Cesium was the first chemical element discovered using spectroscopy.
German scientists Robert Bunsen and Gustav Kirchhoff identified cesium in 1860 by examining the spectrum of mineral-water residues. They saw two previously unknown blue lines, which led to the element’s name from the Latin caesius, meaning sky blue.
Cesium is an alkali metal with atomic number 55. It is extremely reactive and has a very low melting point for a metal, although it is not the element that melts at about room temperature in the excluded comparison.
Cesium is especially important in timekeeping. The cesium-133 atomic transition defines the SI second, making cesium atomic clocks foundational to high-precision navigation, telecommunications, and scientific measurement.