Cesium was the first chemical element identified through its spectral lines, in 1860.
German scientists Robert Bunsen and Gustav Kirchhoff discovered cesium by examining mineral-water samples with a spectroscope. They observed two previously unknown blue spectral lines, which led them to infer the presence of a new element. The name cesium comes from the Latin caesius, meaning sky blue.
Cesium is an alkali metal with atomic number 55. It is soft, pale gold in appearance and highly reactive, especially with water. Because it has a very low melting point, it can become liquid in a warm environment, although its reactivity makes casual handling unsafe.
Cesium-133 is important in timekeeping. The international definition of the second is based on a specific microwave transition in cesium-133 atoms. Cesium is distinct from rubidium, another alkali metal discovered later through spectroscopy, and from helium, which was first identified in the Sun’s spectrum.