Imagine an element so rare that less of it exists on Earth than a grain of salt. Francium is the scarcest naturally occurring element on the planet. At any given moment, only 20-30 grams sit in the entire Earth's crust. That is because its most stable isotope survives for just 22 minutes. Blink, and it has already decayed.
Francium was discovered in 1939 by French scientist Marguerite Perey, who noticed unfamiliar radiation during the decay of actinium. She named the element after France. As the heaviest alkali metal, francium should theoretically be the most reactive metal in the periodic table. But there is no way to test that: nobody has ever held a visible amount of francium in their hands.
The largest collection of francium atoms ever assembled — about 300,000 — weighs roughly 10⁻¹⁶ grams. That is a billion times less than a grain of sand. Such tiny quantities are captured in magneto-optical traps cooled to near absolute zero.
In theory, francium is a soft silvery metal with a melting point of only 27 °C. On contact with water it should explode far more violently than cesium. But no experiment has ever confirmed this, because gathering enough material is physically impossible.
Francium is extraordinarily radioactive. Any visible quantity would instantly heat up from its own radiation and vaporize. In practice, scientists work with a few hundred atoms inside specialized laser traps at nuclear laboratories. Direct contact with francium is impossible — it simply does not exist in visible amounts. The main hazard is the intense beta and gamma radiation released during decay.
At any given moment, only 20-30 grams of francium exist on the entire Earth. It is the rarest naturally occurring element on the planet.
The most stable isotope, francium-223, lasts just 22 minutes. After one hour, less than 15% of the original sample remains.
Marguerite Perey discovered francium in 1939 at age 30. She went on to become the first woman elected to the French Academy of Sciences.
In theory, francium explodes on contact with water more violently than cesium or rubidium. But nobody has ever been able to test this — there is simply not enough material.
The most francium scientists have ever gathered in one place is 300,000 atoms. That is 10⁻¹⁶ grams — a billion times lighter than a grain of sand.
Francium has the lowest electronegativity of all elements — just 0.7. It is the most eager atom in the periodic table to give up its electron.
| Isotope | Mass (u) | Abundance | Half-life | Decay |
|---|---|---|---|---|
212Fr☢ | 211.996202 | synthetic | 20.0 minutes | α/β− |
223Fr☢ | 223.019736 | synthetic | 22.00 minutes | β− |
Radioactive decay of actinium