Cesium is a Group IA or alkali metal in the 6th period. Other members of the group are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and francium (Fr).
Cs was discovered in 1860 by Bunsen and Kirchhoff, and was the first element discovered spectroscopically. The name, from the Latin word caesius for "sky blue", refers to the characteristic blue lines in the emission spectrum of the element.
Pure Cs is a silvery-white, soft, ductile metal. Of the five Group IA metals it:
Cs, with a melting point of only 28.6°C, is one of only three metals - Hg, Cs, and Ga - that can easily be liquids at room temperature.
Cs is widely distributed in the earth's crust, but at very low concentrations. However, it is an important constituent of the mineral pollucite, a hydrated cesium aluminosilicate. One of the world's richest sources is in Manitoba, Canada where there are an estimated 300,000 tons of pollucite averaging 20% cesium.
The element is prepared by the reduction of one of its salts, often CsCl, with an electropositive metal, usually Ca.
2 CsCl + Ca CaCl2 + 2 Cs
The element is similar in its reactivity to potassium and rubidium, except that it is oxidized much more readily than any other Group IA metal. It reacts rapidly with air to give the monoxide (Cs2O), the peroxide (Cs2O2), but primarily the superoxide (CsO2,).
Cs(s) + O2(g) CsO2(s)
Formation of peroxides is characteristic of potassium and rubidium as well.
The element reacts violently with water to give cesium hydroxide, CsOH, and H2.
2 Cs(s) + 2 H2O(liq) H2(g) + 2 CsOH(aq)
The hydroxide is the strongest base known; it readily attacks glass.
Salts of cesium are similar to those of other alkali metals and are generally quite water-soluble.
Because it reacts so readily with air, Cs is used as a "getter" in vacuum tubes (see also rubidium). Furthermore, since it is so electropositive, it has been used in photocells.
The element is also the basis of so-called "atomic clocks", which are accurate to 5 seconds in 300 years and serve as the international time standard.
The principal use of the element has been in research on ion propulsion engines for use in outer space. It is known that, in principle, 1 lb of Cs will propel a vehicle 140 times farther than by burning the same quantity of any other known liquid or solid.