Cesium (Cs)

Soft, silvery-gold metal

Cesium is the most reactive stable metal, exploding on contact with water and defining the second, and with a single 6s¹ electron and the lowest ionization energy of any stable element (375.7 kJ/mol), cesium-133's hyperfine transition (9,192,631,770 Hz) defines the international second and powers atomic clocks.

Key metrics

Density1.93g/cm³

Melting point302K

Boiling point944K

Electronegativity0.79Pauling

Discovery

1860discovered

Discovered by: Robert Bunsen
Reactivity profile: Explodes in water
Reactivity: High
Bonding: Metallic

Electron configuration

\textcolor{#6b7280}{[Xe]}\; \textcolor{#ef4444}{6_{s}^{1}}

[Xe] 6s1

Wave function

\Psi_{\text{Cs}} \approx \frac{1}{\sqrt{55!}} \det \left| \chi_1(\mathbf{r}_1) \dots \chi_{55}(\mathbf{r}_{55}) \right|

Probability density $|\Psi|^2$ across the Cs valence shell.

Crystal structure

Body-Centered Cubic

Lattice constantsa = 1.00

Cell anglesα = β = γ = 90°

Coordination number8

Atoms per cell2

Atomic packing factor0.68

Physical properties

Atomic radius: 298 pm
Ionization energy: 376 kJ/mol
Electron affinity: 46 kJ/mol
Crystal structure: Body-centered cubic (bcc)

Oxidation states

Acid/base

Basic

Strongly acidicStrongly basic

State at 25 °CSolid

302 K944 K298 K

Emission spectrum

3 transitions

380450500550600650700780

VioletBlueGreenYellowRed

455.5nmCs IBlue90%

459.3nmCs I (blue)Blue100%

522.1nmCs I (green)Green50%

Dominant line 459.3 nmRange 456–522 nmTransitions 3