Alkali Metals

Rubidium (Rb), the fourth element in the alkali metal group, has an atomic number of 37 and an atomic weight of 85.4678 amu. Its melting point is 102.8°F (39.31°C), and its boiling point is 1,270.4°F (688°C). Cesium (Cs), the second to last element in the group, has an atomic number of 55, an atomic weight of 132.9054 amu, a melting point of 83.12°F (28.40°C), and a boiling point of 1,239.8°F (671°C). Both were discovered in 1860-1861 by R.W. Bunsen and G.R. Kirchoff. They were the first two elements to be discovered with a spectroscope.

Both rubidium and cesium are rare in the earth's crust (rubidium at 90 parts per million and cesium only 3 parts per million.) The main mineral in which cesium can be found is pollucite. Rubidium can be found in pollucite, lepidolite, and carnallite. Rubidium is used almost exclusively for research, but cesium has some highly specialized industrial uses, including special glasses and radiation detection equipment.