Rubidium is a very soft, silvery alkali metal that ignites in air and reacts violently with water.
Rubidium has the ground-state configuration \([Kr]5s^1\). The single valence electron in the 5s orbital is weakly held (low ionization energy), so Rb readily loses it to form Rb+, making rubidium highly reactive like other alkali metals.
With water, rubidium reacts violently, generating heat, hydrogen gas, and a strongly basic solution:
\(\mathrm{2\,Rb(s) + 2\,H_2O(l) \rightarrow 2\,RbOH(aq) + H_2(g)\uparrow}\)
In air, Rb rapidly forms oxides; heavier alkali metals (K, Rb, Cs) favor superoxides:
\(\mathrm{Rb(s) + O_2(g) \rightarrow RbO_2(s)}\)
Hence it must be stored under dry mineral oil or in inert atmospheres.
Rubidium gives a characteristic reddish-violet (red-purple) flame. Heating excites the valence electron; as it relaxes, it emits light at specific wavelengths corresponding to Rb atomic lines (notably in the red region).
With halogens:
\(\mathrm{2\,Rb + Cl_2 \rightarrow 2\,RbCl}\)
With dilute acids (vigorous):
\(\mathrm{2\,Rb + 2\,HCl \rightarrow 2\,RbCl + H_2\uparrow}\)
All reactions are highly exothermic; perform only as thought experiments in the classroom—never attempt in open lab settings.
Rb-85 (stable) and Rb-87 (partly radioactive, very long half-life) are the most abundant. Rb-87 undergoes beta decay to \(^{87}\!\mathrm{Sr}\), enabling the Rb–Sr radiometric dating method for ancient rocks. Rb-87 is also widely used in cold-atom physics and atomic clocks.
They lock a microwave oscillator to the hyperfine transition of Rb-87 in its ground state (frequency \(\nu \approx 6.834\,\text{GHz}\)). The resonance condition is enforced by optical pumping and microwave interrogation, yielding a highly stable time/frequency standard.
Ultra-cold Rb-87 atoms were among the first gases to reach Bose–Einstein condensation. Laser cooling on the D-lines (near 780–795 nm) and magnetic/evaporative cooling bring the gas to nanokelvin temperatures, creating a macroscopic quantum state.
Elemental Rb’s main hazards are chemical reactivity and fire/explosion risk with air/water. Salts of Rb have low acute toxicity but should be handled as laboratory chemicals. Never touch Rb metal; keep it under dry mineral oil/inert gas, use non-sparking tools, and maintain Class D extinguishers (do not use water/CO2 on alkali metal fires).
Rubidium occurs in trace amounts in minerals like lepidolite and pollucite, often mixed with cesium and potassium. It is obtained as a by-product during lithium/cesium processing and purified by fractional crystallization/ion exchange, then reduced from Rb salts to the metal.