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About Cesium

Cesium Bohr

With one electron in its sixth and outermost shell, cesium (or caesium) is the most electropositive of all stable elements on the periodic table: the metal is extremely pyrophoric, spontaneously igniting when it comes into contact with air and exploding violently in water or ice at any temperature above -116 °C. Though cesium is only mildly toxic, it is classified as hazardous because of its high reactivity and is usually packaged in glass ampoules in a vacuum or under an inert gas such as argon. The heaviest of the stable alkali metals, cesium is silvery-gold in appearance, ductile, and the softest element on the periodic table at 0.2 on the Mohs scale. Cesium melts at 28 °C, making it one of three elements (the others being gallium and mercury) that are liquid at or near room temperature; mercury is the only element with a lower melting point. Only one of cesium’s known isotopes is stable (133Cs), but its total of 39 makes it tied with xenon as the two elements with the most amount of known isotopes. Cesium was also the first element to be discovered via spectroscopy by German scientists Robert Bunsen and Gustav Kirchhoff, who had invented the technology the previous year. Bunsen and Kirchhoff gave the new element a name based on the Latin word caesius (sky or heavenly blue) for the two brilliant blue lines emitted in its spectrum.

Cesium is the 45th most abundant element on earth, found in the minerals pollucite, avogadrite, pezzottaite, londonite, rhodizite, beryl, and some potassium ores; the primary commercial source of metallic cesium is from the mining of pollucite, while cesium radioisotopes are produced from nuclear reactor waste. Oxygen-free metallic cesium can also be produced via the thermal decomposition of cesium azide (CsN3). Laboratories use cesium compounds for various organic chemistry functions such as the hydrogenation of organic compounds or, in the case of cesium fluoride, as a source of the fluorine anion. X-ray radiotherapy in cancer treatment often employs radioactive isotope cesium-137. In commercial and industrial applications, cesium salts can serve as catalyst promoters, glass strengtheners, components of photoelectric cells, crystals in scintillation counters, and “getters” in vacuum tubes; cesium formate brines are commonly utilized in oil drilling to lubricate drill bits and maintain pressure. Thermionic energy converters use a vapor of cesium ions to determine the work function of the electrodes. Early ion propulsion engines for space exploration used cesium as a propellant until xenon became the standard. The most accurate commercially available atomic clocks keep time using the oscillation of the cesium-133 atom’s 9193 MHz hyperfine transition frequency. Known as the “cesium standard,” this frequency is the primary time standard for the definition of the second and is critical to the data transmission infrastructures of cell phone networks, GPS, and the internet.

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Cesium is used as an oxygen "getter" in vacuum and electronic tubes and as a component of photoelectric cells. Atomic clocks use the microwave spectral line emitted by Cesium Isotope 133 for reference. Radioactive Cesium 137 is an artificial isotope that is used for cancer radiotherapy. High Purity (99.999%) Cesium Oxide (Cs2O) PowderLaboratories use various cesium compounds for their ability to hydrogenate organic compounds. Cesium is available as metal and compounds with purities from 99% to 99.999% (ACS grade to ultra-high purity). Nanoparticles and nanopowders provide ultra-high surface area which nanotechnology research and recent experiments demonstrate function to create new and unique properties and benefits. Oxides are available in forms including powders and dense pellets for such uses as optical coating and thin film applications. Oxides tend to be insoluble. Fluorides are another insoluble form for uses in which oxygen is undesirable such as metallurgy, chemical and physical vapor deposition and in some optical coatings. Cesium is available in soluble forms including chlorides, nitrates and acetates. These compounds are also manufactured as solutions at specified stoichiometries.

Cesium Properties

Cesium(Cs) and molecular weight, atomic number and elemental symbolCesium (or Caesium ) is a Block S, Group 1, Period 6 element. The number of electrons in each of Cesium's shells is 2, 8, 18, 18, 8, 1 and its electronic configuration is [Xe] 6s1. The cesium atom has a radius of and its Van der Waals radius is In its elemental form, CAS 7440-46-2, cesium has a silvery gold appearance. Cesium is a member of the alkali group of metals. It is one of three metals that occur as a liquid at room Cesium Bohr Model temperature - the others are mercury and gallium. Elemental CesiumCesium's main commercial source is pollucite ore; however, it is also found in beryl, avogadrite, pezzottaite, and londonite. Cesium was first discovered by Robert Bunsen and Gustav Kirchhoff in 1860. Cesium is named for the vibrant blue lines in its spectrum. The word Cesium originates from the Latin word caesius, meaning "sky blue". Cesium information, including technical data, safety data, high purity properties, research, applications and other useful facts are discussed below. Scientific facts such as the atomic structure, ionization energy, abundance on earth, conductivity and thermal properties are also included.

Symbol: Cs
Atomic Number: 55
Atomic Weight: 132.9055
Element Category: alkali metal
Group, Period, Block: 1 , 6, s
Color: silvery gold/yellow
Other Names: Caesium
Melting Point: 28.44 °C, 83.19 °F, 301.59 K
Boiling Point: 671 °C, 1240 °F, 944 K
Density: 1.93 g/cm3
Liquid Density @ Melting Point: 1.843 g/cm3
Density @ 20°C: 1.873 g/cm3
Density of Solid: 1879 kg·m3
Specific Heat: 0.24J/g/K
Superconductivity Temperature: N/A
Triple Point: N/A
Critical Point: 1938 K, 9.4 MPa
Heat of Fusion (kJ·mol-1): 2.09
Heat of Vaporization (kJ·mol-1): 66.5
Heat of Atomization (kJ·mol-1): 77.58
Thermal Conductivity: 35.9 W·m-1·K-1
Thermal Expansion: (25 °C) 97 µm·m-1·K-1
Electrical Resistivity: (20 °C) 205 nΩ·m
Tensile Strength: N/A
Molar Heat Capacity: 32.210 J·mol-1·K-1
Young's Modulus: 1.7 GPa
Shear Modulus: N/A
Bulk Modulus: 1.6 GPa
Poisson Ratio: N/A
Mohs Hardness: 0.2
Vickers Hardness: N/A
Brinell Hardness: 0.14 MPa
Speed of Sound: N/A
Pauling Electronegativity: 0.79
Sanderson Electronegativity: 0.22
Allred Rochow Electronegativity: 0.86
Mulliken-Jaffe Electronegativity: 0.62 (s orbital)
Allen Electronegativity: N/A
Pauling Electropositivity: 3.21
Reflectivity (%): N/A
Refractive Index: N/A
Electrons: 55
Protons: 55
Neutrons: 78
Electron Configuration: [Xe] 6s1
Atomic Radius: 265 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius: 244±11 pm
Covalent Radius (Å): 2.38
Van der Waals Radius: 343 pm
Oxidation States: 1 (strongly basic oxide)
Phase: Solid
Crystal Structure: body-centered cubic
Magnetic Ordering: paramagnetic
Electron Affinity (kJ·mol-1) 45.541
1st Ionization Energy: 375.71 kJ·mol-1
2nd Ionization Energy: 2234.37 kJ·mol-1
3rd Ionization Energy: 3400 kJ·mol-1
CAS Number: 7440-46-2
EC Number: 231-155-4
MDL Number: MFCD00134037
Beilstein Number: N/A
SMILES Identifier: [Cs]
InChI Identifier: InChI=1S/Cs
PubChem CID: 5354618
ChemSpider ID: 4510778
Earth - Total: 15.3 ppb 
Mercury - Total: 2.5 ppb 
Venus - Total: 17.0 ppb 
Earth - Seawater (Oceans), ppb by weight: 0.5
Earth - Seawater (Oceans), ppb by atoms: 0.023
Earth -  Crust (Crustal Rocks), ppb by weight: 1900
Earth -  Crust (Crustal Rocks), ppb by atoms: 290
Sun - Total, ppb by weight: 8
Sun - Total, ppb by atoms: 0.07
Stream, ppb by weight: 0.05
Stream, ppb by atoms: 0.0004
Meterorite (Carbonaceous), ppb by weight: 140
Meterorite (Carbonaceous), ppb by atoms: 20
Typical Human Body, ppb by weight: 20
Typical Human Body, ppb by atom: 1
Universe, ppb by weight: 0.8
Universe, ppb by atom: 0.1
Discovered By: Robert Bunsen and Gustav Kirchhoff
Discovery Date: 1860
First Isolation: Carl Setterberg (1882)

Health, Safety & Transportation Information for Cesium

Safety Data
Material Safety Data Sheet MSDS
Signal Word Danger
Hazard Statements H260-H314
Hazard Codes F,C
Risk Codes 11-14/15-34
Safety Precautions 8-16-26-36/37/39-43-45
RTECS Number FK9225000
Transport Information UN 1415 4.3/PG 1
WGK Germany 3
Globally Harmonized System of
Classification and Labelling (GHS)
Corrosion-Corrosive to metals Flame-Flammables

Cesium Isotopes

Caesium (Cs) has 40 known isotopes with atomic masses ranging from 112 to 151. 133Cs is the only stable isotope.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
112Cs 111.95030(33)# 500(100) µs p to 111Xe; a to 108I 1+# N/A 889.68 -
113Cs 112.94449(11) 16.7(7) µs p to 112Xe; ß+ to 113Xe 5/2+# N/A 907.08 -
114Cs 113.94145(33)# 0.57(2) s ß+ to 114Xe; ß+ + p to 113I; ß+ + a to 110Te; a to 110I (1+) N/A 915.16 -
115Cs 114.93591(32)# 1.4(8) s ß+ to 115Xe; ß+ + p to 114I 9/2+# N/A 932.55 -
116Cs 115.93337(11)# 0.70(4) s ß+ to 116Xe; ß+ + p to 115I; ß+ + a to 112Te (1+) N/A 940.63 -
117Cs 116.92867(7) 8.4(6) s ß+ to 117Xe (9/2+)# N/A 958.03 -
118Cs 117.926559(14) 14(2) s ß+ to 118Xe; ß+ + p to 117I; ß+ + a to 114Te 2 N/A 966.1 -
119Cs 118.922377(15) 43.0(2) s ß+ to 119Xe; ß+ + a to 115Te 9/2+ N/A 974.18 -
120Cs 119.920677(11) 61.2(18) s ß+ to 120Xe; ß+ + a to 116Te; ß+ + p to 118I 2(-#) N/A 982.26 -
121Cs 120.917229(15) 155(4) s ß+ to 121Xe 3/2(+) N/A 999.66 -
122Cs 121.91611(3) 21.18(19) s ß+ to 122Xe; ß+ + a to 118Te 1+ N/A 1007.74 -
123Cs 122.912996(13) 5.88(3) min ß+ to 123Xe 1/2+ N/A 1015.81 -
124Cs 123.912258(9) 30.9(4) s ß+ to 124Xe 1+ N/A 1023.89 -
125Cs 124.909728(8) 46.7(1) min ß+ to 125Xe 1/2(+) N/A 1041.29 -
126Cs 125.909452(13) 1.64(2) min ß+ to 126Xe 1+ N/A 1049.37 -
127Cs 126.907418(6) 6.25(10) h ß+ to 127Xe 1/2+ N/A 1057.45 -
128Cs 127.907749(6) 3.640(14) min ß+ to 128Xe 1+ N/A 1065.52 -
129Cs 128.906064(5) 32.06(6) h EC to 129Xe 1/2+ 1.49 1073.6 -
130Cs 129.906709(9) 29.21(4) min EC to 130Xe; ß- to 130Ba 1+ 1.46 1081.68 -
131Cs 130.905464(5) 9.689(16) d EC to 131Xe 5/2+ 3.54 1089.76 -
132Cs 131.9064343(20) 6.480(6) d EC to 132Xe; ß- to 132Ba 2+ 2.22 1097.84 -
133Cs 132.905451933(24) STABLE - 7/2+ 2.582024 1105.92 100
134Cs 133.906718475(28) 2.0652(4) y EC to 134Xe; ß- to 134Ba 4+ 2.994 1114 -
135Cs 134.9059770(11) 2.3(3)E+6 y ß- to 135Ba 7/2+ 2.732 1122.08 -
136Cs 135.9073116(20) 13.16(3) d ß- to 136Ba 5+ 3.71 1130.15 -
137Cs 136.9070895(5) 30.1671(13) y ß- to 137Ba 7/2+ 2.84 1138.23 -
138Cs 137.911017(10) 33.41(18) min ß- to 138Ba 3- N/A 1136.99 -
139Cs 138.913364(3) 9.27(5) min ß- to 139Ba 7/2+ N/A 1145.07 -
140Cs 139.917282(9) 63.7(3) s ß- to 140Ba 1- N/A 1153.15 -
141Cs 140.920046(11) 24.84(16) s ß- to 141Ba; ß- + n to 140Ba 7/2+ N/A 1151.91 -
142Cs 141.924299(11) 1.689(11) s ß- to 142Ba; ß- + n to 141Ba 0- N/A 1159.99 -
143Cs 142.927352(25) 1.791(7) s ß- to 143Ba; ß- + n to 142Ba 3/2+ N/A 1168.07 -
144Cs 143.932077(28) 994(4) ms ß- to 144Ba; ß- + n to 143Ba 1(-#) N/A 1166.83 -
145Cs 144.935526(12) 582(6) ms ß- to 145Ba; ß- + n to 144Ba 3/2+ N/A 1174.91 -
146Cs 145.94029(8) 0.321(2) s ß- to 146Ba; ß- + n to 145Ba 1- N/A 1173.68 -
147Cs 146.94416(6) 0.235(3) s ß- to 146Ba; ß- + n to 147Ba (3/2+) N/A 1181.75 -
148Cs 147.94922(62) 146(6) ms ß- to 147Ba; ß- + n to 146Ba N/A N/A 1189.83 -
149Cs 148.95293(21)# 150# ms [>50 ms] ß- to 148Ba; ß- + n to 147Ba 3/2+# N/A 1188.6 -
150Cs 149.95817(32)# 100# ms [>50 ms] ß- to 149Ba; ß- + n to 148Ba N/A N/A 1196.67 -
151Cs 150.96219(54)# 60# ms [>50 ms] ß- to 150Ba; ß- + n to 149Ba 3/2+# N/A 1195.44 -