Cesium
Atomic Data
| Atomic Number | 55 |
| Symbol | Cs |
| Atomic Weight | 132.91 u |
| Density (STP) | 1.873 g/cm³ |
| Melting Point | 28.44 °C (301.59 K) |
| Boiling Point | 670.85 °C (944 K) |
| Electronegativity | 0.79 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1 |
| Oxidation States | +1 |
| Phase at STP | Solid |
| Category | Alkali Metal |
| Period / Group | 6 / 1 |
| CAS Number | 7440-46-2 |
Electron Configuration
[Xe] 6s1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| O | 5 | 5s | 2 | 48 |
| O | 5 | 5p | 6 | 54 |
| P | 6 | 6s | 1 | 55 |
| Total | 55 | 55 | ||
Isotopes of Cesium
Cesium has two naturally occurring stable isotopes. The most abundant is ¹³³Cs, comprising 100% of all naturally occurring Cesium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Cesium-133 | ¹³³Cs | 55 | 78 | 100 | Stable |
| Cesium-137 | ¹³⁷Cs | 55 | 82 | trace | Stable |
Abundance & Occurrence
Cesium is present in Earth's crust at approximately 3 ppm by mass and at approximately 0.1 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Extreme water reactivity: Caesium reacts explosively with water: even more violently than rubidium or potassium: generating hydrogen gas, caesium hydroxide, and sufficient heat to ignite the hydrogen instantly; contact with even trace moisture can be fatal.
- Spontaneous ignition in air: Caesium ignites spontaneously in air and must be handled exclusively in an inert atmosphere (dry argon or helium glove box); contact with air must be prevented at all times.
- Severe caustic burns: Caesium hydroxide (CsOH) is one of the strongest bases known; it causes rapid, deep alkaline burns to skin, eyes, and mucous membranes.
- Caesium-137: radiation: Cs-137 (t½ = 30 years, beta/gamma) is a major fission product and significant radiological hazard; it is responsible for most of the long-term contamination after nuclear accidents (Chernobyl, Fukushima) and orphaned source incidents (Goiânia accident, 1987).
- Storage: Caesium metal is stored under dry mineral oil or inert gas in sealed, clearly labelled containers; inventories must be maintained and orphaned sources reported to the national nuclear regulator.
Cesium in the Real World
Real-World Uses
- Atomic clocks and GPS: Caesium-133 defines the SI second: one second is exactly 9,192,631,770 oscillations of the Cs-133 hyperfine transition. Primary caesium atomic clocks underpin GPS, the internet, financial transaction timestamps, and global telecommunications synchronisation.
- Oil and gas drilling fluids: Caesium formate brine (density up to 2.3 g/mL) is used as a high-density, solids-free drilling and completion fluid in high-pressure, high-temperature (HPHT) oil and gas wells.
- Photoelectric cells: Caesium metal coatings on photocathodes in photomultiplier tubes exploit the photoelectric effect at low work function energies to detect UV and visible photons in scientific instruments and night-vision image intensifiers.
- Cancer radiation therapy: Caesium-137 (t½ = 30.2 yr) sealed sources are used in brachytherapy for treating gynaecological cancers and in teletherapy machines in countries where linear accelerators are unavailable.
- Optical systems research: Caesium vapour cells are used in quantum optics experiments, atomic magnetometers, and the development of optical quantum memory for quantum communication networks.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is caesium used for?
Caesium's most important application is in atomic clocks: the SI definition of the second is based on the transition frequency of caesium-133 atoms. Caesium-based atomic clocks are the world's most accurate timekeepers and are the foundation of GPS, mobile networks, and financial transaction timestamps. Caesium is also used in drilling fluids for oil and gas wells (caesium formate) and in photocathodes.
Is caesium the most reactive metal?
Caesium is among the most reactive of all metals. It ignites spontaneously in air, reacts explosively with water, and must be stored sealed under an inert atmosphere. Its reaction with water is more violent than that of any other alkali metal because of its large atom size and very low ionisation energy. Francium is theoretically more reactive but is too rare to observe macroscopically.
How was caesium discovered?
Caesium was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff using flame spectroscopy: the same method they used to discover rubidium. They detected two distinctive blue spectral lines in the mineral water from Dürkheim, Germany, that matched no known element. Caesium was the first element discovered by spectroscopy. Its name comes from the Latin 'caesius', meaning sky blue.
How does a caesium atomic clock define the second?
The SI second is defined as exactly 9,192,631,770 oscillations of the radiation corresponding to the transition between the two hyperfine ground states of caesium-133. A caesium atomic clock works by microwave-irradiating a beam of Cs-133 atoms and measuring when the frequency of the radiation maximally excites the hyperfine transition. Modern caesium fountain clocks are accurate to about one second in 300 million years.