Atomic number 55
Cesium
Alkali metal
In the real world
Quick facts
- Atomic mass
- 132.91
- Group
- 1
- Period
- 6
- State (STP)
- solid
- Electronegativity
- 0.79
- Common ox. states
- +1
Properties
Electron configuration
[Xe] 6s¹
- Ionization energy
- 375.7 kJ/mol
- Electron affinity
- 45.5 kJ/mol
- Melting point
- 301.70 K (28.6 °C)
- Boiling point
- 944.00 K (670.9 °C)
How dangerous is it?
5/5 · Extreme
A gold-colored metal so reactive that the moisture on your skin sets it on fire. It only exists safely sealed away in a lab.
- FlammableImmediate
The reaction with moisture releases hydrogen and instantly sets it alight.
Kept far from water; a cesium fire is smothered with dry sand, never water.
- ReactiveImmediate
Drop it in water and it reacts explosively; even damp air or skin is enough to start the reaction.
Sealed in glass tubes under argon gas, and only opened inside a dry, airless glovebox.
How it bonds
Cesium has the lowest ionization energy of any stable element - its 6s electron is so far from the nucleus that almost anything will pull it off. Cs⁺ is the only common form. Atomic clocks count cesium-133 transitions to define the second.
