---
title: "Cesium (Cs) — element 55"
description: "Cesium is element 55, symbol Cs, atomic weight 132.91. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/cesium/
site: "At One Place"
---

# Cesium (Cs)

**Cesium — element 55: Cs**

132.91 u · Alkali metal · period 6, group 1

The second is defined as exactly 9,192,631,770 oscillations of the radiation from a caesium-133 hyperfine transition, so every clock on Earth, every GPS fix and every financial timestamp is ultimately counting caesium. It is the most reactive metal in practical terms — it ignites on contact with air and reacts explosively with water, even with ice at −116 °C — and has the lowest electronegativity of any stable element at 0.79. It is also faintly golden, one of only a handful of coloured metals alongside gold and copper.

## Properties

- **Atomic number:** 55
- **Symbol:** Cs
- **Standard atomic weight:** 132.91 u
- **Category:** Alkali metal
- **Group:** 1
- **Period:** 6
- **Block:** s-block
- **Electron configuration:** [Xe] 6s¹
- **Electrons per shell:** 2, 8, 18, 18, 8, 1
- **Melting point:** 301.59 K — 28.4 °C — 83.2 °F
- **Boiling point:** 944 K — 670.9 °C — 1239.5 °F
- **Density:** 1.93 g/cm³
- **Electronegativity:** 0.79 (Pauling)
- **Oxidation states:** +1
- **Discovered:** 1860 by Robert Bunsen and Gustav Kirchhoff

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹
- **Noble gas shorthand:** [Xe] 6s¹
- **Shells:** K: 2 · L: 8 · M: 18 · N: 18 · O: 8 · P: 1

## What it is used for

- Caesium atomic clocks, which define the second
- Dense caesium formate drilling fluids for deep oil wells
- Photoelectric cells and infrared night-vision photocathodes
- Caesium-137 gamma sources for industrial gauges and sterilisation
- Getters that scavenge residual gas in vacuum tubes

## Hazards

- Ignites spontaneously in air; reacts explosively with water including ice
- Caesium-137 is a major fallout and reactor-accident contaminant with a 30-year half-life

> Spelled "cesium" here following US usage and IUPAC's accepted alternative; "caesium" is the primary IUPAC spelling and is carried as an alias. Melts at 28.4 °C, so like gallium it liquefies in a warm hand — through the sealed ampoule it has to be kept in.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 53 | Iodine | I | 126.9 |
| 54 | Xenon | Xe | 131.29 |
| 55 | Cesium ← | Cs | 132.91 |
| 56 | Barium | Ba | 137.33 |
| 57 | Lanthanum | La | 138.91 |

## Group 1

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Cesium sits in group 1 with hydrogen, lithium, sodium, potassium, rubidium, francium.

## Common questions

### What is the atomic number of cesium?

55. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for cesium?

Cs.

### What is the electron configuration of cesium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹, or [Xe] 6s¹ in noble-gas shorthand.

### What is the melting point of cesium?

301.59 K — 28.4 °C — 83.2 °F

## Related pages

- [Iodine (I)](https://atoneplace.net/element/iodine/)
- [Xenon (Xe)](https://atoneplace.net/element/xenon/)
- [Barium (Ba)](https://atoneplace.net/element/barium/)
- [Lanthanum (La)](https://atoneplace.net/element/lanthanum/)
- [Hydrogen (H)](https://atoneplace.net/element/hydrogen/)
- [Lithium (Li)](https://atoneplace.net/element/lithium/)
- [Sodium (Na)](https://atoneplace.net/element/sodium/)
- [Potassium (K)](https://atoneplace.net/element/potassium/)
- [Rubidium (Rb)](https://atoneplace.net/element/rubidium/)
- [Francium (Fr)](https://atoneplace.net/element/francium/)

## Sources

- Official vendor product documentation — At One Place (/about/)