Rubidium

Rubidium is a soft, silvery metal that melts at about 39 °C and bursts into flame in air. Robert Bunsen and Gustav Kirchhoff discovered it in 1861 with a spectroscope, naming it after the deep red lines in its light. It occurs in small amounts in minerals such as lepidolite and pollucite, and is recovered as a byproduct of lithium and cesium production.

Rubidium has few large uses, but some important small ones. Rubidium atomic clocks are compact and precise, and they help keep time in satellite navigation and phone networks. Physicists also chill rubidium atoms almost to absolute zero to study strange quantum states of matter.