Francium is a chemical element with the symbol Fr and atomic number 87. It is extremely radioactive; its most stable isotope, francium-223 (originally called actinium K after the natural decay chain in which it appears), has a half-life of only 22 minutes. It is the second-most electropositive element, behind only caesium, and is the second rarest naturally occurring element (after astatine). Francium's isotopes decay quickly into astatine, radium, and radon. The electronic structure of a francium atom is [Rn] 7s1; thus, the element is classed as an alkali metal.

Bulk francium has never been seen. Because of the general appearance of the other elements in its periodic table column, it is presumed that francium would appear as a highly reactive metal if enough could be collected together to be viewed as a bulk solid or liquid. Obtaining such a sample is highly improbable since the extreme heat of decay resulting from its short half-life would immediately vaporize any viewable quantity of the element.

Francium was discovered by Marguerite Perey in France (from which the element takes its name) in 1939. Before its discovery, francium was referred to as eka-caesium or ekacaesium because of its conjectured existence below caesium in the periodic table. It was the last element first discovered in nature, rather than by synthesis. Outside the laboratory, francium is extremely rare, with trace amounts found in uranium ores, where the isotope francium-223 (in the family of uranium-235) continually forms and decays. As little as 200–500 g exists at any given time throughout the Earth's crust; aside from francium-223 and francium-221, its other isotopes are entirely synthetic. The largest amount produced in the laboratory was a cluster of more than 300,000 atoms.

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Atomic properties

Atomic mass(223.019 u)

Atomic radii

Covalent radius(260 pm)
Van der Waals radius(348 pm)

Atomic shell

Electron configurationRn 7s1
Ionization energy(1st) 4.072741 eV
(2nd) 22.4 eV
(3rd) 33.5 eV
(4th) 39.1 eV
(5th) 50 eV
Shell model

Physical properties

Density(2.9 g·cm−3)
Molar volume(7.7·10-5 m3·mol−1)


Melting point300 K
Boiling point950 K
Liquid range650 K


Melting enthalpy(2 kJ·mol-1)
Enthalpy of vaporization(65 kJ·mol-1)
Binding energy(64 kJ·mol-1)

Heat and conductivity

Thermal conductivity(15 W·m-1·K-1)



Chemical properties

Oxidation state+1
Standard potential(-2.92 V) (Fr+ + e- → Fr)


Pauling scale0.7
Allred-Rochow scale0.86
Mulliken scale0.68
Nagle scale0.82

Other properties

Natural occurrenceDecay product
Crystal structureBody-centered cubic
Goldschmidt Classificationsynthetic
Superconductorwithout transition tempperature
Radioactivityhighly radioactive