Cesium is a shiny, silvery-gold metallic element. Its atomic number is 55 and its symbol is Cs. It belongs to a group of elements called the alkali metals. Robert Bunsen and Gustov Kirchhoff from Germany discovered it in 1860 when they were studying the minerals left by the evaporation of mineral waters. Pure cesium metal, however, was not prepared until 1882 by another scientist named Setterburg.

The physical properties of cesium are very interesting. It is the softest of all metals and can even be cut with a knife. Some describe it as being like wax. Cesium is one of three metals that are liquid at or near room temperature (the other two are gallium and mercury). It is also a very reactive metal. For instance, when it mixes with cold water, there is an explosive reaction. Its melting point is so low that it will melt if it is held in the hand. However, because it is so reactive, it can seriously burn the skin, so it must be handled with great care. Cesium hydroxide is the strongest base known.

Only a few thousand kilograms of cesium is used each year. This fact, plus the fact that cesium is so reactive to air and water, results in very high prices for cesium and cesium compounds.

Cesium is not beneficial to animals or plants.