Atomic Properties of Cs
- Atomic Number of Cs
- 55
- Atomic Mass of Cs
- 132.9100 u
- Electron Configuration
- [Xe] 6s1
- Electronegativity
- 0.79
- Block
- s-block
- Group
- 1
- Period
- 6
Cesium looks almost designed to be memorable. It is a pale golden metal, soft enough to cut and able to melt at about 28.5 degrees Celsius, so a warm room can turn a solid sample into a liquid. That transformation must happen inside a sealed container. Exposed cesium reacts rapidly with air and explosively with water, producing hydrogen and enough heat to ignite it.
The same loose hold on its outer electron that drives this reactivity also makes cesium useful in photoelectric devices and vacuum equipment. Yet its most famous achievement happens without any splash or flame. Cesium-133 provides the reference for the SI second. One second is defined using exactly 9,192,631,770 periods of radiation associated with a particular transition between two hyperfine levels in the atom's ground state.
Atomic clocks do not work by watching cesium decay. Cesium-133 is stable. They tune microwave radiation to the frequency at which the atoms change energy state, then use that extraordinarily consistent response as a pendulum made by quantum mechanics. Precise timekeeping supports navigation, telecommunications, and measurements that compare clocks across the world.
Cesium was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff through two bright blue lines in a spectrum. Its name comes from a Latin word for sky blue, even though the metal itself looks golden. Cesium starts fires in water, marks time without ticking, and carries one color in its name while wearing another.