Francium – facts and properties
- Symbol
- Fr
- Atomic number
- 87
- Atomic mass
- 223 u
- Period
- 7
- Group
- 1
- Category
- alkali metal
- Electron configuration
- [Rn] 7s¹
- Electronegativity
- 0.7
- Valence electrons
- 1
- State at standard conditions
- solid
- Melting point
- 27 °C
- Discovered
- 1939
Francium is an extremely radioactive alkali metal with atomic number 87. After astatine it is the second rarest element found in nature, and nobody has ever seen or weighed a sample of it. The physicist Marguerite Perey discovered it in 1939 at the Curie Institute in Paris and named it after her home country, France. Today it serves basic research alone: scientists cool single francium atoms with laser light in traps to test atomic physics with great precision.
Position in the periodic table
Occurrence and extraction
In nature, francium arises only as a rare side branch of the uranium-235 decay series. The intermediate product actinium-227 decays almost always by beta emission to thorium-227. In a small share of cases it emits an alpha particle instead and becomes francium-223. This isotope has a half-life of about 22 minutes, which makes it the longest-lived isotope of the element. At any moment the whole of the Earth's crust therefore holds only traces, an amount that can be neither seen nor collected.
Marguerite Perey was working at the Curie Institute with purified actinium-227 when she noticed radiation that could not come from the actinium itself. She let caesium salts crystallise out of the solution and found that the unknown substance came down with them. That showed her a new alkali metal behaving like a heavier caesium. She announced the find in 1939. Francium was the last element to be discovered in nature before it was ever made in a laboratory. Perey later became a professor at Strasbourg. She died in 1975 of a cancer linked to the radiation exposure in her work.
Properties and reactions
Francium is in period 7 and group 1, below caesium, and as the heaviest alkali metal it closes the group. Radium is its neighbour to the right in the period. The atom has one valence electron, and its electron configuration is [Rn] 7s¹. Its atomic mass is given as 223 u, the mass number of the longest-lived isotope, francium-223.
Almost none of its properties have been measured. The electronegativity of 0.7 and the melting point of 27 °C are estimates, derived from the trends in the group and from calculations. There are no measured values for the boiling point, the density or the atomic radius. Even its chemistry is known only from studies with tiny traces: francium ions resemble caesium ions and are carried down with some sparingly soluble caesium salts.
A very reactive metal is expected, one that forms a singly charged positive ion and would react violently with water. Whether it is even more reactive than caesium has not been shown, and calculations suggest that relativistic effects change the behaviour of such heavy atoms noticeably.
Francium-223 mostly changes by beta decay into radium-223, an alpha emitter that continues the decay series.
Research and synthesis
Because francium cannot be obtained from nature, it is made artificially. In a particle accelerator, a beam of ions or protons hits a target, for example of gold or uranium carbide. The atoms that form must be collected in a trap within a few minutes, before they decay. There are no everyday uses, and it would not be possible to gather enough material for one.
As the heaviest alkali metal, francium has a single outer electron, as sodium and caesium do, and its spectrum is comparatively easy to interpret. Research groups at accelerator facilities in North America, Europe and Japan cool the atoms with laser light to a fraction of a degree above absolute zero and hold them in magneto-optical traps. In these traps they measure the energy levels of the atom very accurately and test how well theory describes them.
Of particular interest are tiny violations of mirror symmetry caused by the weak interaction, which are larger in heavy atoms. The search for an electric dipole moment of the electron is another goal. It would point to physics beyond today's Standard Model.
Quick question
What is the atomic number of francium (Fr)?
FAQ
Why has nobody ever seen francium?
Half of any francium-223, the longest-lived isotope, has decayed after about 22 minutes. In nature it occurs only in traces, and in the laboratory researchers make single atoms rather than visible amounts. A sample to look at or weigh has never existed.
What is francium used for?
Nothing in everyday life. Researchers catch francium atoms in traps with laser light and measure the atomic shell very precisely. The results test theories of atomic physics and help in the search for new physics.
Where does the name francium come from?
Marguerite Perey named the element after France, the country of her birth and of her research. The name was accepted internationally a few years after the discovery.