Rubidium

A grayish mica called lepidolite was discovered in the 1760s. It would subsequently be shown to contain lithium and potassium, and 101 years later, rubidium.

In 1861, rubidium was discovered by two famed German scientists—Robert Bunsen, inventor of the Bunsen burner, and Gustav Kirchhoff, a pioneer in electric circuits and “blackbody” radiation from hot objects. Both were experts in flame spectroscopy, a way to analyze a sample based on what colors are emitted when the sample is burned. Each element has a unique color signature. They saw that lepidolite emitted multiple bright ruby-red colors that had not been seen before. They figured out that it was a new element and named it rubidium. (Rubidium and ruby have the same Latin root, rubidus, referring to the deep-red color.)

Isolating rubidium from lepidolite proved quite complicated and challenging. But Bunsen and Kirchhoff succeeded… sort of. Rubidium spontaneously ignites and oxidizes in air, so they managed to isolate it from one compound only to immediately create a new one, an oxide. Still, they were able to measure the atomic weight, the density, and the melting point all to within a hair of their modern values. Pure rubidium metal wasn’t obtained until 1928.

Rubidium melts to a liquid metal at a little above body temperature. It is a soft, silvery-white, and ductile, being very pliable. It is the second-most electropositive element (readily donating electrons in bonding) after cesium, making it highly reactive.

It would be exciting (and dangerous) to have a sample of pure rubidium metal because of two major oxidation reactions. First, it ignites spontaneously in air. Second, like potassium and cesium (above and below it on the periodic table), it reacts violently in water. It bonds with a hydrogen and oxygen atom from the H₂O, making RbOH, and sets fire to the remaining hydrogen! Therefore, storing rubidium is tricky. It must be kept in mineral oil or in an inert atmosphere.

Rubidium isn’t produced in great quantities, and its most important use is probably for physics research. However, there are some practical applications. For example, some atomic clocks use rubidium instead of cesium. Although cesium is considered a better timekeeper, rubidium atomic clocks paired with GPS have excellent accuracy at lower cost. They are frequently used in the telecommunications industry.

Rubidium is useful in medicine, too. It is processed in the body like potassium (which lies directly above it on the periodic table) and is faintly radioactive, so it can be used to track where potassium is absorbed. It can also be used to locate brain tumors because it collects in tumors more than in regular cells. Rubidium can also be used to make special types of glass and gives fireworks a red-purple color.

The largest source of rubidium comes from the mineral pollucite. Pollucite ore is particularly abundant at Bernic Lake in Manitoba, Canada. The ore is primarily used as a source of cesium, but rubidium is also obtained as a byproduct. Similarly, rubidium is recovered as a byproduct of lithium extraction from lepidolite. It can also be sourced from other potassium minerals and brines.