Francium
Atomic Data
| Atomic Number | 87 |
| Symbol | Fr |
| Atomic Weight | 223 u |
| Density (STP) | N/A |
| Melting Point | 26.85 °C (300 K) |
| Boiling Point | 676.85 °C (950 K) |
| Electronegativity | 0.7 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s1 |
| Oxidation States | +1 |
| Phase at STP | Solid |
| Category | Alkali Metal |
| Period / Group | 7 / 1 |
| CAS Number | 7440-73-5 |
Electron Configuration
[Rn] 7s1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| P | 6 | 6s | 2 | 80 |
| P | 6 | 6p | 6 | 86 |
| Q | 7 | 7s | 1 | 87 |
| Total | 87 | 87 | ||
Isotopes of Francium
Francium is monoisotopic: ²²³Fr is its only naturally occurring stable isotope, accounting for 100% of all natural Francium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Francium-223 | ²²³Fr | 87 | 136 | trace | Stable |
Abundance & Occurrence
Francium is present in Earth's crust at approximately trace amounts by mass and at approximately trace amounts by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Extreme radioactivity: Francium-223 (t½ = 21.8 min) is the most unstable of the first 103 elements; it decays by beta emission, making it a radiation hazard during handling; the total inventory of francium ever in existence is tiny.
- Alkali metal reactivity: As the heaviest alkali metal, francium is expected to be the most reactive with water and air: more violent than caesium; however, quantities are too small (typically atom-counting experiments) to make this a practical chemical hazard.
- Handling context: Francium is studied only in trace quantities using ion traps and laser cooling; it is never handled as a bulk chemical, and all work occurs in specialised nuclear physics facilities with full radiation controls.
- Regulatory controls: Any production and handling of francium requires a nuclear regulatory authority licence; the quantities involved are so small that the radiological hazard is primarily theoretical rather than practical.
Francium in the Real World
Real-World Uses
- Atomic physics and quantum electrodynamics research: Francium's simple atomic structure (one valence electron) and heavy nucleus make it ideal for precision spectroscopic measurements that test quantum electrodynamic corrections and search for parity non-conservation effects at the nuclear level.
- Nuclear structure studies: Francium isotopes produced in accelerator reactions are studied using laser spectroscopy and decay measurements to probe nuclear moments, deformations, and shell structure in the region near the doubly-magic lead-208 nucleus.
- Fundamental symmetry searches: Experiments on trapped francium atoms search for permanent electric dipole moments, which would indicate CP violation beyond the Standard Model and could help explain the matter-antimatter asymmetry of the universe.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
Has francium ever been observed in large quantities?
No. Francium is the second rarest naturally occurring element on Earth (after astatine). The total amount in Earth's crust at any moment is estimated at less than 30 grams, produced only as a decay product of actinium. Its longest-lived isotope, Fr-223, has a half-life of only 22 minutes. The largest quantity ever assembled in a laboratory was about 300,000 atoms, accumulated using laser trapping techniques at SUNY Stony Brook.
Is francium the most reactive metal?
Francium is predicted to be the most reactive alkali metal, but this is theoretical: its extreme rarity and very short half-life mean a macroscopic reaction with water has never been observed. Based on trends down Group 1 and relativistic calculations, francium would react explosively with water, but the few hundred thousand atoms that can be assembled in a trap are far too few to see a reaction macroscopically.
How was francium discovered?
Francium was discovered in 1939 by French physicist Marguerite Perey at the Curie Institute in Paris, while purifying actinium-227 samples. She noticed an unexpected alpha-decay branch in actinium and identified the product as a new alkali metal element with atomic number 87. She named it francium after France. Marguerite Perey was the last person to discover a naturally occurring element, and one of the first women to be elected to the French Académie des Sciences.
What are francium's predicted chemical properties?
Francium is directly below caesium in Group 1 and is predicted to be the most electropositive and least electronegative of all elements. Relativistic effects are significant at Z=87, causing the 7s orbital to contract. Theoretical and limited experimental studies suggest francium's ionic radius is similar to or slightly smaller than caesium's (contrary to simple trends) and that it forms Fr+ ions easily. Its chemistry, to the extent it has been studied in tracer quantities, closely follows caesium.