The Spontaneous Element Cesium

cesium element

The Element Cesium

The element cesium belongs to the alkali metal group, or Group 1, of the periodic table. This element is shiny and has a silvery-goldish finish. It is one of the most reactive metals on Earth; It is extremely reactive and highly-flammable. It is also one of only 5 metals that can be a liquid in a warm room. Let’s learn more about this exciting, spontaneous element. 

Cool facts about the Element Cesium

  1. Cesium was the first element to be discovered through spectroscopy.  
  2. Out of all the metallic elements, cesium has the second lowest melting point. It is one of just four metals that are liquid at or near room temperature. 
  3. The name cesium comes from the Latin word “caesium”, which means “sky blue”. 
  4. The element cesium is the most electropositive and alkaline element on the periodic table. 
  5. Only two other metals, in addition to cesium, are liquid at or near room temperature. These metals include gallium and mercury
  6. Cesium is a very powerful element. Theoretically, 1 kg of cesium can force a vehicle 140 times as far as the burning of the same amount of any liquid or solid.  
  7. The Chernobyl Accident of 1986 took place at the Chernobyl nuclear power station in Ukraine. Four reactors stood at this station. Due to flawed design and deviation from safety regulations, the nuclear power plant exploded. About 27 kg of radioactive cesium-137 contaminated the air as a result.  
  8. If the reaction of cesium metal and water takes place in a glass bottle, the bottle will shatter. Cesium is the most reaction metal on the periodic table that is stable.

The Element Cesium on the Periodic Table

Cesium has atomic symbol Cs, and atomic number 55. It lies below the element rubidium and above francium in group 1 of the periodic table, the alkali metals. It is a soft, gold-colored metal.

The cesium atom has an electron configuration of 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1 or [Xe]6s1 . It has the largest atomic radius, and the smallest electronegativity, of all the stable elements.

The Element Cesium is a softy (but beware!)

Is cesium a metal?

The element cesium is the softest of all the metals. It is also very beautiful. It has a very low melting point of 28.5˚C (83.3˚F), which is just above room temperature. The metal will melt in your hands, into a candle wax-like consistency. While this sounds interesting, do NOT hold it. This element is sensitive to moisture and may explode in your hands. It also explodes when it reacts with cold water. Additionally, the metal oxidizes rather quickly from air exposure. This results in the formation of superficial superoxide, which can be harmful. Does an element seemingly as dangerous and spontaneous as cesium have any use? Let’s find out.

How dangerous is cesium? 

As stated, Cesium-137 is a radioactive isotope that comes from nuclear fission. It behaves similarly to sodium, as it reacts well with chlorides to form a crystalline powder. While low levels of this element exist in the air, radioactive cesium has also been identified in both surface water and foods. It dissolves easily in water and binds to the soil, which then contaminates growing plants. Nuclear power plants and nuclear weapon testing can release this radioactive isotope. Exposure to radioactivity is never healthy and can have dire effects, including cancer. 

Why is the element cesium so reactive? 

The element cesium has the largest atoms out of the whole periodic table (excluding Francium which has a half-life of only 22 minutes). It has an atomic radius of 298 pm. To put this into perspective, carbon has an atomic radius of 67 pm. Why is the atomic radius so significant? The atomic radius describes the size of different atoms; It is defined as half the distance between the nuclei of two identical bonded atoms. It helps us to understand why some molecules fit together and why some do not.

Cesium has a large valence electron shell and a low effective nuclear charge. The size of the valence shell affects how tightly bound the outermost electrons are to the nucleus. Since cesium’s valence shell is large, its valence electrons are loosely bound, and therefore, fall off easily in chemical reactions. The effective nuclear charge is the net positive charge of an electron in a multi-electron atom. What does this mean? The higher the effective nuclear charge is, the greater the attraction is between electrons and the nucleus. Cesium’s low effective nuclear charge indicates that the nucleus does a poor job of attracting its electrons. Both of these factors make cesium extremely reactive. 

Cesium’s Applications in Today’s World

Cesium combines well with the element oxygen. This makes it an ideal getter for vacuum and electron tubes. Getters work to clear out traces of trapped oxygen in the tube by binding to the oxygen molecules. Additionally, cesium is very photoelectric, meaning it loses electrons when struck by light. It is used for atomic clocks, photoelectric cells, and as a catalyst promoter. As a catalyst promoter, it increases the activity of other metal oxides and the hydrogenation of organic compounds. 

Cesium atomic clocks are extremely accurate and tell the world’s standard measure of time. They have an error of only 1 second in up to 100 million years. Cesium is the ideal atom for atomic clocks because of the following characteristics: It vaporizes easily, possesses a magnetic field, had a stable isotope, and has a high frequency for the transition. 

Cesium-137, also called radiocesium, is an isotope that is the byproduct of nuclear fission processes. It is used in radiation-detection equipment, medical radiation therapy devices, and industrial devices that detect liquid flow through pipes and measure the thickness of materials like paper and sheets of metal. 

In the future, spacecraft may use cesium to fuel their ion engines. Cesium is the ideal element for this job because its ions can accelerate to great speeds, making it favorable for high exhaust engines. 

History of the Element Cesium

In 1860, Robert Wilhelm Bunsen and Gustav Robert Kirchhoff discovered the element cesium while observing the spectroscopic analysis of mineral water from Durkheim. The scientists obtained seven grams of cesium chloride compound. Unfortunately, they were not able to isolate pure cesium from this sample. Carl Theodor Setterberg, from the University of Bonn, is credited for later accomplishing the extraction of the pure metal through the electrolysis of molten cesium cyanide. However, cesium could have potentially been discovered 14 years earlier! Carl Plattner almost found the element while studying pollucite, a cesium-containing mineral. While trying to break down the components of the mineral, he ran out of material and his experiment was cut short, and therefore, cesium was not identified.

Cesium Reactions

Solid cesium is very reactive with water. When the two combine, it forms an aqueous solution of cesium hydroxide and hydrogen gas. This exothermic reaction occurs rapidly and continues even when the solution turns basic.

Upon exposure to air, the surface of cesium oxidizes and tarnishes. It also results in the formation of cesium superoxide on the surface of the metal, which presents itself in an orange-yellow color. This product is a stable salt that is known to be harmful to molecules in its environment. It is involved in the pathogenesis of common diseases, including atherosclerosis, and is able to damage DNA.

Cesium Compounds

Cesium is extremely reactive and forms a variety of compounds.


Cesium oxides are inorganic compounds that form when cesium metal reacts with oxygen. Interestingly, as you go further down the alkali metals on the periodic table, you will notice that cesium oxide is the darkest in color; While lithium is white, cesium is yellow-orange. This is due to the fact that the alkali metal’s bandgap decreases as you move down the periodic table. This allows for more blue and green wavelengths to absorb. As a result, the color darkens.

There are three kinds of binary oxide compounds, which are dicesium oxide, Cs2O, cesium superoxide, CsO2, and dicesium peroxide, Cs2O2

Dicesium oxide is the most common and simplest oxide. Upon heating, this orange-red metal turns dark red, and eventually black. This color change occurs as a result of more oxygen reacting with the compound. While dicesium oxide is stable in dry air, it is basic and sensitive to moisture and will react aggressively with water to produce the hydroxide, CsOH.

Cesium superoxide is a more yellow-orange metal that forms when cesium metals ignite in the air and react with oxygen. This superoxide is a strong oxidant that, while stable at room temperature, will decompose when heated. Additionally, cesium superoxide is a stable salt of the oxygen anion and exhibits attractive magnetic properties.

Lastly, dicesium peroxide is a thermally stable solid. When it decomposes, it forms dicesium oxide. Dicesium peroxide and oxide are the most stable out of the three cesium oxides. How so? Their cesium atoms are larger in size compared to cesium superoxide. 


Cesium metal reacts with halogens to form the following inorganic compounds: cesium fluoride, cesium chloride, cesium bromide, and cesium iodide. All of these compounds are soluble, meaning they readily dissolve in water. Additionally, they all possess the +1 oxidation state. 

Cesium fluoride is a slightly basic, ionic, white crystalline solid. Compared to other fluoride compounds, cesium fluoride is more soluble and dissociates more readily. When exposed to acid, it forms hydrofluoric acid, which is toxic and corrosive. Cesium fluoride forms intermediates that react rapidly with water. This makes it ideal for desilylation reactions, which remove silicon groups. Additionally, it provides a source of fluoride anions. Therefore, it is used in reactions that form soluble fluoride compounds. 

When cesium metal reacts with diluted hydrochloric acid, its forms this dense salt, called cesium chloride. This is one of the most common compounds. Cesium chloride is an ionic, soluble, and fairly stable crystalline compound. Interestingly, while this compound is only slightly electrically conductive, it is still used in the production of electrically conducting glass. When placed in water, cesium chloride forms a non-viscous solute, making it ideal for centrifugation processes. When cesium chloride reacts with strong acids like sulfuric acid, it releases the corrosive hydrochloric acid. Furthermore, cesium chloride is vital in the drug development industry because of its vasoconstrictive properties. While this compound has low toxicity, it can have adverse effects when ingested in large amounts, such as hypokalemia (the lack of potassium ions) and arrhythmias. 

Cesium bromide is a stable alkali metal salt that has the appearance of either a white or translucent powder. When heated, it decomposes to cesium oxide. Its main use is in the optics industry as a beamsplitter component, which helps improve optical quality. 

Cesium iodide can also come from hydroiodic acid reacting with cesium hydroxide. This forms a cesium iodide salt, which can then go through a recrystallization process to be further purified. This compound is a white crystalline compound that can also come in submicron and nanopowder forms. It is soluble in both water and ethanol, but only slightly soluble in methanol. It possesses hygroscopic properties, meaning it readily attracts water molecules. Like cesium bromide, it is also used in beamsplitters, as well as fluoroscopy equipment. 


Cesium sulfide, Cs2S is a type of inorganic, binary compound. It presents itself as a whitish-yellow crystalline that is soluble in both water and acid. It is the product of cesium reacting with sulfur in tetrahydrofuran (THF). One of the biggest cons of cesium sulfide is that it smells terrible. In the air, cesium sulfide will release hydrogen sulfide, which smells like rotten eggs.   

Other binary compounds include cesium selenide and cesium telluride, which both exist as +1 oxidation states.


Along with cesium chloride, cesium nitrate is the most common cesium compound. This white, crystalline solid is also soluble in water and acts as a strong oxidizing agent. Therefore, it is highly flammable and exposure to any organic material may result in a fire. For example, mixing hydrocarbon with cesium nitrate results in a burst of flames. It also has the ability to decompose explosively when heated. This compound synthesizes other chemicals and compounds, such as cesium nitratocuprate, Cs2[Cu(No3)4]. When mixed with an alkyl ester, it will form an alkyl nitrate; When mixed with other alkali metal nitrates, it will produce cesium nitrite. Additionally, it acts as a colorant and oxidizer for pyrotechnics, otherwise known as fireworks.

Isolation of Elemental Cesium

Cesium is a difficult element to obtain. It is not an abundant metal and is never alone. It exists with rubidium, another alkali metal. To get cesium, the cesium and rubidium ores go through a long process. It involves the ores getting crushed and heated with sodium at high temperatures. This then forms an alloy, which goes further goes through a process called fractional distillation so that the two metals can get separated. 

Another method of isolating cesium is through an alkali metal converter. This device takes cesium chloride and heats it until reduced cesium distills and re-solidifies. Learn about it here:

*Note: This is the work from own awesome team at ChemTalk!

Cesium Oxidation States

The most common oxidation state for cesium is +1. It also exhibits an oxidation state of -1, but it is rare and less stable. All alkali metals have one electron in their valence shell. When that electron is “released”, the alkali metal becomes more stable. Thus, all alkali metals have the +1 oxidation state.

Properties of the Element Cesium

  • Cesium’s symbol: Cs
  • Melting point: 301.7 K; 28.5°C; 83.3°F   
  • Boiling point: 944 K; 671°C; 1240°F
  • Density: 1.873 g/cm³
  • Atomic weight: 132.905
  • Atomic number: 55
  • Electronegativity: 0.79
  • Classification: Alkali metal
  • Natural abundance in the Earth’s crust: 0.00019%
  • Electron shell configuration: [Xe] 6s1
  • Isotopes: Cesium-133 is the only stable isotope
  • Found naturally in the minerals: Pollucite, rhodizite, and lepidolite
  • Toxicity: Toxic when ingested or exposed to skin

Where can I buy the Element Cesium?

Cesium is relatively expensive and produced in limited amounts. It is usually sold in ampules or in mineral oils. This element has the potential to be dangerous, and you likely do not want it in your home.

Explore more

Learn how amazing chemists, such as Alan Katz, work with such dangerous chemicals.

If you enjoyed learning about cesium, check out the other elements on our Interactive Periodic Table!