cesium133atomin

Cesium-133 is the only stable isotope of the alkali metal cesium (atomic number 55). It has 55 protons and 78 neutrons, giving a mass number of 133. In nature, cesium is essentially Cs-133. The nucleus has a spin of 7/2, and the ground-state electronic configuration is [ Xe ] 6s1. The first ionization energy is about 375.7 kJ/mol.

Cesium-133 is best known for its role in timekeeping. The hyperfine transition between the two ground-state hyperfine levels lies at exactly 9,192,631,770 hertz, and this frequency defines the length of a second in the International System of Units since 1967. This standard underpins modern atomic clocks and precise time measurement used in navigation, communication, and science.

Chemistry and physical properties: Cesium is a soft, highly reactive alkali metal. It has a melting point of 28.5 °C and a low ionization energy, which leads to vigorous reactions with water to form cesium hydroxide and hydrogen gas. In compounds, cesium typically exists as the Cs+ cation.

Occurrence and history: Cesium was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff from mineral water. The name derives from the Latin caesius, meaning “sky blue,” in reference to a characteristic blue spectral line produced by cesium gas. Cs-133’s stability and its role in the clock standard have made it central to metrology and many technologies that rely on precise timekeeping.