Rubidium

Rubidium is an alkali metal with a single electron outside a krypton core ([Kr]5s1). That electron sits far from the nucleus and is well shielded, so it takes only 4.18 electronvolts to remove it, less than for any lighter alkali metal. As a result rubidium is extremely reactive: it ignites spontaneously in air and reacts explosively with water. The metal is soft and silvery-white and melts at just 39.3 °C, barely above body temperature. Its compounds contain the Rb+ ion, which behaves much like K+.

Rubidium makes up about 90 ppm of the crust, more than copper, yet it forms no minerals of its own, hiding instead in potassium minerals such as lepidolite. Its electrons have a hyperfine transition at a precisely fixed frequency of about 6.8 GHz, which is the basis of rubidium atomic clocks in GPS satellites and telecommunications networks. Rubidium atoms are easy to cool with lasers, so they became the workhorse of ultracold physics; the first Bose–Einstein condensate in 1995 was made from rubidium-87.

Discovery

Robert Bunsen and Gustav Kirchhoff discovered rubidium in 1861 at Heidelberg, using the flame spectroscope they had invented a year earlier. Analyzing the mineral lepidolite, they saw two deep red spectral lines no one had recorded before, and named the element after the Latin rubidus, deep red. Bunsen later prepared the metal by heating a rubidium compound with carbon.