Francium (Fr)

Unknown (presumed silvery-white metal)

Francium is the most unstable of all naturally occurring elements, with about 30 grams existing on Earth at any moment, and with a single 7s¹ electron, francium has the lowest ionization energy theoretically predicted (around 380 kJ/mol). Its longest-lived isotope (Fr-223) has a half-life of 22 minutes, making bulk study impossible.

Key metrics

Density1.87g/cm³predicted

Melting point300Kpredicted

Boiling point950Kpredicted

Electronegativity0.7Paulingpredicted

Discovery

1939discovered

Discovered by: Marguerite Perey
Reactivity profile: Extremely reactive
Reactivity: High
Bonding: Metallic

Electron configuration

\textcolor{#6b7280}{[Rn]}\; \textcolor{#ef4444}{7_{s}^{1}}

[Rn] 7s1

Wave function

\Psi_{\text{Fr}} \approx \frac{1}{\sqrt{87!}} \det \left| \chi_1(\mathbf{r}_1) \dots \chi_{87}(\mathbf{r}_{87}) \right|

Probability density $|\Psi|^2$ across the Fr 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: 260 pmpredicted
Ionization energy: 380 kJ/molpredicted
Electron affinity: 47 kJ/molpredicted
Crystal structure: Body-centered cubic

Oxidation statespredicted

Acid/base

Basic

Strongly acidicStrongly basic

State at 25 °CSolid

300 K950 K298 K

predicted

Emission spectrum

3 transitions

380450500550600650700780

VioletBlueGreenYellowRed

465.4nmFr I (blue)Blue60%

568.4nmFr I (yellow)Yellow40%

612.4nmFr I (orange)Orange100%

Dominant line 612.4 nmRange 465–612 nmTransitions 3