Francium
[Rn] 7s¹ · 2 · 8 · 18 · 32 · 18 · 8 · 1
The most unstable of the first hundred elements and the rarest natural metal. About 30 grams of francium exist in the entire crust at any moment, and nobody has ever seen a weighable amount.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 87
- Atomic mass
- 223 а.е.м. ≈
- Electron configuration
- [Rn] 7s¹
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 8 · 1
- Block
- s-block
- Group
- Alkali metals
- Period
- 7
- Electronegativity (Pauling)
- 0.79
- Electronegativity (Allen)
- 0.67
- Atomic radius
- 348 pm
- Covalent radius
- 260 pm
- Van der Waals radius
- 348 pm
- Ionisation energy 1
- 393 kJ/mol
- Electron affinity
- 46.9 kJ/mol
- Common oxidation states
- +1
- Oxidation states
- +1
Physical properties
- State at 25 °C
- solid
- Density
- 1.87 g/cm³ ≈
- Melting point
- 300 K · 26.9 °C ≈
- Boiling point
- 950 K · 676.9 °C ≈
- Speed of sound
- no data
Thermal properties
- Heat of fusion
- 2 kJ/mol ≈
- Heat of vaporisation
- 65 kJ/mol ≈
- Specific heat capacity
- no data
- Thermal conductivity
- no data
Mechanical properties
- Young's modulus
- no data
- Shear modulus
- no data
- Bulk modulus
- no data
- Poisson ratio
- no data
- Mohs hardness
- no data
- Brinell hardness
- no data
Electrical and magnetic properties
- Electrical resistivity
- no data
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- body-centred cubic (bcc)
- Lattice constants
- no data
Abundance
- In the crust
- does not occur
- In the ocean
- no data
- In the universe
- no data
- In the human body
- no data
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 221Fr | — | 4.77 min | α |
| 223Fr | — | 22 min | β− |
The longest-lived isotope is francium-223 at 22 minutes. All the rest live seconds.
Discovery
- Year of discovery
- 1939
- Discovered by
- Marguerite Perey
- Where
- France
- Origin of the name
- named after France
History
Marguerite Perey discovered francium in 1939 at the Curie Institute while studying the decay of actinium-227. She became the first woman elected to the French Academy of Sciences.
Where it occurs
Formed as a minor branch of actinium decay. Estimates put 20–30 grams of francium on the whole planet at once.
How it is produced
In accelerators, by irradiating gold or thorium. Yields are around a million atoms at a time — enough for spectroscopy in a magneto-optical trap.
Role in living things
No biological role.
Safety
Radiotoxic, but the quantities are so small that it poses a real hazard only in the laboratory.
Uses
- Precision tests of the Standard Model: francium is the heaviest alkali atom, where parity violation effects are largest
- Fundamental research into atomic structure
Curiosities
- Francium was the last element found in nature rather than synthesised first.
- Marguerite Perey began as Marie Curie's laboratory assistant and ended a professor and academician.
- Nobody has ever seen francium: even a million atoms is invisible to the eye.
- Francium is formally the most electropositive element, but the claim cannot be tested experimentally.