On the periodic table, cesium was the first element discovered through spectroscopy.
Cesium has atomic number 55 and the symbol Cs. German chemists Robert Bunsen and Gustav Kirchhoff discovered it in 1860 while analyzing mineral-water samples with a spectroscope. They identified two distinctive blue spectral lines, leading to the element’s name from the Latin caesius, meaning sky-blue.
Spectroscopy separates light into wavelengths and can reveal elements through characteristic emission lines. The discovery of cesium demonstrated that an element could be identified from its light signature before it was isolated as a pure substance. Bunsen later helped isolate cesium compounds, but pure cesium metal was obtained in 1882 by Carl Setterberg.
Cesium is an alkali metal in group 1 and period 6. The closely related element rubidium was also discovered by spectroscopy, in 1861, but it came after cesium. Cesium’s most famous modern use is in atomic clocks, whose frequency helps define the SI second.