Cesium (or caesium) is a soft, shiny, pale gold metal with the chemical symbol Cs and atomic number 55. It is the most electropositive and least electronegative element using the Pauling scale. Cesium melts just above room temperature (28.5 °C or 83.3 °F), meaning you could theoretically melt it in your hand (but do not try this, as it reacts explosively with moisture).
Discovered in 1860 through flame spectroscopy, cesium is best known for its role in atomic clocks, oil drilling, cancer therapy, and radioactive tracing. Here’s a complete look at this fascinating alkali metal.
Key Takeaways: Cesium Facts
- Element Name: Cesium (American) / Caesium (IUPAC)
- Symbol: Cs
- Atomic Number: 55
- Category: Alkali metal (Group 1)
- Color: Pale gold
- Melting Point: 28.5 °C (83.3 °F) — a liquid metal at body temperature
- Most notable use: Cesium-133 defines the SI second in atomic clocks
- Highly reactive: Ignites in air, explodes in water
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 and Naming
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 of Cesium
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 of Cesium
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
Covalent Radius: 244±11 pm
Atomic Volume: 70.0 cc/mol
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
⏱ Cesium and Atomic Clocks
- Cesium-133 defines the second: One second = 9,192,631,770 cycles of radiation from Cs-133
- Cesium clocks are accurate to 1 second in millions of years.
- Used in GPS, telecommunications, and time standards.
☢️ Environmental Impact and Remediation
- Cesium-137 from nuclear accidents (e.g., Chernobyl, Fukushima) contaminates soil.
- Immobilized using phytoremediation and clay minerals (e.g., illite).
- Soil monitoring uses Cs-137 as a dating tool for post-1950s sediment layers.
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.
References
- 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