Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium is an alkali metal and has physical and chemical properties similar to those of rubidium and potassium.
Three reasons Cesium stands out
- 1
It's used in timekeeping, as the atomic clock element of choice.
- 2
Atomic clocks using this element are accurate to within one second over 300 million years.
- 3
It reacts even more explosively with water than sodium, producing a violent burst of heat and hydrogen gas.
Keeping time with cesium
The second is defined using a microwave transition in cesium-133 atoms, turning this reactive metal into the foundation of modern timekeeping. Cesium melts near room temperature and reacts violently with water. In vacuum devices and atomic clocks, those difficult properties are controlled rather than encountered casually.