Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
[Xe] 6s¹
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
2, 8, 18, 18, 8, 1
Number of electrons in each shell, from the nucleus outwards.
28.4 °C (301.59 K)
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
671 °C (944 K)
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
1.93 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
0.79 Pauling
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
3.894 eV
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
0.472 eV
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
244 pm
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
+1
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Caesium (Cs) is a chemical element with atomic number 55. It sits in group 1 and period 6 of the periodic table, in the s-block. It is an alkali metal. At room temperature it is a solid. Its standard atomic weight is 132.91.
Origin of the name
Caesium was the first element discovered with the spectroscope. In 1860 Robert Bunsen and Gustav Kirchhoff, who had just invented the instrument, saw two bright blue lines in the light of mineral water from Dürkheim and named the element after them, from the Latin caesius, ‘sky blue’.
Where is caesium found?
Caesium is rare. It is obtained mainly from the mineral pollucite; one of the world’s richest deposits lies at Bernic Lake in Manitoba, Canada.
What is caesium used for?
Caesium keeps the world’s time: since 1967 the second has been defined by the vibrations of caesium atoms, and caesium atomic clocks are among the most accurate clocks there are. Caesium is also used in photocells and to remove the last gas from vacuum tubes.
Interesting facts
Caesium explodes on contact with cold water and reacts even with ice.
Radioactive caesium-137, released in the Chernobyl accident, is still one of the main sources of radioactivity in the area around it.