Cs
132.910Cesium
Sezyum
Cesium is a very soft alkali metal with atomic number 55. It is famous for the cesium atomic clock definition of the second, its low melting point, high reactivity, photoelectric behavior and radioactive cesium-137 fallout concerns.
55
132.91 g/mol
[Xe] 6s¹
2 | 8 | 18 | 18 | 8 | 1
0.79
1.873 g/cm³
28.44 °C
670.9 °C
Robert Bunsen
Cesium was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff using spectroscopy. Its name comes from caesius, Latin for sky-blue, because of the blue spectral lines that revealed it.
Cesium-133
Natural abundance: 100%. Only stable natural cesium isotope; used in the SI second definition.Cesium-137
Half-life: About 30.1 years. Radioactive fission product and important long-lived fallout contaminant.CsCl
Cesium ChlorideDensity-gradient centrifugation and research chemistry.CsI(Tl)
Thallium-Doped Cesium IodideScintillation detector crystals for radiation measurement.Cs₂CO₃
Cesium CarbonateOrganic synthesis base and electronics materials.CsOH
Cesium HydroxideVery strong base used mainly in specialized chemistry.- Cesium atomic clocks that define the SI second through the hyperfine transition of cesium-133
- Photoelectric cathodes and photomultiplier applications
- Cesium-137 gamma sources for gauges, calibration and some older medical or industrial uses
- Cesium iodide scintillation crystals for radiation detection
- High-density cesium formate brines for oil and gas drilling fluids
Cesium has no essential biological role. Cs⁺ can behave somewhat like K⁺ in organisms, and radioactive cesium-137 can spread through soft tissues after nuclear contamination.
- The SI second is defined by 9,192,631,770 cycles of radiation from a cesium-133 hyperfine transition.
- Cesium melts just above room temperature, around 28.5 °C.
- Cesium reacts violently with water and must be stored away from air and moisture.
- Cesium-137 contamination became a major monitoring concern after nuclear accidents and weapons fallout.
- Cesium has atomic number 55 and belongs to group 1, the alkali metals.
- Its electron configuration is [Xe] 6s¹.
- Cesium commonly forms +1 compounds.
- For memory: Cs-133 defines the second, while Cs-137 is the fallout isotope.
What is the atomic number of cesium?
The atomic number of cesium is 55.
What is the electron configuration of cesium?
Cesium has the electron configuration [Xe] 6s¹.
What is cesium used for?
Cesium is used in atomic clocks, photoelectric devices, radiation detectors, drilling fluids and specialized compounds.
Why is cesium used in atomic clocks?
Cesium-133 has a precisely defined hyperfine transition frequency, which is used to define the SI second.
Is cesium radioactive?
Natural cesium is stable cesium-133, but radioactive cesium-137 is an important fission product and contamination concern.
Is cesium the same as caesium?
Yes. Cesium is the American spelling, while caesium is the British spelling.