Caesium Cs

Discovery

Caesium was the first element discovered with a spectroscope. In 1860 Robert Bunsen and Gustav Kirchhoff in Heidelberg evaporated mineral water from Dürkheim, placed the residue in a flame and examined the light.

They saw two bright blue lines that matched no known element. The element was so dilute that tens of thousands of litres of water had to be concentrated to obtain a few grams of its salts, but the spectral lines alone convinced them it was new.

Metallic caesium was first obtained in 1882 by the Swedish chemist Carl Setterberg, who electrolysed caesium cyanide. After caesium atomic clocks were developed in the mid-twentieth century, the International System of Units redefined the second in 1967 in terms of the caesium-133 atom.

Origin of the name

The name comes from the Latin caesius, "sky blue" or "bluish grey", after the blue lines in its spectrum. The IUPAC spelling is caesium, while American usage prefers cesium. The symbol is Cs.

Main uses

• Atomic clocks and time standards: one second is defined as 9,192,631,770 periods of the radiation emitted by the caesium-133 atom as it switches between two energy states. Caesium clocks underpin national time standards and satellite navigation.
• Drilling fluids: dense, relatively benign caesium formate brine is used in high-pressure gas wells.
• Radiation sources: caesium-137 was used in radiotherapy and blood irradiators and is still used in industrial thickness and density gauges.
• Photoelectric devices and detectors: caesium readily releases electrons when struck by light and was used in phototubes, and caesium iodide crystals detect radiation.

Isotopes

Caesium has a single stable isotope, caesium-133. Radioactive caesium-137 (half-life about 30 years) and caesium-134 (about 2 years) are produced by fission and were major sources of soil and food contamination after the Chernobyl accident in 1986 and the Fukushima accident in 2011. Because caesium behaves like potassium, it dissolves easily in water and is readily taken up by living things.

In everyday life

Caesium is extracted mainly from the mineral pollucite, and the Tanco mine in Manitoba, Canada, was long a principal source. Few people ever see the pure metal, which laboratories store in glass ampoules under argon.

In 1987 in Goiânia, Brazil, a caesium-137 chloride source taken from an abandoned radiotherapy machine was mistaken for a glowing curiosity and spread among many people; four people died. The accident led to much stricter control of radioactive sources. Internal contamination with caesium-137 can be treated with Prussian blue, which helps the body excrete it.