Atomic number:55

Group numbers:1

Period:6

Electronic configuration:[Xe] 6s¹

Formal oxidation number:+1

Electronegativities:0.79

Atomic radius / pm:265.5

Relative atomic mass:132.9054519 ± 0.0000002

 

Caesium (or cesium) was discovered by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff (DE) in 1860. The origin of the name comes from the Latin word caesius meaning sky blue or heavenly blue. The spelling of caesium was recommended instead of cesium (the spellings in common use in the USA) by the International Union of Pure and Applied Chemistry (IUPAC) in 1990. It is a very soft, light grey, ductile metal that reacts readily with oxygen and reacts explosively with water. Caesium is found in pollucite [(Cs4Al4Si9O26).H2O] and as trace in lepidolite. It is used as a 'getter' to remove air traces in vacuum and cathode-ray tubes, in producing photoelectric devices and atomic clocks. Since it ionises readily, it is used as an ion rocket motor propellant.

Density / g dm^-3:1873 (293 K)

 1843 (m.p.)

Molar volume / cm³mol^-1:70.96 (293 K)

 72.11 (m.p.)

Electrical resistivity / μΩcm:20 (20 °C)

Thermal conductivity / W m^-1K^-1:35.9

Melting point / °C:28.44

Boiling point / °C:671

Heat of fusion / kJ mol^-1:2.09

Heat of vaporization / kJ mol^-1:66.5

Heat of atomization / kJ mol^-1:77.58

1st ionization energy / kJ mol^-1:375.71

2nd ionization energy / kJ mol^-1:2234.37

3rd ionization energy / kJ mol^-1:-

in the atmosphere / ppm:-

in the Earth's crust / ppm:1

in the oceans / ppm:0.0005