Cesium Facts – Caesium or Cs

Cesium Facts - Caesium
Cesium or caesium is an alkali metal with atomic number 55 and element symbol Cs.

Cesium or caesium is a shiny, pale gold metal with element symbol Cs and atomic number 55. Cesium is the softest metal that is a solid at room temperature. It becomes a liquid element at 28.5 °C or ​83.3 °F. Like gallium, it is a metal you can melt in the palm of your hand (but please don’t). Here is a collection of interesting cesium facts, including the element’s properties, uses, and sources.

Basic Cesium Facts

Name: Cesium (ASC, American) or Caesium (IUPAC, international)

Symbol: Cs

Atomic Number: 55

Atomic Weight: 132.905451

Group: group 1 (alkali metal)

Period: period 6

Block: s-block

Electron Configuration: [Xe] 6s1

Electrons per Shell: 2, 8, 18, 18, 8, 1

Discovery: Robert Bunsen and Gustav Kirchhoff discovered cesium in 1860 in mineral water. They discovered the element using a spectroscope and identified the element by its sky blue emission spectrum lines. Carl Setterberg first isolated the pure element in 1882.

Name Origin: Cesium comes from the Latin word caesius, which means “sky blue.” This refers to the element’s sky blue spectra lines.

Isotopes: There are 39 isotopes of cesium, ranging in mass numbers from 112 to 151. The only stable isotope is cesium-133. The isotope with the longest half-life is cesium-135, with a half-life of 2.3 million years. Cesium-137 is a noteworthy isotope produced by nuclear weapons testing.

Biological Role and Toxicity: Cesium does not serve any known biological function in any organism. The non-radioactive isotope of cesium is mildly toxic. For example, cesium chloride is approximately as toxic as sodium chloride (table salt) or potassium chloride. Symptoms of toxicity include low blood pressure, upset stomach, and tingling lips. Excessive intakes of cesium lead to hypokalemia (low potassium), hypomagnesia (low magnesium), tachycardia (fast heart rate), and sometimes heart attack. Touching pure cesium is ill-advised because it ignites or reacts explosively with water. Radioactive isotopes do not readily accumulate in the body, but are very dangerous. Experiments with dogs indicate 4.1 μg of cesium-137 per kilogram of body weight is lethal. Lower doses can cause infertility and cancer.

Uses: The main use of cesium-137 is in the petroleum industry as a tracer in drilling fluids. Cesium-137 is also used in food irradiation, cancer treatment, atomic clocks, as a catalyst, in photoelectric cells, and as a getter in vacuum tubes. The non-radioactive isotope and its compounds find use in specialty glass, infrared flares, and beer brewing.

Sources: Cesium is a relatively rare element that occurs with an abundance of about 3 parts per million in the Earth’s crust. This makes it the 45th most abundant element in the crust. Because of the atom’s large size, it gets excluded from many mineral crystals. The few minerals that contain cesium include pollucite, sylvite, carnallite, beryl, avogadrite, pezzottaite, rhodizite, and londonite. The richest source of cesium-bearing ore is the Tanco Mine in Manitoba, Canada. Other sources are the Bikita deposit in Zimbabwe and the Karibib Desert in Namibia. From pollucite, extraction steps are acid digestion, alkaline decomposition, or direct reduction.

Physical Data

Element Classification: alkali metal

Density: 1.93 g/cm3

Melting Point: 301.7 K ​(28.5 °C, ​83.3 °F)

Boiling Point: 944 K ​(671 °C, ​1240 °F)

Phase at STP: solid

Appearance: pale gold metal

Fusion Heat: 2.09 kJ/mol

Molar Heat Capacity: 32.210 J/(mol·K)

Crystal Structure: body-centered cubic (bcc)

Thermal Expansion: 97 µm/(m⋅K) (at 25 °C)

Thermal Conductivity: 35.9 W/(m⋅K)

Electrical Resistivity: 205 nΩ⋅m (at 20 °C)

Magnetic Ordering: paramagnetic

Young’s Modulus: 1.7 GPa

Bulk Modulus: 1.6 GPa

Mohs Hardness: 0.2

Brinell Hardness: 0.14 MPa

Atomic Data

Atomic Radius: 265 pm

Atomic Volume: 70.0 cc/mol

Covalent Radius: 244±11 pm

Van der Waals Radius: 343 pm

Ionic Radius: 167 (+1e)

Pauling Electronegativity: 0.79

First Ionization Energy: 375.7 kJ/mol

Oxidation States: -1, +1

Interesting Cesium Facts

  • Most people think the only element that is gold-colored is gold. However, cesium has a distinct golden cast.
  • Cesium is highly reactive. It spontaneously ignites in air and explodes upon contact with water. For this reason, people store cesium in sealed containers, within an inert gas or liquid or a vacuum.
  • Cesium is so soft and ductile that it easily draws into extremely fine wires.
  • While melting cesium is your hand is far too dangerous, you can hold a container of solid cesium in your hand. Also, as the metal cools it forms beautiful crystals.
  • Cesium is the most electronegative element on the periodic table, using the Allen electronegativity scale. Depending on conditions, it also has the largest covalent radius.
  • Cesium hydroxide (CsOH) is such a strong base that it eats through glass.
  • In 2009, the price for 99.8% pure cesium was about $10 per gram or $280 per ounce.
  • Cesium-137 released during nuclear weapons testing settled into soil and is reliable marker of “modern” sediment.


  • Bick, Manfred; Prinz, Horst (2005). “Cesium and Cesium Compounds” in Ullmann’s Encyclopedia of Industrial Chemistry 2005. Weinheim: Wiley-VCH. doi:10.1002/14356007.a06_153
  • Greenwood, N. N.; Earnshaw, A. (1984). Chemistry of the Elements. Oxford, UK: Pergamon Press. ISBN 978-0-08-022057-4.
  • Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.
  • Melnikov, P.; Zanoni, L. Z. (June 2010). “Clinical effects of cesium intake”. Biological Trace Element Research. 135 (1–3): 1–9. doi:10.1007/s12011-009-8486-7
  • Weeks, Mary Elvira (1932). “The discovery of the elements. XIII. Some spectroscopic discoveries”. Journal of Chemical Education. 9 (8): 1413–1434. doi:10.1021/ed009p1413