Francium is a short-lived radioactive alkali element whose study depends on small quantities produced or detected through nuclear processes. It has no assigned standard atomic weight. Its position below cesium provides a useful comparison, but that position should not be mistaken for measured bulk-material properties.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
Why can a few trapped francium atoms test atomic theory?
Laser-cooled and trapped francium atoms
FOLLOW THIS RELATIONSHIP
Define an atomic observable
Francium research can examine transitions between electronic states. The question concerns the behavior of individual atoms, so a visible sample of metal is unnecessary.
Explore the model (optional)
FrFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
Optional: check your understanding
USE THE EXPLANATION
Why is a macroscopic metal specimen unnecessary here?
Atomic identity, reference properties and structure models
INTERACTIVE 3D87 visible sites
An electron censusDrag to rotate · scroll or pinch to zoom
[Rn] 7s1. Grouping electrons by principal shell connects the neutral-atom configuration to periodic-table patterns.
What this representation means
A three-dimensional counting diagram. Dots are count markers, not measured electron positions; spherical placement and spacing are arbitrary. There are no electron trajectories here. Orbitals describe quantum states.
Marguerite Perey identified francium in 1939 while examining the radioactive products of actinium. Instead of finding a rich of a new metal, she recognized evidence for a small branch producing element 87. Naming it for France followed a tradition also visible in polonium. Francium became a reminder that a naturally occurring element can be real and identifiable without ever existing as a durable macroscopic specimen.
1939
Natural discovery
Perey identifies element 87 among actinium decay products.
2013
Trapping facility reported
Researchers report commissioning a francium trapping facility at TRIUMF.
Francium appears briefly in natural radioactive decay chains, especially through actinium. The atoms disappear too quickly to accumulate into a mineable material or a piece of metal for display. Laboratory work instead uses freshly produced , whose limited availability shapes what researchers can measure and which questions they can ask.
One outer places francium in the alkali-metal family, making atomic comparisons with cesium especially useful. Experiments work with small numbers of atoms rather than ordinary metal samples. Measured can test electronic structure, while quoted bulk melting points or appearances require caution because direct macroscopic measurements are unavailable.
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
CITE THIS PAGE
Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
solid
293.15 K, 101.325 kPa
Melting point in K
300 K
at standard pressure unless noted Estimated; francium has never been isolated in bulk.
Boiling point
Not recorded
101.325 kPa
Density
Not recorded
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity
Not recorded
Pauling scale
First ionization energy in kJ/mol
380 kJ/mol
gas-phase neutral atom, ground state
Covalent radius in pm
260 pm
single-bond covalent radius
Common oxidation states
+1
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust
Not recorded
upper continental crust, mass fraction
Has stable isotopes
Noderived
Discovery year in CE
1939 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 7
Ra 0.9 Pauling · Radium
Electronegativity across period 7. Exact values.
Ra
0.9
Ac
1.1
Th
1.3
Pa
1.5
U
1.38
Np
1.36
Pu
1.28
Am
1.13
Cm
1.28
Bk
1.3
Cf
1.3
Es
1.3
Fm
1.3
Md
1.3
No
1.3
Lr
1.3
Electronegativity down group 1
H 2.2 Pauling · Hydrogen
Electronegativity down group 1. Exact values.
H
2.2
Li
0.98
Na
0.93
K
0.82
Rb
0.82
Cs
0.79
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
