Periodic Table > Francium


Some elements exist in several different structural forms, these are called allotropes.

For more information on Murray Robertson’s image see Uses and properties facts below.


Fact box terminology

Elements appear in columns or ‘groups’ in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

Elements are laid out into rows or ‘periods’ so that similar chemical behaviour is observed in columns.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp, principal, diffuse, and fundamental.

Atomic Number
The number of protons in the nucleus.

Atomic Radius/non -bonded (Å)
based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties, for more details please refer to the CRC Handbook of Chemistry and Physics.

Electron Configuration
The arrangements of electrons above the last (closed shell) noble gas.

Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.

Melting Point (°C)
The temperature at which the solid–liquid phase change occurs.

Melting Point (K)
The temperature at which the solid–liquid phase change occurs.

Melting Point (°F)
The temperature at which the solid–liquid phase change occurs.

Boiling Point (°C)
The temperature at which the liquid–gas phase change occurs.

Boiling Point (K)
The temperature at which the liquid–gas phase change occurs.

Boiling Point (°F)
The temperature at which the liquid–gas phase change occurs.

Elements that do not possess a liquid phase at atmospheric pressure (1 atm) are described as going through a sublimation process.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative Atomic Mass
The mass of an atom relative to that of Carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists the value given is the abundance weighted average.

Key Isotopes (% abundance)
An element must by definition have a fixed number of protons in its nucleus, and as such has a fixed atomic number, however variants of an element can exist with differing numbers of neutrons, and hence a different atomic masses (e.g. 12C has 6 protons and 6 neutrons and 13C has 6 protons and 7 neutrons).

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems (where several isotopes exist, a value is presented for the most prevalent isotope).

Fact box

Group Melting point 21°C, 70°F, 294 K 
Period Boiling point 650°C, 1202°F, 923 K 
Block Density (g cm−3) Unknown 
Atomic number 87  Relative atomic mass [223]  
State at 20°C Solid  Key isotopes 223Fr 
Electron configuration [Rn] 7s1  CAS number 7440-73-5 
ChemSpider ID 4886484 ChemSpider is a free chemical structure database

Uses and properties terminology

Image Explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.

Natural Abundance

Where this element is most commonly found in nature.

Biological Roles

The elements role within the body of humans, animals and plants. Also functionality in medical advancements both today and years ago.


The description of the element in its natural form.

Uses and properties

Image explanation
The image reflects the ancient cultural ‘Gallic’ iconography of France, the country that gives the element its name.
An intensely radioactive metal.
Francium has no uses, having a half life of only 22 minutes.
Biological role
Francium has no known biological role. It is toxic due to its radioactivity.
Natural abundance
Francium is obtained by the neutron bombardment of radium in a nuclear reactor. It can also be made by bombarding thorium with protons.
Atomic data terminology

Atomic radius/non -bonded (Å)
Based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties,for more details please refer to the CRC Handbook of Chemistry and Physics.

Electron affinity (kJ mol−1)
The energy released when an additional electron is attached to the neutral atom and a negative ion is formed (where several isotopes exist, a value is presented for the most prevalent isotope). *

Electronegativity (Pauling scale)
The degree to which an atom attracts electrons towards itself, expressed on a relative scale as a function bond dissociation energies, Ed in eV. χA - χB =(eV)-1/2sqrt(Ed(AB)-[Ed(AA)+Ed(BB)]/2), with χH set as 2.2 (where several isotopes exist, a value is presented for the most prevalent isotope).

1st Ionisation energy (kJ mol−1)
The minimum energy required to remove an electron from a neutral atom in its ground state (where several isotopes exist, a value is presented for the most prevalent isotope).

Covalent radius (Å)
The size of the atom within a covalent bond, given for typical oxidation number and coordination (where several isotopes exist, a value is presented for the most prevalent isotope). ***

Atomic data

Atomic radius, non-bonded (Å) 3.48 Covalent radius (Å) 2.42
Electron affinity (kJ mol−1) 44.38 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)

Mining/Sourcing Information

Data for this section of the data page has been provided by the British Geological Survey. To review the full report please click here or please look at their website here.

Key for numbers generated

Governance indicators

1 (low) = 0 to 2

2 (medium-low) = 3 to 4

3 (medium) = 5 to 6

4 (medium-high) = 7 to 8

5 (high) = 9

Reserve distribution (%)

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %

(Where data are unavailable an arbitrary score of 2 was allocated. For example, Be, As, Na, S, In, Cl, Ca and Ge are allocated a score of 2 since reserve base information is unavailable. Reserve base data are also unavailable for coal; however, reserve data for 2008 are available from the Energy Information Administration (EIA).)

Production Concentration

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %

Crustal Abundance

1 (low) = 100 to 1000 ppm

2 (medium-low) =10 to 100 ppm

3 (medium) = 1 to 10 ppm

4 (medium-high) = 0.1 to 1 ppm

5 (high) = 0.1 ppm

(Where data are unavailable an arbitrary score of 2 was allocated. For example, He is allocated a score of 2 since crustal abundance data is unavailable.)

Explanations for terminology

Crustal Abundance (ppm)

The abundance of an element in the Earth's crust in parts-per-million (ppm) i.e. The number of atoms of this element per 1 million atoms of crust.


The country with the largest reserve base.

Reserve distribution (%)

This is a measure of the spread of future supplies, recording the percentage of a known resource likely to be available in the intermediate future (reserve base) located in the top three countries.

Production Concentrations

This reports the percentage of an element produced in the top three countries. The higher the value, the larger risk there is to supply.

Political stability of top producer

The World Bank produces a global percentile rank of political stability. The scoring system is given below, and the values for all three production countries were summed.

Relative Supply Risk Index

The Crustal Abundance, Reserve distribution (%), Production Concentration and Governance Factor scores are summed and then divided by 2, to provide an overall Relative Supply Risk Index.


Oxidation states and isotopes

Key for Isotopes

Half Life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture


Common Oxidation states
The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Free atoms have an oxidation state of 0, and the sum of oxidation numbers within a substance must equal the overall charge.

Important Oxidation states
The most common oxidation states of an element in its compounds.

Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.

Oxidation states and isotopes

Common oxidation states 1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  223Fr 223.020 - 22.0 m  β- 

Pressure and temperature - advanced terminology

Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus (GPa)

Young's modulus is a measure of the stiffness of a substance, that is, it provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus (GPa)

The shear modulus of a material is a measure of how difficult it is to deform a material, and is given by the ratio of the shear stress to the shear strain.

Bulk modulus (GPa)

The bulk modulus is a measure of how difficult to compress a substance. Given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour Pressure (Pa)

Vapour pressure is the measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
Unknown Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) Unknown
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - - - - - - - -
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Mendeleev said there should be an element like caesium waiting to be discovered. Consequently, there were claims, denials, and counterclaims by scientists who said they had found it. During the 1920s and 30s, these claims were made on the basis of unexplained radioactivity in minerals, or new lines in their X-ray spectra, but all eventually turned out not to be evidence of element 87.

Francium was finally discovered in 1939 by Marguerite Perey at the Curie Institute in Paris. She had purified a sample of actinium free of all its known radioactive impurities and yet its radioactivity still indicated another element was present, and which she rightly deduced was the missing element 87. Others challenged her results too, and it was not until after World War II that she was accepted as the rightful discoverer in 1946.
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Listen to Francium Podcast
Transcript :

Chemistry in its Element - Francium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry

(End promo)

Chris Smith

This week, we're hunting for the chemical that was named after France. But you'll need very good eyesight because there's less than a kilogram of it across the entire earth. Know what it is yet? Here's Peter Wothers.

Peter Wothers

In 1929, Madame Curie hired a newly-qualified, twenty-year old technician, Marguerite Catherine Perey, to act as her lab assistant. Ten years later, this remarkably skilled woman discovered the much sought after element francium. As a result, she was encouraged to study for a degree, and then for her PhD. Despite the fact that her mother was convinced she would fail, in March 1946 Perey successfully defended her thesis on Element 87. Sixteen years later, she became the first woman to be elected a member of the French Academy of Sciences, an honour not even awarded to her mentor Madame Curie.

But the story of element 87 begins much earlier than its discovery date in 1939. When Mendeleev first proposed his periodic table in 1869 he left gaps for elements not yet discovered, but that he predicted should exist. One such gap was one beneath caesium, a position later found to belong to francium.

More than 40 years later, it was suggested that each element has a unique numbered position in the periodic table, known as its atomic number. Physicist Henry Moseley proved the existence of the "atomic number" and also suggested that it represented the number of positively-charged protons in the nucleus of the atom. Once the atomic numbers for all the known elements were assigned, it was clear seven elements were missing from the periodic table between hydrogen with atomic number 1, and uranium, number 92. Francium, number 87, was the last of these elements to be discovered in nature.

From its position in the table, it was clear that element 87 would be a reactive alkali metal, the heaviest member of the family lithium, sodium, potassium, rubidium and caesium. Consequently, many researchers started looking for the new metal in ores which contained these related elements. Many false claims were made before it was realised that the missing element would be radioactive with no stable form. Then the search focused on looking at the decay sequences of other radioactive elements.

Two simple rules dictate what elements are formed in a decay series. For each alpha particle a radioactive sample emits, the atomic number of the product element formed is two less than the element from which it formed. For each beta particle emitted, the atomic number of the product increases by one. The element with atomic number 89, two places to the right of our 87, is actinium which had been discovered in 1899.

Studying decay products is not an easy task and Perey's skill allowed her to swiftly purify a sample of an actinium salt so she could observe only the emissions from this element. She eventually realised that most actinium (almost 99% in fact) slowly decays with the emission of a beta particle, forming element 90, thorium. This then decays by emitting an alpha particle to form element 88, radium. However, about 1% of the actinium doesn't do this and instead emits an alpha particle to form the missing alkali metal element 87. This was made even more difficult to spot by the fact francium has a half life of just 21 minutes, because it quickly emits a beta particle to once again form radium.

During these initial investigations, Perey referred to her element as actinium-K; a reference to the route by which it is formed. However, she needed a proper name for her element. During her PhD exam, she suggested the name "catium" since she thought it would be the metal that most readily loses an electron to form a cation. Fortunately, this name was met with little enthusiasm - one of her examiners even suggested that English-speaking people might think it was named after a cat. Perey then suggested the name Francium, after her native country, and this name was accepted.

Whilst it is naturally occurring, or to be more precise, naturally formed - albeit briefly - during radioactive decay of other elements, the amount of francium on earth is tiny. It has been estimated that at any one time there is less than a kilogram of the element in the entire earth's crust. What's more, to the surprise of most chemists and going against the well-known trends of the periodic table, it turns out that francium is not the most reactive metal. On descending a group in the periodic table, on average the outermost electrons get further and further away from the nucleus and as a result, become easier to remove from the atom. This is the trend for the elements lithium, sodium, potassium, rubidium and caesium. However, for the really heavy elements, the presence of so many positively charged protons in the nucleus has the affect of causing the electrons to move round at incredibly fast speeds approaching the sound of light. As Einstein realised at such speeds strange things being to happen. The electrons become a little closer to the nucleus than expected and they also become slightly harder to remove than expected.

Remarkably considering its short half-life, it has been possible to measure experimentally the energy needed to remove an electron from francium to form a positively charged francium cation. The energy needed is 393 kJ mol -1 some 17 kJ mol-1 more than for caesium. This means that francium is not the metal that most easily forms a cation, as Perey was suggesting with her proposed name catium; this honour goes to francium's lighter family member, caesium.

Chris Smith

So the experiment that we all wanted to do at school and drop a lump of Francium into wouldn't have actually been that impressive after all. I'll just stick to caesium then. Next week, you say tomato, I say tomato, but when it comes to element number 13, who's actually right?

Kira Weissman

Sir Humphrey Davy, the Cornish chemist who discovered the metal, called it 'aluminum', after one of its source compounds, alum. Shortly after, however, the International Union of Pure and Applied Chemistry (or IUPAC) stepped in, standardizing the suffix to the more conventional 'ium'. In a further twist to the nomenclature story, the American Chemical Society resurrected the original spelling in 1925, and so ironically it is the Americans and not the British that pronounce the element's name as Davy intended.

Chris Smith

And you can catch up with the story of the substance that's given us super light aircraft and the eponymous drink can on next week's Chemistry in its Element. I'm Chris Smith, thank you for listening and goodbye.


Chemistry in its elementis brought to you by the Royal Society of Chemistry and produced by thenakedscientists dot com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld dot org forward slash elements.

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Description :
A teaching resource on The Alkali Metals supported by video clips from the Royal Institution Christmas Lectures® 2012.
Description :
A series of short, fun videos exploring the chemistry of the alkali metals, taken from a lecture by Dr. Peter Wothers at the University of Cambridge.
Description :
In this experiment you will be looking to see whether precipitates form when you add drops of solutions of sulphates or carbonates to drops of solutions of group 1 or 2 metal ions.
Description :
How do alkali metals react with water?
Description :
A collection of visually stimulating and informative infographics about the elements, which would make a valuable addition to any science classroom.
Description :
These demonstrations show the similarity of the physical and chemical properties of the alkali metals and the trend in reactivity down group 1 of the periodic table.

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Visual Elements images and videos
© Murray Robertson 2011.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J.J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions(version 3.0), 2010, National Institute of Standards and Technology, Gaithersburg, MD, accessed December 2014.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

© John Emsley 2012.



Produced by The Naked Scientists.


Periodic Table of Videos

Created by video journalist Brady Haran working with chemists at The University of Nottingham.