# Definition:Caesium

## Chemical Element

Caesium is the chemical element that has $55$ protons in the nuclei of its atoms.

Its chemical symbol is $\text {Cs}$.

## Isotopes

### Cs-$133$

Caesium-$133$ is the isotope of caesium which has $78$ neutrons in addition to its $55$ protons.

Cs-$133$ is the only stable isotope of caesium.

## Linguistic Note

The word caesium derives the name from the Latin word caesius, meaning sky-blue. from the bright blue lines in its emission spectrum.

In informal US English, the word is spelt cesium.