Cesium

Caesium is the most reactive of all stable metals, it explodes instantly in water, ignites in air, and melts barely above room temperature. Yet it is also the world's most precise timekeeper: the caesium atomic clock defines the international standard second with accuracy that would not lose a second in 300 million years.

• Atomic Number5555 protons, 55 electrons
• Atomic Mass132.9054520 u55× heavier than hydrogen
• State at Room TempSolidSolid
• Density1.93 g/cm³
• Melting / Boiling28.4°C / 670.9°C
• Discovered1860

What is Cesium?

Caesium is an alkali metal in Group 1 with 55 protons. Its single outer electron is so loosely held that caesium reacts more violently with water than any other stable element. Pure caesium is a soft, golden-yellow metal that must be handled under inert gas.

Named from the Latin caesius meaning sky-blue or grey-blue: the colour of two spectral lines that revealed its existence. Discovered in 1860 by Robert Bunsen and Gustav Kirchhoff using the newly invented technique of flame spectroscopy.

Fact The second is defined by caesium. Since 1967, one second is officially defined as exactly 9,192,631,770 oscillations of the microwave radiation emitted by caesium-133 transitioning between two hyperfine states.

Where you find Cesium

On Earth

Caesium is very rare at approx. 3 parts per million in the crust, concentrated mainly in a few pegmatite mineral deposits.

• Pollucite. A caesium aluminium silicate found in lithium pegmatites. The Tanco mine in Manitoba, Canada, holds a large fraction of world reserves.
• By-product of lithium mining. Some caesium is recovered from lithium mineral processing waste streams.

How we use Cesium

• Atomic clocks.. The caesium atomic clock defines the SI second as exactly 9,192,631,770 oscillations of the caesium-133 atom. GPS, internet synchronisation and scientific measurement all depend on caesium clocks.
• Drilling fluid.. Caesium formate brine is used as a high-density, non-damaging drilling fluid in deep oil and gas wells.
• Research.. Ultra-cold caesium atoms are used in quantum computing and precision measurement experiments.

Did you know? A caesium atomic clock running since the Big Bang 13.8 billion years ago would still only have lost approx. 4 seconds. GPS satellites carry caesium atomic clocks; without them, GPS position errors would grow by kilometres per day.

How it was discovered

Bunsen and Kirchhoff discovered caesium spectroscopically in 1860 while analysing mineral water from Bad Dürkheim. The pure metal was isolated by Carl Setterberg in 1882 using electrolysis of caesium cyanide. It was the first element discovered by spectroscopy.

Deeper dive: cesium properties and applications

Caesium atomic clocks work by detecting microwave radiation that causes caesium-133 atoms to transition between two hyperfine ground states, states differing in the relative spin orientation of the electron and nucleus. The transition frequency (9,192,631,770 Hz) is extraordinarily stable, unaffected by temperature, pressure or chemical environment. Modern optical lattice clocks using ytterbium are even more accurate but caesium defines the international second.

Moving to 56 protons on the periodic table brings us to Barium.