Césium

Cäsium (Cs, atomic number 55) is a soft, silvery‑gold alkali metal found in group 1 of the periodic table. It has a very low melting point of 28.5°C and a density of about 1.93 g/cm³. It is highly reactive, readily igniting in air and reacting vigorously with water to form cesium hydroxide and hydrogen gas. Because of its reactivity, elemental cesium is typically stored under mineral oil or in an inert atmosphere.

Occurrence and production: Cesium is present in trace amounts in the Earth's crust and does not occur as a free element. The principal commercial sources are cesium-bearing minerals, particularly pollucite. Commercial processing yields cesium salts such as cesium chloride, which can be converted to metallic cesium by electrolysis. Major producing regions include Canada, Namibia, and Zimbabwe.

Isotopes: The only stable isotope is Cs‑133; many radioisotopes exist, with Cs‑137 being the most notable in industry and medicine. Cs‑137 is produced by nuclear fission and has a half-life of about 30 years. Radioactive cesium isotopes are used in radiography, sterilization, and cancer therapy in some applications, but require stringent handling and shielding.

Applications: The principal use of cesium is as the time standard for the definition of the second, based on the hyperfine transition of Cs‑133 in atomic clocks. Cesium compounds serve in various chemical syntheses and in drilling fluids. Radioactive cesium sources are employed in industrial radiography and medical therapies, where appropriate containment is essential.

Safety and history: Cesium is highly reactive and caustic; contact with moisture or air requires careful handling and appropriate safety measures. It was discovered in 1860 by Bunsen and Kirchhoff through flame spectroscopy and was named from the Latin caesius, meaning blue, in reference to its spectral lines.