Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

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 (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

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

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

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.

Atoms of the same element with different numbers of 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.

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


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.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

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.
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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.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

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)


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. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

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

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- 


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.



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

A measure of the stiffness of a substance. 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

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A 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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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 element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at

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Visual Elements images and videos
© Murray Robertson 1998-2017.



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 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.
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

Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.



Produced by The Naked Scientists.


Periodic Table of Videos

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