Cs
Caesium
Atomic number: 55Mass: 132.905 uCategory: Alkali Metal
About Caesium
Caesium is a soft, silvery-gold alkali metal with a melting point of 28.5°C — it becomes liquid on a warm day. It is highly reactive. It has the lowest electronegativity of all non-radioactive elements.
Properties
Period
6
Group
1
Block
S
Category
Alkali Metal
Electronegativity
0.79 (Pauling)
Ionization Energy
375.7 kJ/mol
Melting Point
302 K (29°C)
Boiling Point
944 K (671°C)
Density
1.873 g/cm³
Oxidation States
+1
Electron Configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹
[Xe] 6s¹
Shell 1:2e⁻
Shell 2:8e⁻
Shell 3:18e⁻
Shell 4:18e⁻
Shell 5:8e⁻
Shell 6:1e⁻
Real-World Uses
Atomic clocks (defining the second)
Drilling fluids
Ion propulsion engines
Medical research
Night-vision devices
Fun Fact
The international definition of a 'second' is based on caesium! One second = 9,192,631,770 oscillations of caesium-133 atoms.