Cesium
Element 55 · Alkali Metal
Cesium sits at the bottom of the alkali metal group, and that position shows: it has a single loosely held outer electron, so it reacts explosively with water and tarnishes in air within seconds. It is so soft it can be cut with a knife, and it melts at just 28.4 °C — warm enough that a sample can melt in a closed hand.
Because cesium atoms vibrate at an extremely steady frequency, they became the heartbeat of atomic clocks. The international definition of the second is anchored to the vibrations of cesium-133 atoms, which is why ultra-precise timekeeping, GPS satellites, and physics laboratories all depend on this metal.
Key facts
| Atomic number | 55 |
|---|---|
| Atomic weight | 132.91 |
| Category | Alkali Metal |
| Group / Period | 1 / 6 |
| Block | s |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1 |
| Common oxidation states | +1 |
| State at STP | Solid |
| Electronegativity | 0.79 |
| Density | 1.93 g/cm³ |
| Melting point | 28.44 °C |
| Boiling point | 671 °C |
Occurrence
Cesium never occurs free in nature. It is extracted mainly from the mineral pollucite and from salty brines, and it is one of the scarcer alkali metals in Earth's crust.
Applications
Atomic clocks and time standards
Dense drilling fluids for oil wells (cesium formate brines)
Photoelectric cells and vacuum tubes
Ion propulsion research for spacecraft
Common compounds
- cesium chloride
- cesium hydroxide
- cesium carbonate
- cesium formate
A cesium-133 atomic clock is so steady it would lose less than a second in millions of years — its atomic vibrations officially define the length of the second.
Data source: IUPAC standard atomic weights 2021