Caesium – facts and properties
- Symbol
- Cs
- Atomic number
- 55
- Atomic mass
- 132.905 u
- Period
- 6
- Group
- 1
- Category
- alkali metal
- Electron configuration
- [Xe] 6s¹
- Electronegativity
- 0.79
- Atomic radius
- 298 pm
- Valence electrons
- 1
- State at standard conditions
- solid
- Melting point
- 28.44 °C
- Boiling point
- 671 °C
- Density
- 1.93 g/cm³
- Discovered
- 1860
Caesium is a soft, golden-tinged alkali metal with atomic number 55. It is one of the most reactive metals and melts just above room temperature. It was discovered in 1860 by spectral analysis and named after two sky-blue lines. Caesium atomic clocks are among the most accurate in the world and define the length of the second. The radioactive caesium-137 plays a role after the reactor accidents at Chernobyl and Fukushima.
Position in the periodic table
Occurrence and extraction
Caesium is rare in the Earth's crust. The most important mineral is pollucite, a caesium aluminium silicate. Major deposits lie in Canada and Zimbabwe. Caesium salts are obtained from the mineral, and the metal from them.
The discovery is closely tied to spectral analysis. The chemist Robert Bunsen and the physicist Gustav Kirchhoff examined the water of a mineral spring from Bad Dürkheim in Heidelberg in 1860. They evaporated large quantities of it and watched the flame through a spectroscope. Besides the known lines, two bright blue ones appeared that matched no known element. It was the first element found by this method. They derived the name from the Latin caesius, meaning sky-blue. Soon afterwards they found rubidium in the same way.
Properties and reactions
Caesium is in period 6 and group 1, below rubidium and above francium. In the period, barium lies to its right. Its electron configuration is [Xe] 6s¹ and its electronegativity is 0.79. The element has 1 valence electron. The metal has a density of 1.93 g/cm³, melts at 28.44 °C and boils at 671 °C. The melting point is so low that caesium melts inside its sealed ampoule from the warmth of a hand or a warm room. The metal is very soft and has a golden sheen.
The single outer electron sits very far from the nucleus and is hardly held. Caesium therefore gives it up more easily than any other element. This explains its reactivity: it ignites in air by itself and reacts violently with water, forming hydrogen and caesium hydroxide, one of the strongest bases. The metal is therefore kept in ampoules under protective gas. In compounds it occurs only as a singly charged positive ion. The metal is dangerous and does not belong in school experiments; in the laboratory only specialists handle it.
In a flame, caesium compounds give a blue to violet colour. Because the electron is given up so readily, caesium releases electrons under light even at low energy, which is used in photocells.
Uses in everyday life and technology
The best-known use is the atomic clock. The isotope caesium-133 absorbs energy in a microwave field at one very specific frequency. This frequency is extremely stable. Since 1967 the second has been defined as the duration of a little over nine billion of these oscillations. The caesium clocks of the Physikalisch-Technische Bundesanstalt in Braunschweig are off by only one second in many millions of years. Their time controls radio-controlled clocks through the DCF77 signal, and similar clocks sit in navigation satellites. Even more accurate optical clocks are expected to replace them, and a new definition of the second is being prepared.
Caesium formate is a salt solution of very high density. It is used as a fluid in deep drilling for oil and gas because it stabilises the borehole and is regarded as comparatively environmentally friendly. Caesium salts are also found in special glasses, catalysts and detectors.
Caesium-137 is a radioactive isotope formed in nuclear fission, with a half-life of about thirty years. It was released in the accidents at Chernobyl in 1986 and Fukushima in 2011. Chemically, caesium resembles potassium and is therefore taken up by plants and fungi. In parts of southern Germany, game meat, especially wild boar, is still measurably contaminated, mainly from Chernobyl and partly from earlier nuclear weapons tests. Radiation sources containing caesium-137 are used in medicine and industry. If they escape control, the consequences can be severe, as in Goiânia, Brazil, in 1987.
Quick question
A high atomic number always makes an element more electronegative - why does that still not hold for cesium (Cs) and fluorine (F)?
FAQ
How is the second defined?
Through caesium-133. The second is the duration of a little over nine billion oscillations of the radiation that this atom gives off in one particular transition. Atomic clocks count these oscillations.
Why is caesium so reactive?
The single outer electron lies far from the nucleus and is hardly held, so caesium gives it up very easily. The metal reacts violently with water and air and may only be stored under protective gas.
What is caesium-137?
A radioactive isotope with a half-life of about thirty years that is formed in nuclear fission. It entered the environment in the reactor accidents at Chernobyl and Fukushima and is taken up by plants and animals in the same way as potassium.