Alkali metalElement 55 · s-block

Cesium (Cs)

Cesium (Cs), also spelled caesium, is an alkali metal with atomic number 55 and a standard atomic weight of 132.91. At room temperature it is a solid that melts at 28 °C and boils at 671 °C.

55CsCesium132.91

Atomic number

protons

Atomic weight

132.91

CIAAW 2024

Melting point

28 °C

301.65 K

Boiling point

671 °C

944.15 K

Density

1.873 g/cm³

near room temperature

Electronegativity

0.79

Pauling scale

State at 25 °C

Solid

Discovered

1860

Germany

Cesium at a glance

Identity

Symbol	Cs
Atomic number (Z)	55
Atomic weight	132.91 ± 0.01 (CIAAW 2024)
Category	Alkali metal
Group	1 — alkali metals (with hydrogen)
Period	6
Block	s-block
CAS number	7440-46-2

Atomic properties

Electron configuration	[Xe] 6s¹ Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹
Electrons per shell	2, 8, 18, 18, 8, 1
Oxidation states	+1 Less common: -1
Electronegativity (Pauling)	0.79
First ionisation energy	3.894 eV (375.7 kJ/mol) 2nd: 23.157 eV · 3rd: 33.195 eV
Electron affinity	0.472 eV
Atomic radius (empirical)	260 pm
Covalent radius	232 pm
Van der Waals radius	343 pm

Physical properties

State at 25 °C	Solid
Melting point	28.5 °C · 83.3 °F · 301.65 K
Boiling point	671 °C · 1,239.8 °F · 944.15 K
Density	1.873 g/cm³
Specific heat capacity	0.242 J/(g·K)
Thermal conductivity	35.9 W/(m·K)
Crystal structure	Body-centred cubic

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	¹³³Cs 100%
Stable isotopes	1
Known isotopes	42
Abundance in Earth's crust	3 mg/kg
Abundance in seawater	0.0003 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Cesium electron configuration

Cesium has 55 electrons. Written in short form, its ground-state configuration is [Xe] 6s¹ — the electrons of the noble gas xenon, plus 6s¹.Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹.

As a member of group 1 it has 1 valence electron in its outer shell (shell 6), which largely decides its chemistry.

6s↑

Discovery and name

Discovered

1860 — Robert Bunsen and Gustav Kirchhoff · Germany.

Name

Latin caesius, sky blue — for its spectral lines.

Known for

The basis of the SI second: one second is 9,192,631,770 cycles of a caesium-133 transition. Caesium formate is used in drilling fluids.

Isotopes of cesium

Naturally occurring cesium is a mix of 1 isotope. Each has 55 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
¹³³Cs	55	78	100%	stable

42 isotopes of cesium are known in our data set (half-lives as published in nuclear data tables).

Cesium compared with group 1

The other elements in group 1 (alkali metals (with hydrogen)), from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Hydrogen	1	1.0080	-259 °C	-253 °C	0.0899 g/L	2.2
Lithium	3	6.94	180 °C	1,342 °C	0.534 g/cm³	0.98
Sodium	11	22.990	98 °C	883 °C	0.97 g/cm³	0.93
Potassium	19	39.098	64 °C	759 °C	0.89 g/cm³	0.82
Rubidium	37	85.468	39 °C	688 °C	1.53 g/cm³	0.82
Cesium	55	132.91	28 °C	671 °C	1.873 g/cm³	0.79
Francium	87	[223]	21 °C (est.)	—	1.87 g/cm³ (pred.)	0.7

Frequently asked questions

6 answers

What is the atomic mass of cesium?

The standard atomic weight of cesium is 132.91 ± 0.01 (CIAAW 2024), so one mole weighs about 132.91 grams.

How many protons, neutrons and electrons does cesium have?

A neutral cesium atom has 55 protons and 55 electrons. Its most common isotope, cesium-133, has 78 neutrons.

What is the electron configuration of cesium?

[Xe] 6s¹ (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹).

What are the melting and boiling points of cesium?

Cesium melts at 28.5 °C (83.3 °F) and boils at 671 °C (1,239.8 °F).

Is cesium a metal?

Cesium is a metal. It is in group 1, period 6, and is a solid at room temperature.

Who discovered cesium?

1860 — Robert Bunsen and Gustav Kirchhoff (Germany).