Francium

Francium (Fr) is an alkali metal with atomic number 87. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight; its longest-lived isotope has mass number 223. Its electron configuration is [Rn] 7s1, which gives it 1 valence electron.

Francium key data

Symbol	Fr
Atomic number	87
Mass number (longest-lived isotope)	223
Protons	87
Neutrons (longest-lived isotope, francium-223)	136
Electrons (neutral atom)	87
Valence electrons	1
Category	Alkali metal
Period	7
Group	1
Block	s-block
Noble gas notation	[Rn] 7s1
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s1
Electrons per shell	2, 8, 18, 32, 18, 8, 1

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

Properties of francium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown. The first ionisation energy is the value for francium from the NIST Atomic Spectra Database, in place of PubChem’s 3.9 eV.

Predicted melting point	300 K (26.85 °C), 90th of 103 by melting point
Predicted state at room temperature	Solid
Predicted electronegativity	0.7 (Pauling), 95th of 95 by electronegativity
Van der Waals radius	348 pm, 1st of 99 by atomic radius
First ionisation energy	4.0727 eV (392.96 kJ/mol), 101st of 102 by first ionisation energy
Predicted electron affinity	0.47 eV (45.3 kJ/mol), 44th of 57 by electron affinity
Common oxidation states	+1
Discovered	1939

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Francium electron configuration

The full electron configuration of francium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s1. With the noble-gas core collapsed it is [Rn] 7s1, and the outermost subshell is 7s1.

Bohr model of francium (Fr): a nucleus and seven shells holding 2, 8, 18, 32, 18, 8 and 1 electrons from the inside out. — Bohr model of francium (Fr). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of francium (Fr), [Rn] 7s1, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 7s. — Orbital diagram of francium (Fr). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one francium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 223 / (6.02214076 × 10²³)
= 3.7030 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 223
= 2.7005 × 10²¹ atoms

Francium has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of francium-223, so treat these as nominal.

Francium on the periodic table

Francium is in period 7, group 1, in the s-block.

The other elements of group 1 are hydrogen, lithium, sodium, potassium, rubidium and caesium.