| Symbol | Cs |
| Atomic number | 55 |
| Group | 1 (Alkali metal) |
| Period | 6 |
| Block | s |
| Classification | Alkali Metal |
| Appearance | Silvery gold |
| Color | Silver |
| Number of protons | 55 p+ |
| Number of neutrons | 78 n0 |
| Number of electrons | 55 e- |
| Phase at STP | Solid |
| Density | 1.93 g/cm3 |
| Atomic weight | 132.905 u |
| Melting point | 301.7 K 28.55 °C 83.39 °F |
| Boiling point | 944 K 670.85 °C 1239.53 °F |
| Heat of vaporization | 67.74 kJ/mol |
| Electronegativity (Pauling Scale) | 0.79 |
| Electron affinity | 45.505 kJ/mol |
| Oxidation states | −1, +1 (a strongly basic oxide) |
| Ionization energies |
|
| Discovery | Robert Bunsen, Gustav Kirchhoff (1860) |
| First isolation | Carl Setterberg (1882) |
Discovery of cesium In 1860, German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in the mineral water from Dürkheim, Germany. Because of the bright blue lines in the emission spectrum, they derived the name from the Latin word caesius, meaning sky-blue. Caesium was the first element to be discovered with a spectroscope, which had been invented by Bunsen and Kirchhoff only a year previously. In 1967, acting on Einstein's proof that the speed of light is the most-constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre. Since then, caesium has been widely used in highly accurate atomic clocks. | |