---
title: "Cesium (Cs): Atomic Mass, Electron Configuration & Properties"
url: https://www.moldraw.com/elements/cesium.html
description: "Cesium (Cs), element 55: atomic mass 132.91, electron configuration [Xe] 6s¹, group 1, period 6, s-block alkali metal. Melting and boiling point, density, electronegativity and shells."
lang: en
publisher: Scidart Academy
generated: 2026-11-03
---
55Cs132.91

# Cesium (Cs) – element 55

Cesium is a chemical element with the symbol **Cs** and atomic number **55**. It is a alkali metal in group 1 (alkali metals), period 6 of the periodic table (s-block), and is a solid at room temperature.

Cesium (Cs) has atomic number 55, atomic mass 132.91 u and electron configuration [Xe] 6s1.

[Download periodic table PDF](https://www.moldraw.com/printables/periodic-table.pdf)[Open interactive periodic table](https://www.moldraw.com/tools/free-chem-tools/interactive-periodic-table.html)

## Cesium facts

| Symbol | Cs |
| --- | --- |
| Atomic number | 55 |
| Atomic mass | 132.91 u (g/mol) |
| Category | alkali metal |
| Group | group 1 (alkali metals) |
| Period | 6 |
| Block | s-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1 |
| Noble-gas notation | [Xe] 6s1 |
| Electrons per shell | 2, 8, 18, 18, 8, 1 |
| Valence electrons | 1 |

## Bohr model and shells

Cesium has 55 electrons in 6 shells: 2, 8, 18, 18, 8, 1. Full configuration and noble-gas notation are in the table; work out any element or ion with the [electron configuration calculator](https://www.moldraw.com/tools/free-chem-tools/electron-configuration-calculator.html).

## Physical and atomic properties

| Melting point | 301.7 K (28.6 °C) |
| --- | --- |
| Boiling point | 944 K (670.9 °C) |
| Density | 1.93 g/cm³ |
| Electronegativity (Pauling) | 0.79 |
| Electron affinity | 45.505 kJ/mol |
| Ionization energies | 375.7, 2234.3, 3400 kJ/mol (1st, 2nd, 3rd) |
| Molar heat capacity | 32.21 J/(mol·K) |
| Discovered by | Robert Bunsen |

Use the atomic mass in the [molar mass calculator](https://www.moldraw.com/tools/free-chem-tools/molar-mass-calculator.html) for any formula containing Cs.

## Position in the periodic table

Tap any element to open its page.

## Frequently asked questions

### What is the atomic mass of cesium?

The atomic mass of cesium is 132.91 u, so one mole of cesium atoms weighs 132.91 g.

### What is the electron configuration of cesium?

Cesium has 55 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹, or [Xe] 6s¹ in noble-gas notation. Electrons per shell: 2, 8, 18, 18, 8, 1.

### How many valence electrons does cesium have?

Cesium has 1 valence electron (group 1).

### Is cesium a metal?

Yes. Cesium is a alkali metal.

### Where is cesium on the periodic table?

Cesium is in group 1 (alkali metals), period 6, in the s-block.

Data: [Periodic-Table-JSON](https://github.com/Bowserinator/Periodic-Table-JSON) (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.