Atomic number 87

Francium

Fr
+ phase
− phase

Shells Orbitals 7s

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87 electrons in 7 shells: 2, 8, 18, 32, 18, 8, 1.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkali metal
Period: 7
Group: 1
Block: s
Atomic mass: 223.01973 Da (longest-lived isotope)
Electron configuration: [Rn] 7s¹
Phase at 25 °C: solid
Melting point: 27 °C
Boiling point: 680 °C
Electronegativity (Pauling): 0.7
Discovered: 1939, Marguerite Perey

Neighbours in the table

Rn Ra

Marguerite Perey of the Curie Institute in Paris found francium in 1939, Los Alamos records, while analysing the decay sequence of actinium, in Jefferson Lab’s account. The element is named in honour of France, Perey’s home country, according to IUPAC, the International Union of Pure and Applied Chemistry. Perey was a physicist in IUPAC’s description and a chemist in Jefferson Lab’s. No weighable amount of francium has ever been made or isolated, Los Alamos states.

Los Alamos also calls francium the heaviest known alkali metal, at the foot of group 1. A radon core with a single 7s electron outside it accounts for all 87 electrons, in shells of 2, 8, 18, 32, 18, 8 and 1. Ionisation energy is the energy needed to pull one electron off a neutral atom. NIST gives francium’s as 4.0727411 eV; Jefferson Lab gives 3.9 eV.

Losing that 7s electron leaves Fr⁺, and +1 is francium’s oxidation state, the charge the atom would carry if every bond it made were ionic. All its known isotopes are highly unstable, so knowledge of its chemistry comes from radiochemical techniques, Los Alamos explains, and it most resembles caesium’s. PubChem gives the mass of its longest-lived isotope, francium-223, 223.01973 Da.

Where it is found

Natural francium exists only as a short stop in a decay chain. Actinium-227 disintegrates by alpha emission into francium-223, the one isotope of francium that occurs in nature, according to Los Alamos. IUPAC places it in uranium minerals, as a result of that actinium decay. Half of any francium-223 is gone within 22.00 minutes in the International Atomic Energy Agency’s (IAEA) evaluation.

The whole crust holds almost none, though francium counts as a natural element, Jefferson Lab notes. It puts the total at no more than one ounce at any one time, and IUPAC at approximately 30 grams. Each figure covers the entire planet’s crust, not a single mine or ore body.

Because so little occurs naturally, francium has to be manufactured before anyone can study it, Jefferson Lab explains. Protons fired at thorium produce it, and so do neutrons fired at radium, in Jefferson Lab’s description.

Why it matters

Francium has no uses outside basic scientific research, Jefferson Lab says, because so little of it is made and its half-life is so short. Nor does francium have isotopic applications beyond research, in IUPAC’s words.

Francium-223 decays in two ways. In the IAEA’s evaluation, 99.994 per cent of its decays are beta-minus emission, in which the nucleus gives out an electron, and 0.006 per cent are alpha emission. The daughters are radium-223 from the beta branch and astatine-219 from the alpha branch, according to Jefferson Lab. Francium-223 is the longest-lived francium isotope, Los Alamos says.

Francium has thirty-three recognised isotopes, Los Alamos says, and none is stable, according to Jefferson Lab. Los Alamos ranks francium as the least stable among the first 101 elements in the periodic table.

As the last member of group 1, francium also caps a periodic trend. Atomic radius grows down a column in Chem1’s account, since each new row adds a shell. PubChem lists francium’s van der Waals radius, measured between atoms that touch without bonding, as 348 pm.

Properties

Properties of francium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	87
Atomic mass	223.01973 Da	Da	PubChem, longest-lived isotope
Melting point	27 °C	°C	PubChem: 300 K
Boiling point	680 °C	°C	Los Alamos: 680 °C
Electronegativity (Pauling)	0.7
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s¹
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Francium’s danger is ionising radiation from its unstable nuclei. Haz-Map, in PubChem’s safety record, classes its hazard as ionising radiation: francium-223 gives out beta particles and, far more rarely, alpha particles. Francium is made only for research, in IUPAC’s account, and no weighable amount has ever been prepared, Los Alamos states. Its reference sheet is the PubChem Laboratory Chemical Safety Summary for francium.

Questions

How much francium is there on Earth?

Very little at any moment. Jefferson Lab estimates no more than one ounce in the whole crust, and IUPAC about 30 grams. All of it is francium-223, which forms when actinium-227 emits an alpha particle, and whose half-life is 22.00 minutes in the evaluation of the International Atomic Energy Agency (IAEA). It turns up in uranium minerals, according to IUPAC.

Who discovered francium and how did it get its name?

Marguerite Perey, working at the Curie Institute in Paris, identified francium in 1939, Los Alamos records. Jefferson Lab says she found it while studying how actinium decays. IUPAC reports that the name honours France, her home country. Francium-223, the isotope formed in actinium's decay, is the only one that occurs in nature, Los Alamos adds, and it is also the longest-lived.

What is francium used for?

Nothing outside basic scientific research. The reasons, in Jefferson Lab's account, are the tiny amounts produced and the short half-life, and in IUPAC's words francium has no isotopic applications beyond research. Laboratories make francium for study by bombarding thorium with protons or radium with neutrons, in Jefferson Lab's description. Los Alamos adds that no sample large enough to weigh has yet been isolated.

What does francium-223 decay into?

Mostly radium-223. The IAEA's evaluation gives 99.994 per cent of decays as beta-minus emission and 0.006 per cent as alpha emission. The beta product is radium-223 and the alpha product astatine-219, according to Jefferson Lab. Half of a sample is gone in 22.00 minutes on the IAEA's figure. PubChem gives the mass of this isotope, francium's longest-lived, as 223.01973 Da.