Definition:Rubidium
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Chemical Element
Rubidium is the chemical element that has $37$ protons in the nuclei of its atoms
Its chemical symbol is $\text {Rb}$.
Isotopes
Rb-$87$
Rubidium-$87$ is the isotope of rubidium which has $50$ neutrons in addition to its $37$ protons.
Rb-$87$ is radioactive, with a half-life of approximately $48.8 \times 10^9$ years.
Linguistic Note
The word rubidium derives from the Latin word rubidus, meaning deep red, after the colour of the lines in its emission spectrum.