Cesium

Caesium (spelled cesium in American English) is a soft, pale-gold alkali metal in group 1, with the configuration [Xe] 6s1. Its single outer electron sits in a large, well-shielded orbital far from the nucleus, so it is removed more easily than from any other stable element. That makes caesium the most electropositive stable metal and the most violently reactive with water. It melts at 28.5 °C, so it turns liquid on a warm day. Along with gold and copper, it is one of the few metals with a distinctly coloured, non-silvery lustre.

Caesium has only one stable isotope, caesium-133, and the energy gap between two arrangements of its outer electron spin defines time itself. Since 1967 the SI second has been defined as exactly 9,192,631,770 oscillations of the radiation from this hyperfine transition, measured by caesium atomic clocks. Such clocks underpin GPS, which relies on time signals accurate to billionths of a second. Caesium formate brines, dense but non-toxic, are used as drilling fluids in high-pressure oil and gas wells. The fission product caesium-137 is a long-lived contaminant from nuclear accidents and also a source for cancer radiotherapy.

Discovery

In 1860 Robert Bunsen and Gustav Kirchhoff in Heidelberg evaporated 40 tonnes of mineral water from Dürkheim and examined the residue with their newly invented flame spectroscope. They saw two bright blue lines that belonged to no known element, making caesium the first element discovered by spectroscopy. They named it from the Latin caesius, meaning 'sky blue'. Carl Setterberg first isolated the metal in 1882 by electrolysing molten caesium cyanide.