Element 55
Cesium (Cs)
Cesium (Cs) is element 55 of the periodic table, an alkali metal in period 6, group 1 (s-block) with an atomic mass of 132.91 and is a solid at standard conditions.
At a glance
- Atomic number
- 55
- Symbol
- Cs
- Atomic mass
- 132.91 u
- Category
- Alkali metals
- Period
- 6
- Group
- 1
- Block
- s-block
- Phase at STP
- solid
Electron structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1
- Shorthand configuration
- [Xe] 6s1
- Electrons per shell
- 2, 8, 18, 18, 8, 1
- Oxidation states
- -1, +1
Physical properties
- Melting point
- 301.59 K (28.44 °C)
- Boiling point
- 944 K (670.85 °C)
- Density
- 1.873 g/cm³
- Phase
- solid
Atomic properties
- Electronegativity (Pauling)
- 0.79
- Atomic radius
- 298 pm
- Covalent radius
- 244 pm
- Van der Waals radius
- 343 pm
- First ionization energy
- 376 kJ/mol
- Electron affinity
- 46 kJ/mol
Notable isotopes
| Isotope | Mass | Abundance | Stability |
|---|---|---|---|
| Cs-133 | 132.90545 | 100% | Stable |
Common uses
- Atomic clocks
- Drilling fluids
- Photoelectric cells
- Cancer treatment
Discovery
- Year
- 1860
- Discovered by
- Robert Bunsen, Gustav Kirchhoff
- Country
- Germany
Frequently asked questions
- Is cesium a metal?
- Yes. Cesium is a metal, classified as an alkali metal on the periodic table.
- How many protons does cesium have?
- Cesium has 55 protons. Its atomic number is 55, which is also the number of electrons in a neutral atom of cesium.
- What group and period is cesium in?
- Cesium is in group 1 and period 6 of the periodic table, in the s-block.
- What is the atomic mass of cesium?
- The atomic mass of cesium is 132.91 u (unified atomic mass units).
Keep exploring
Cesium's row is on the printable element property sheet, and its tile is on the periodic table wall chart - the same numbers as above, laid out for A4 or A3 paper.