Caesium

Caesium (Cs) is an alkali metal with atomic number 55 and an atomic mass of 132.91. Its electron configuration is [Xe] 6s1, which gives it 1 valence electron.

Caesium key data

Symbol	Cs
Atomic number	55
Atomic mass (standard atomic weight)	132.91
Molar mass	132.91 g/mol
Protons	55
Neutrons (most abundant isotope, caesium-133)	78
Electrons (neutral atom)	55
Valence electrons	1
Category	Alkali metal
Period	6
Group	1
Block	s-block
Noble gas notation	[Xe] 6s1
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1
Electrons per shell	2, 8, 18, 18, 8, 1

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of caesium

From PubChem, the US National Institutes of Health. Public domain.

Melting point	301.59 K (28.44 °C), 89th of 103 by melting point
Boiling point	944 K (670.85 °C), 76th of 93 by boiling point
Density	1.93 g/cm³, 77th of 96 by density
State at room temperature	Solid
Electronegativity	0.79 (Pauling), 94th of 95 by electronegativity
Van der Waals radius	343 pm, 2nd of 99 by atomic radius
First ionisation energy	3.894 eV (375.7 kJ/mol), 102nd of 102 by first ionisation energy
Electron affinity	0.472 eV (45.5 kJ/mol), 43rd of 57 by electron affinity
Common oxidation states	+1
Discovered	1860

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Caesium electron configuration

The full electron configuration of caesium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1. With the noble-gas core collapsed it is [Xe] 6s1, and the outermost subshell is 6s1.

Bohr model of caesium (Cs): a nucleus and six shells holding 2, 8, 18, 18, 8 and 1 electrons from the inside out. — Bohr model of caesium (Cs). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of caesium (Cs), [Xe] 6s1, with the xenon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 6s. — Orbital diagram of caesium (Cs). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one caesium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 132.91 / (6.02214076 × 10²³)
= 2.2070 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 132.91
= 4.5310 × 10²¹ atoms

Caesium on the periodic table

Caesium is in period 6, group 1, in the s-block.

The other elements of group 1 are hydrogen, lithium, sodium, potassium, rubidium and francium.