Discovered in 1861 by R.W. Bunsen and G. Kirchoff in Heidelberg, Germany. Rubidium is a soft and highly reactive member of the alkali group of metals. It is widely distributed throughout nature, but only occurs in small amounts (its abundance in the earth's crust is 310 ppm), the main source being carnallite, the hydrated chloride of magnesium and potassium. The metal is obtained by the electrolysis of the fused halide, although it can be obtained on a laboratory scale by heating the chloride with calcium metal and distilling out the metal. The metal ignites spontaneously in air and reacts violently with water and applications are limited.