Category: Alkali metal
Atomic Number: 55
Period: 6 | Group: 1
Block: s-block
The most reactive metal, the softest, and the atom that defines the second.
| Property | Value |
|---|---|
| Phase at STP | Solid |
| Density | N/A |
| Melting Point | 301.59 K (28.44 °C) |
| Boiling Point | 944.00 K (670.85 °C) |
| Heat Capacity | N/A |
| Property | Value |
|---|---|
| Atomic Mass | 132.9055 u |
| Electronegativity (Pauling) | N/A |
| Oxidation States | -1, +1 |
| Electron Configuration |
| Discovered By | Unknown |
|---|---|
| Discovery Year | 1860 |
| Named After | Latin caesius, "sky blue" |
Found with the spectroscope (1860)
In 1860 Robert Bunsen and Gustav Kirchhoff in Heidelberg evaporated large volumes of mineral water from Dürkheim and examined the flame of the residue with their new spectroscope. They saw two bright blue lines that matched no known element.
The name
They named the element after the Latin caesius, sky blue, for the color of those lines. Cesium was the first element found with the spectroscope; rubidium followed in 1861.
The metal (1882)
Bunsen could not isolate the metal. Carl Setterberg first obtained it in 1882 by electrolysis of molten cesium cyanide.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Cs-133 | 78 | 100% | Stable | - |
| Cs-134 | 79 | - | 2.0652 yrs | β⁻ |
| Cs-137 | 82 | - | 30.08 yrs | β⁻ |
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Atomic clocks
Since 1967 the second has been defined by cesium-133: it is the duration of 9,192,631,770 periods of the radiation emitted in a transition between two energy levels of the cesium atom. Cesium atomic clocks keep national time standards, and GPS and other navigation satellites carry cesium and rubidium clocks.
Catalysts and drilling fluids
By quantity, cesium carbonate and cesium hydroxide, used mainly as catalysts, are the leading cesium products, followed by cesium formate brine, a dense, reusable fluid used when drilling high-pressure, high-temperature oil and gas wells (USGS).
Other uses
Electronics and optics: cesium is used in photoelectric cells, photomultiplier tubes and night vision devices.
Chemistry: cesium compounds serve as catalysts and in specialty glasses.
Radiation sources: cesium-137 has been used in cancer radiotherapy, blood irradiators and industrial gauges, though X-ray sources are replacing it because of security concerns.
| Earth's Crust | N/A |
|---|---|
| Universe | N/A |