Atomic number 87 · Alkali metal
Francium Fr
Francium (Fr), element 87, is an alkali metal so unstable that every isotope decays within minutes. It is often described as the last element discovered in nature rather than first made in a laboratory, and the Earth's crust is estimated to contain only a few tens of grams at any moment.
Key properties
| Atomic number | 87 |
|---|---|
| Atomic weight | [223] (No stable isotope; mass number of a representative isotope in brackets) |
| Category | Alkali metal |
| Group | 1 |
| Period | 7 |
| Block | s |
| State (25 °C, 1 atm) | Solid |
| Melting point | 300 K (26.9 °C) |
| Boiling point | — |
| Density | — |
| Electronegativity (Pauling) | 0.7 |
| Atomic radius (van der Waals) | 348 pm |
| First ionization energy | 3.9 eV |
| Electron affinity | 0.47 eV |
| Oxidation states | +1 |
| Discovered | 1939 |
Electron configuration
- Condensed
- [Rn]7s1
- Full
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s1
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 8 · 1
Orbital diagram
Position in the table
Discovery
Mendeleev predicted an alkali metal below caesium and called it eka-caesium. Claims in the 1920s and 1930s, such as "virginium" and "moldavium", all proved mistaken.
In 1939 Marguerite Perey at the Curie Institute in Paris was purifying actinium-227 when she detected radiation that the known decay of actinium could not explain. She showed that a small fraction of actinium-227 alpha-decays to form element 87. Perey had started as Marie Curie's assistant, and in 1962 she became the first woman elected to the French Academy of Sciences, as a corresponding member.
Origin of the name
Perey named the element after her country, France. She first proposed "catium", but it was felt to be too easily confused with "cation", so francium was chosen instead.
Main uses
Francium decays too quickly and can be made in amounts too tiny for any practical use. It matters in research:
- Fundamental physics: francium atoms have a simple electronic structure but a heavy nucleus, so laser-trapped francium is used to measure how the weak nuclear force affects atoms (atomic parity violation). Groups such as one at Stony Brook University in New York have gathered hundreds of thousands of francium atoms in magneto-optical traps.
- Spectroscopy: precise measurements of its energy levels provide benchmarks for atomic theory.
Isotopes
All francium isotopes are radioactive. The longest-lived, francium-223, has a half-life of about 22 minutes and forms occasionally when actinium-227 decays in the uranium-235 series. In the laboratory, isotopes such as francium-210 and francium-211 are made by nuclear reactions such as firing oxygen ions at gold.
In everyday life
No one will ever handle francium. By the trends of the alkali metals it should react violently with water, but enough has never been collected to try. Relativistic effects make francium's measured ionisation energy slightly higher than caesium's, so many chemists regard caesium, not francium, as the most reactive metal in practice. Its melting point has never been measured and is only estimated to lie near room temperature.
Good to know
- Francium is often called the last element discovered in nature before being made artificially.
- The Earth's crust holds only a few tens of grams of francium at any moment.
- Its discoverer, Marguerite Perey, became the first woman elected to the French Academy of Sciences, as a corresponding member.
Same group (1)
Data sources · Properties: PubChem Periodic Table (US NIH/NLM public data) · Atomic weights: CIAAW 2024 abridged standard atomic weights · Names: Wikidata (CC0); Korean names follow the Korean Chemical Society. Retrieved 2026-09-23.