55Cs132.91
Caesium
Alkali metalsSoft, very reactive metals with one outer electron. They react with water to make alkaline solutions.
[Xe] 6s¹
- Atomic number
- 55
- Atomic mass
- 132.91
- Protons
- 55
- Neutrons
- 78 (most common)
- Electrons
- 55
- Shells
- 2, 8, 18, 18, 8, 1
- State at 25 °C
- Solid
- Position
- Group 1, period 6
Proton (+)Neutron (0)Electron (−)
Discovered byRobert Bunsen and Gustav Kirchhoff, 1860Robert Bunsen: 2 elementsGustav Kirchhoff: 2 elements
The first element found by spectroscopy. It's named from the Latin caesius, sky blue, after its blue spectral lines.
Did you know?
- It melts at about 28.5 °C, just above room temperature.
- It explodes on contact with water.
Where it's found
- The mineral pollucite
What it's used for
- Atomic clocks, which define the second
- Drilling fluids
Try it
There's no safe home experiment for Caesium. Explore it with the animated atom above instead.