Alkali metalElement 87 · s-block

Francium (Fr)

Francium (Fr) is an alkali metal with atomic number 87; its most stable known isotope has mass number 223. At room temperature it is a solid that melts at 21 °C (estimated).

87FrFrancium[223]

Atomic number

protons

Atomic weight

[223]

most stable isotope

Melting point

21 °C

estimated

Boiling point

—

Density

1.87 g/cm³

predicted

Electronegativity

0.7

Pauling scale

State at 25 °C

Solid

Discovered

1939

France

Francium at a glance

Identity

Symbol	Fr
Atomic number (Z)	87
Atomic weight	[223] mass number of the most stable isotope
Category	Alkali metal
Group	1 — alkali metals (with hydrogen)
Period	7
Block	s-block
CAS number	7440-73-5

Atomic properties

Electron configuration	[Rn] 7s¹ Full: 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
Oxidation states	+1
Electronegativity (Pauling)	0.7
First ionisation energy	4.073 eV (393 kJ/mol) 2nd: 22.4 eV · 3rd: 33.5 eV
Electron affinity	0.486 eV
Covalent radius	223 pm
Van der Waals radius	348 pm

Physical properties

State at 25 °C	Solid
Melting point	21 °C · 69.8 °F · 294.15 K estimated
Density	1.87 g/cm³ (predicted)
Crystal structure	Body-centred cubic

Isotopes & abundance

Natural occurrence	From radioactive decay
Radioactive	Yes
Most stable isotope	²²³Fr — half-life 22 min
Known isotopes	37

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Francium electron configuration

Francium has 87 electrons. Written in short form, its ground-state configuration is [Rn] 7s¹ — the electrons of the noble gas radon, plus 7s¹.Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 7s¹.

As a member of group 1 it has 1 valence electron in its outer shell (shell 7), which largely decides its chemistry.

7s↑

Discovery and name

Discovered

1939 — Marguerite Perey · France.

Name

France.

Known for

The last element discovered in nature. Extremely rare and short-lived — its longest-lived isotope has a half-life of 22 minutes.

Isotopes of francium

Francium has no stable isotopes. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
²²³Fr	87	136	22 min

37 isotopes of francium are known in our data set (half-lives as published in nuclear data tables).

Francium compared with group 1

The other elements in group 1 (alkali metals (with hydrogen)), from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Hydrogen	1	1.0080	-259 °C	-253 °C	0.0899 g/L	2.2
Lithium	3	6.94	180 °C	1,342 °C	0.534 g/cm³	0.98
Sodium	11	22.990	98 °C	883 °C	0.97 g/cm³	0.93
Potassium	19	39.098	64 °C	759 °C	0.89 g/cm³	0.82
Rubidium	37	85.468	39 °C	688 °C	1.53 g/cm³	0.82
Cesium	55	132.91	28 °C	671 °C	1.873 g/cm³	0.79
Francium	87	[223]	21 °C (est.)	—	1.87 g/cm³ (pred.)	0.7

Frequently asked questions

6 answers

What is the atomic mass of francium?

Francium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [223], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does francium have?

A neutral francium atom has 87 protons and 87 electrons. Its most stable isotope, francium-223, has 136 neutrons.

What is the electron configuration of francium?

[Rn] 7s¹ (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 7s¹).

What are the melting and boiling points of francium?

Francium melts at 21 °C (69.8 °F) (estimated).

Is francium a metal?

Francium is a metal. It is in group 1, period 7, and is a solid at room temperature.

Who discovered francium?

1939 — Marguerite Perey (France).