Francium · element 87
The last element found in nature rather than made in a lab, with barely an ounce of it on Earth at any moment
Key facts
Francium sits at the bottom of the alkali metals, beneath lithium, sodium and caesium, and on paper it should be the most reactive metal of all. In practice nobody has ever tested that, because nobody has ever had a visible piece. The most stable isotope, francium-223, loses half its atoms every 22 minutes.
It was discovered in 1939 by Marguerite Perey, who began her career at nineteen as Marie Curie's laboratory assistant. Francium was the last element to be found in nature before the era of artificially made elements began, and Perey named it after her country.
Where it is found
Francium-223 is produced constantly, and destroyed almost as fast, in uranium ores. Actinium-227, a rare member of the uranium-235 decay chain, decays about one time in seventy by emitting an alpha particle rather than an electron, and the product is francium, which itself decays within minutes. The result is a vanishingly small population: roughly one francium atom for every billion billion atoms of uranium, and an estimated 20 to 30 grams on the whole planet at any time.
Physicists who want francium make it themselves, by firing protons at thorium or heavy ions at gold, then trap the handful of atoms produced with lasers and magnetic fields. Clouds of a few hundred thousand atoms have been held this way, the most francium anyone has ever seen at once.
How it was discovered
Element 87 had been claimed several times under names like russium, virginium and moldavium, and every claim collapsed. Marguerite Perey found the real thing while doing routine work at the Radium Institute in Paris. Purifying samples of actinium-227, she noticed a faint extra radioactivity that did not fit actinium or any of its known decay products.
By careful chemistry she showed that the mystery substance behaved like an alkali metal, following caesium through her separations, and decayed with a half-life of about 21 minutes. She published in 1939, first proposing the name catium, which colleagues rejected, and settled on francium in 1946. In 1962 Perey became the first woman elected to the French Academy of Sciences. Like her mentor, she died of cancer linked to decades of radiation exposure.
What we use it for
Francium has no commercial uses and never will: there is too little, it disappears too quickly, and everything it could do, caesium does more cheaply. In the 1950s researchers noticed that francium concentrated in certain tumours in rats and briefly considered it for cancer diagnosis, but the idea was impractical.
Its value is to fundamental physics. A single outer electron orbiting a very heavy nucleus makes francium a beautifully simple system for testing theories. Experiments with trapped atoms probe the weak nuclear force and search for tiny asymmetries in the electron that would hint at physics beyond the standard model; the heavy nucleus amplifies these effects, so francium can reveal things lighter atoms cannot.
Reading the numbers
Every physical property of francium is an estimate. The melting point of about 27 °C would mean it melts in a warm room, much like caesium at 28 °C and gallium at 30 °C. The boiling point near 680 °C and density of 1.87 grams per cubic centimetre, roughly twice that of water, are extrapolated from the trend down group 1. The atomic mass of 223 is the mass number of the longest lived isotope, not a measured average.
The electron configuration [Rn] 7s1 could not be simpler: a full radon core with a single electron in the seventh shell. That lone electron defines the alkali metals and is given up easily, and francium's first ionisation energy of 380 kJ/mol is among the lowest of any element. Yet it is slightly higher than caesium's 376, breaking the trend that ionisation energy always falls down a group. In such a heavy atom the 7s electron moves fast enough for relativistic effects to pull it closer to the nucleus. Its electronegativity of 0.79 matches caesium. Francium opens period 7, the last row of the table.
Safety and the environment
Francium's only hazard is radiological, and even that is theoretical, since no laboratory holds more than a few hundred thousand atoms at once. Experiments are shielded and monitored, but the total radioactivity involved is tiny.
There is no francium pollution and no francium waste. The natural traces in uranium ore are in equilibrium, created and destroyed at the same rate, and have been for billions of years. If a lump of francium could ever be assembled, it would be expected to react violently with air and water like caesium, but it would vaporise from its own decay heat first.
Did you know?
- Marguerite Perey joined the Radium Institute at nineteen as Marie Curie's personal laboratory assistant and discovered francium ten years later.
- The total amount of francium in the Earth's crust at any moment is estimated at only 20 to 30 grams.
- Francium was the last element discovered in nature; every element found after 1939 was first made artificially.
- Perey originally proposed the name catium for the new element, but colleagues objected and francium was chosen instead.
- Francium's first ionisation energy is slightly higher than caesium's, because relativistic effects in the heavy atom pull its outer electron inward.
- Physicists have trapped clouds of a few hundred thousand francium atoms with lasers and photographed their faint glow, the largest quantity ever seen together.
More in the alkali metals
These share a column with francium, so their outer electrons are arranged in a similar way and they tend to behave alike.
See all 6 alkali metals on one page, or browse all 118 elements.
Compare Francium with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Is francium the most reactive metal?
It is often described that way because it sits at the bottom of group 1, but its outer electron is actually held slightly more tightly than caesium's due to relativistic effects. Caesium may therefore be marginally more reactive. The question has never been settled by experiment because no visible sample of francium exists.
What happens if francium touches water?
Nobody knows from experience. Based on caesium, a piece of francium would be expected to react explosively, producing hydrogen and francium hydroxide. In reality a visible piece could never be assembled, because its intense radioactivity would boil it away first.
Why is francium so rare?
It forms only as a rare side branch in the decay of actinium-227, itself a scarce element, and every francium isotope decays within minutes. Atoms are destroyed almost as fast as they are made, so the planet-wide total never exceeds a few tens of grams.
Is francium dangerous?
In principle it is highly radioactive, but no one is ever exposed to meaningful amounts. Laboratory experiments work with a few hundred thousand atoms, and natural traces in rock are far too small to matter. It poses no practical hazard to the public.
Read next
Guess the mystery element in six tries. A new one every day, free, no sign-up.
Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Francium reference article. Spotted an error? Tell us.