The element discovered in 1860 by Robert Bunsen and Gustav Kirchhoff with the symbol Cs is cesium. The spelling “caesium” is standard in international and British usage, while “cesium” is the American English form; both refer to the same element.
Bunsen and Kirchhoff discovered cesium through flame spectroscopy, a method that identifies elements by the characteristic colors and lines in their emitted light. They examined mineral water from Dürkheim, Germany, and noticed two previously unknown bright blue spectral lines. The element’s name comes from the Latin caesius, meaning sky blue.
Cesium is an alkali metal with atomic number 55. It is unusually soft and has a melting point near room temperature, so it can melt in a warm environment. Its isotope cesium-133 is central to the modern definition of the second, used in atomic clocks.
Rubidium is the most plausible mix-up because Bunsen and Kirchhoff discovered it soon afterward using spectroscopy. However, rubidium’s symbol is Rb, while Cs identifies cesium.