October 11, 2011, 12:53 pm


caption Cesium metal. (Source: chemsoc.org)

Cesium is a shiny, silvery-gold metallic element. Its atomic number is 55 and its symbol is Cs. It belongs to a group of elements called the alkali metals. Robert Bunsen and Gustov Kirchhoff from Germany discovered it in 1860 when they were studying the minerals left by the evaporation of mineral waters. Pure cesium metal, however, was not prepared until 1882 by another scientist named Setterburg.

Previous Element: Xenon

Next Element: Barium


Physical Properties
Color yellow/silvery
Phase at Room Temp. solid
Density (g/cm3) 1.9
Hardness (Mohs) 0.2
Melting Point (K) 301.6
Boiling Point (K) 978
Heat of Fusion (kJ/mol) 2.092
Heat of Vaporization (kJ/mol) 66
Heat of Atomization (kJ/mol) 79
Thermal Conductivity (J/m sec K) 35.9
Electrical Conductivity (1/mohm cm) 48.9
Source Li extract by-product
Atomic Properties
Electron Configuration [Xe]6s1
Number of Isotopes 56 (1 natural)
Electron Affinity (kJ/mol) 121.85
First Ionization Energy (kJ/mol) 1086.4
Second Ionization Energy (kJ/mol) 2352.6
Third Ionization Energy (kJ/mol) 4620.4
Electronegativity 45.506
Polarizability (Å3) 59.6
Atomic Weight 132.91
Atomic Volume (cm3/mol) 70
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 265
Ionic Radius1+ (pm) 181
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) ---
Common Oxidation Numbers +1
Other Oxid. Numbers ---
In Earth's Crust (mg/kg) 3.00×100
In Earth's Ocean (mg/L) 3.00×10-4
In Human Body (%) 0.000009%
Regulatory / Health
CAS Number 7440-46-2
OSHA Permissible Exposure Limit (PEL) No limits
OSHA PEL Vacated 1989 No limits
NIOSH Recommended Exposure Limit (REL) No limits
University of Wisconsin General Chemistry
Mineral Information Institute
Jefferson Accelerator Laboratory

The physical properties of cesium are very interesting. It is the softest of all metals and can even be cut with a knife. Some describe it as being like wax. Cesium is one of three metals that are liquid at or near room temperature (the other two are gallium and mercury). It is also a very reactive metal. For instance, when it mixes with cold water, there is an explosive reaction. Its melting point is so low that it will melt if it is held in the hand. However, because it is so reactive, it can seriously burn the skin, so it must be handled with great care. Cesium hydroxide is the strongest base known.

Only a few thousand kilograms of cesium is used each year. This fact, plus the fact that cesium is so reactive to air and water, results in very high prices for cesium and cesium compounds.

Cesium is not beneficial to animals or plants.


When cesium burns, the light spectrum created contains two bright blue lines. Based on these blue lines, this element was named cesium after the Latin word caesius which means sky blue.


Most rocks typically contain very little cesium. Seawater also contains very little. Springs of mineral waters can contain as much as 9 mg/liter of cesium. However, there are a number of minerals that contain significant amounts of cesium, including mica, beryl, feldspar, petalite, and pollucite.

Most cesium is retrieved from the mineral pollucite. This mineral is typical of a special igneous rock known as a pegmatite.

The United States has low-grade deposits of cesium ore in South Dakota and Maine, from which it is presently too expensive to get the cesium out. As a result, the U.S. imports 100% of the cesium it uses and it imports nearly all of this cesium from Canada. Other nations producing cesium are Southwest Africa and Zimbabwe.


Cesium and cesium compounds have a number of interesting uses and applications. For example, they are used as catalysts in chemical reactions. Because it is easily ionized by light, metallic cesium is used in photoelectric cells and infrared detectors. (An element that is ionized is transformed from a neutrally charged element into an electrically charged ion.) Cesium compounds are used in specialized alkaline batteries that are designed to work in subzero climates. Cesium carbonate is used in the production of special glass and glass products.

The most accurate clock in the world, the "atomic clock," measures time based on the very precise vibration of the electrons in the outer shell of the cesium atom. This clock is accurate within 5 seconds every 300 years!

Cesium-137 is radioactive and may be used for radiation therapy to treat certain cancers.

Space travel engineers have discovered that burning cesium in space is a very efficient form of fuel. It is determined to be 140 times more efficient than any other fuel.

Substitutes and Alternative Sources

Estimates of the world resources of cesium have not been calculated. Presently, the supply meets demand and it appears it will do so for many years to come.

In addition, rubidium and rubidium compounds are as effective as cesium and can be used in place of cesium and its compounds.

Further Reading



Saundry, P. (2011). . Retrieved from http://www.eoearth.org/view/article/151043


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