135Cstrace2.3×106 yβ135Ba 
137Cssyn30.17 y[1]β137Ba 

Source: Wikipedia


List of Caesium Compounds 

Caesium forms binary compounds with halides, hydrides with hydrogen molecules, and oxides in reactions with O2. The oxidation states of this chemical are 1, −1. 

Caesium Minerals

  • Avogadrite
  • Galkhaite
  • Margaritasite
  • Pezzottaite
  • Pollucite


Caesium Compounds

  • Caesium acetate
  • Caesium auride
  • Caesium azide
  • Caesium bicarbonate
  • Caesium bisulfate
  • Caesium bromide
  • Caesium cadmium bromide
  • Caesium cadmium chloride
  • Caesium carbonate
  • Caesium chloride
  • Caesium chromate
  • Caesium cyanide
  • Caesium dodecaborate
  • Caesium fluoride
  • Caesium hexa-fluoro-cobaltate(IV)
  • Caesium hexa-fluoro-cuprate(IV)
  • Caesium hydride
  • Caesium hydroxide
  • Caesium iodide
  • Caesium lithium borate
  • Caesium nitrate
  • Caesium oxalate
  • Caesium oxide
  • Caesium perchlorate
  • Caesium sulfate
  • Caesium sulfide
  • Caesium titanate
  • Caesium tungstate

5 Interesting Facts and Explanations

  1. Caesium is one of the five chemical elements in the periodic system that are classified as metals but occur in a liquid aggregate state. 
  2. This alkali metal is the 50th most common element found as accumulation in the Earth’s crust.
  3. Bromine (Br) and mercury (Hg) are the only elements that occur in a constant liquid state at a standard temperature, while caesium (Cs), rubidium (Rb), francium (Fr), and gallium (Ga) change their solid aggregate state into liquid only at or above room temperature.  
  4. The chemical elements that spontaneously flare up in contact with air are labeled as pyrophoric. 
  5. The flame spectroscopy method of chemical analysis was invented in 1859 by the German scientists Robert Bunsen and Gustav Kirchhoff. From then on, the new chemical elements and compounds were analyzed by using specialized instruments, like the spectroscope. 

Chemical Property and physical property of element Cesium

Symbol of Cesium: Cs
Name: Cesium
Atomic Number of Cesium: 55
Atomic Mass of Cesium: 132.90543
Uses of Cesium: Used as a ‘getter’ to remove air traces in vacuum and cathode-ray tubes. Also used in producing photoelectric devices and atomic clocks. Since it ionizes readily, it is used as an ion rocket motor propellant.
Description of Cesium: Soft, slightly malleable, silvery-white metal.
Melting Point of Cesium:
Boiling Point of Cesium:
Group of Cesium: Alkali Metal
Shells of Cesium: 2,8,18,18,8,1
Orbitals of Cesium: [Xe] 6s1
Valence of Cesium: 1
Crystal Structure of Cesium: Cubic: Body centered
Electro Negativity of Cesium: 0.79
Covalent Radius of Cesium: 2.35 Å
Ionic Radius of Cesium: 1.74 (+1) Å
Atomic Radius of Cesium: 3.34 Å
Atomic Volume of Cesium: 71.07 cm³/mol
Name Origin of Cesium: Latin: coesius (sky blue); for the blue lines of its spectrum.
Discovered of Cesium By: Gustov Kirchoff, Robert Bunsen
Year: 1860
Location: Germany
Pronounced of Cesium: SEE-zi-em
Oxydation States of Cesium: 1
Density of Cesium: 1.873 g/cm³