Caesium (Cs)
Alkali metal · s-block · period 6 · group 1
Soft, gold-tinted metal that is liquid just above room temperature.
Caesium (symbol Cs) is chemical element number 55, a alkali metal in group 1, period 6 of the periodic table. Its standard atomic weight is 132.91 and its electron configuration is [Xe] 6s1. At room temperature it is a solid. The most reactive metal that is not radioactive — it explodes on contact with water.
Properties
| Atomic number | 55 |
|---|---|
| Atomic mass | 132.91 |
| Category | Alkali metal |
| Block | s-block |
| Group · Period | 1 · 6 |
| Electron configuration | [Xe] 6s1 |
| Electrons per shell | 2, 8, 18, 18, 8, 1 |
| State at 25 °C | solid |
| Melting point | 301.7 K |
| Boiling point | 944 K |
| Density | 1.93 g/cm³ |
| Electronegativity (Pauling) | 0.79 |
| Radioactive | No |
| Origin | primordial |
| Discovered | 1860 · Bunsen & Kirchhoff |
Key facts about Caesium
- The most reactive metal that is not radioactive — it explodes on contact with water.
- Named for the two sky-blue lines in its spectrum.
- One second is officially defined by 9,192,631,770 vibrations of a caesium atom.
- The most electropositive stable element (lowest electronegativity).
What Caesium is used for
Atomic clocks and time standards, oil-drilling fluids, photoelectric cells, ion propulsion, specialty glass.
Where it sits in the periodic table
Caesium is in group 1, period 6, of the s-block. Its electron configuration [Xe] 6s1 means its outermost shell holds 1 electron, which is what places it here and explains much of its chemistry. See Caesium on the interactive table →
Caesium — frequently asked questions
What is Caesium used for?
Atomic clocks and time standards, oil-drilling fluids, photoelectric cells, ion propulsion, specialty glass.
Is Caesium radioactive?
No. Caesium has at least one stable isotope and is not radioactive.
What is the electron configuration of Caesium?
Caesium has the electron configuration [Xe] 6s1, which is 2, 8, 18, 18, 8, 1 electrons per shell.
What group and period is Caesium in?
Caesium is in group 1, period 6, in the s-block. Its outer shell holds 1 electron.
Who discovered Caesium?
Caesium was discovered in 1860 by Bunsen & Kirchhoff.