Francium

Francium is the most unstable naturally occurring element and the second rarest after astatine. It is so radioactive that only tiny amounts exist on Earth at any moment, no one has ever seen enough francium to observe its physical appearance. What little we know suggests it is the most reactive alkali metal of all.

• Atomic Number8787 protons, 87 electrons
• Atomic Mass223.01973 u87× heavier than hydrogen
• State at Room TempSolidSolid
• DensityNot measured
• Melting / Boiling26.9°C
• Discovered1939

What is Francium?

Francium is an alkali metal in Group 1 with 87 protons. All of its isotopes are radioactive. The most stable, francium-223, has a half-life of just 22 minutes. Any primordial francium formed when the Earth was born decayed long ago. The tiny amount that exists today is produced by the radioactive decay of actinium.

Named after France, discovered by the French physicist Marguerite Perey in 1939 at the Curie Institute in Paris. She found it as a decay product of actinium-227. Perey became the first woman elected to the French Academy of Sciences. The symbol Fr comes from the name.

Fact Francium is the most unstable naturally occurring element and the second rarest. Only approx. 30 grams exist in the Earth's crust at any moment, yet chemists can trap individual francium atoms using laser cooling and study their quantum properties.

Where you find Francium

On Earth

Francium does not occur in any usable quantity, only a few thousand atoms exist anywhere in the Earth at any moment. It is produced in nuclear reactions involving actinium.

• Tiny amounts occur naturally. Only approx. 30 grams of francium exist naturally in the Earth's entire crust at any one time, a consequence of its 22-minute half-life.

How we use Francium

• Research only.. Francium has no practical uses. Small numbers of francium atoms are trapped in laser cooling experiments to study atomic structure and probe for violations of fundamental physical symmetries.

Did you know? Marguerite Perey discovered francium in 1939 while carefully studying the decay products of actinium. She noticed a previously unknown alpha-decay mode producing a new radioactive element. It was the last element to be discovered by a single person rather than a team. Perey was a student of Marie Curie and was exposed to high levels of radiation throughout her career, eventually dying of bone cancer in 1975.

How it was discovered

Discovered by Marguerite Perey in Paris in 1939, who identified it as a decay product of actinium-227. She was working at the Radium Institute founded by Marie Curie. Francium was the last naturally occurring element to be discovered before synthetic elements were produced artificially.

Deeper dive: francium properties and applications

Despite its extreme rarity and radioactivity, francium atoms can be trapped and cooled using lasers. At temperatures close to absolute zero, the atomic properties of francium can be measured with extraordinary precision, allowing physicists to search for tiny violations of fundamental symmetries in nature, experiments that probe physics beyond the Standard Model.

Moving to 88 protons on the periodic table brings us to Radium.