Definition:Caesium
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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.
It is pronounced see-zee-um.