Caesium (Cs): electron configuration, atomic mass and facts
Caesium (also spelled cesium) (Cs) is element 55 of the periodic table, an alkali metal in period 6, group 1. Its electron configuration is [Xe] 6s1, with electrons arranged in shells 2, 8, 18, 18, 8, 1, and its standard atomic weight is 132.91 u.
▶ Watch Caesium’s electrons orbit in the interactive periodic tableCaesium at a glance
| Symbol | Cs |
|---|---|
| Atomic number | 55 |
| Atomic mass | 132.91 u |
| Category | Alkali metal |
| Group | 1 |
| Period | 6 |
| Block | s-block |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1 |
| Noble-gas shorthand | [Xe] 6s1 |
| Electrons per shell | 2, 8, 18, 18, 8, 1 |
| Outer-shell electrons | 1 |
| Electronegativity (Pauling) | 0.79 |
| State at 20 °C | solid |
| Flame test colour | blue-violet |
Where is Caesium on the periodic table?
Caesium is in period 6, group 1, in the s-block. The period number matches its 6 electron shells, and the block names the subshell its last electrons fill. In the same group, Rubidium sits directly above it and Francium sits directly below it. Elements in one group have the same number of outer-shell electrons (1 here), which is why they react in similar ways.
How to work out the electron configuration of Caesium
Put 55 electrons into subshells in order of rising energy, filling each before starting the next: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), 3d¹⁰ (30), 4p⁶ (36), 5s² (38), 4d¹⁰ (48), 5p⁶ (54), 6s¹ (55). In shorthand, [Xe] stands for the 54 electrons of xenon, giving [Xe] 6s¹.
How electronegative is Caesium?
Caesium has a Pauling electronegativity of 0.79, ranking 99 of the 100 elements with a known value (fluorine is highest at 3.98). That is low: it tends to give up electrons and form positive ions.
Caesium in the virtual lab
Faradium’s chemistry workbench includes these chemicals that contain caesium: Caesium chloride. Open the Chemistry Lab to experiment with them.
Frequently asked questions
What is the electron configuration of Caesium?
The electron configuration of Caesium (Cs) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1. In noble-gas shorthand it is written [Xe] 6s1.
How many electrons does Caesium have in each shell?
Caesium has 55 electrons arranged in shells as 2, 8, 18, 18, 8, 1. It has 1 electron in its outer shell.
What is the atomic mass of Caesium?
The standard atomic weight of Caesium is 132.91 u, so one mole of Caesium atoms has a mass of about 132.91 g.
Is Caesium a metal?
Yes. Caesium is an alkali metal.
Explore nearby elements
More alkali metals: Lithium, Sodium, Potassium, Rubidium, Francium. See all elements on the periodic table.