Atomic number: 55

Symbol: Cs

Category: alkali metal

Density at 25°C: solid: density: 1.93 g·cm−3

Caesium, sometimes spelled cesium, is an alkali metal so it tends to be quite reactive with with water at -116°C. It is quite often used in industry and also in medical imaging. It is mostly found in pollucite ore. Caesium compounds are used as part of drilling fluids. 

Tube of caesium held in an inert gas, note the slightly gold colour.
 image: adapted from wikipedia

Use, natural applications or common combinations: 

  • drilling
  • atomic clocks and used to define the second
  • ion thrusters for spacecraft

Basic characteristics:

  • Standard atomic weight: 32.905 4519(2)
  • Melting point:  28.44 °C
  • Boiling point: 671 °C
  • Radioactivity: mostly naturally stable
  • some isotopes (137Cs) are used industrially with a half-life of around 30 years emitting beta particles and gamma rays. It is used for radiotherapy. Also used for sterilisation disinfection.