Francium (Fr)

Francium (symbol Fr) is the chemical element with atomic number 87: an alkali metal, solid at 0 °C, radioactive, with an atomic mass of 223 u. Discovered in 1939 (Marguerite Perey).

Francium is an alkali metal, a cousin of sodium and caesium, but so radioactive that it never survives more than a few moments. Its most stable isotope, francium-223, has a half-life of 22 minutes: by then, half the atoms in a sample have already decayed. No one has ever seen a piece of it with the naked eye. It was discovered in 1939 by a Frenchwoman, Marguerite Perey.

Key facts

Atomic number: 87
Symbol: Fr
French name: Francium
Atomic mass: 223 u
Molar mass: [223] g/mol
Family: Alkali metals
Position: period 7, group 1, s-block
State at 0 °C: Solid
Melting point: 26.9 °C
Boiling point: 676.9 °C
Electronegativity (Pauling): 0.79
First ionization energy: 393 kJ/mol
Electron configuration: [Rn] 7s¹
Radioactive: yes
Discovery: 1939, Marguerite Perey

Origin of the name

From France, the homeland of the woman who discovered it.

What does francium look like?

Never observed in visible amounts: a sample would vaporize from the heat of its own radioactivity. By analogy with caesium, it is thought to be a soft metal with a golden or silvery sheen, melting just above room temperature. The largest “samples” ever assembled are clouds of a few hundred thousand atoms held in laser traps.

Where is francium found?

It appears in vanishingly small traces in uranium and thorium ores, as a short-lived link in their decay chains. At any given moment, the entire Earth's crust is estimated to hold a few tens of grams at most, scattered atom by atom. It is one of the rarest naturally occurring elements.

Natural sources

Traces in uraninite (pitchblende)
Traces in thorium ores

How is francium produced?

It is made in the lab, atom by atom, by firing a beam of oxygen ions at a gold target in a particle accelerator, or from the decay of actinium-227. The atoms produced are then trapped and cooled with lasers so they can be studied for a few seconds or minutes.

What is francium used for?

Basic research only
Precision tests of atomic physics (structure of heavy atoms)
Studies of the weak interaction inside the nucleus

In everyday life

None — it lives far too briefly for any practical use

Role in living things

No biological role. It has never existed in large enough amounts for its behavior in a living organism to be studied in detail; it would probably behave like potassium or caesium.

Hazards of francium

In theory very dangerous, both for its intense radioactivity and for its alkali-metal reactivity (violent reaction with water). In practice, the risk to the public is zero: there is never enough of it to cause a problem.

Cosmic origin

None of it survives from the formation of the Solar System: it is constantly being created by the radioactive decay of uranium and thorium.

History of francium

Mendeleev had predicted an “eka-caesium” below caesium. After several false discoveries (“virginium,” “moldavium”), Marguerite Perey, a former lab assistant to Marie Curie, identified it in 1939 at the Radium Institute in Paris while studying the decay of actinium. She named it francium in honor of France.

Did you know?

It was the last element to be discovered in nature first; every element found since was made by humans before possibly being spotted in trace amounts.
In 1962, Marguerite Perey became the first woman elected (as a corresponding member) to the French Academy of Sciences, which Marie Curie had never been able to join.
Perey died of cancer, very likely linked to the radiation she absorbed during her career.

Isotopes of francium

No stable isotopes, and the longest-lived one survives only 22 minutes. It exists in nature only in minuscule amounts: a few dozen grams at any given moment in the entire Earth's crust.

francium-221: radioactive, half-life 4.8 min. Produced by actinium-225 during alpha-radiation cancer treatments.
francium-223: radioactive, half-life 22 min. The longest-lived; isolated in 1939 by Marguerite Perey among the decay products of actinium.

Related elements

Same family (alkali metals): Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs).

Same column (group 1): Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs).