Periodic Table > Fermium


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 Actinides  Melting point 1527°C, 2781°F, 1800 K 
Period Boiling point Unknown 
Block Density (g cm−3) Unknown 
Atomic number 100  Relative atomic mass [257]  
State at 20°C Solid  Key isotopes 257Fm 
Electron configuration [Rn] 5f127s2  CAS number 7440-72-4 
ChemSpider ID 22434 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 aims to suggest a self-propagating nuclear chain reaction, such as occurs in nuclear reactors and atomic bombs.
A radioactive metal obtained only in microgram quantities.
Fermium has no uses outside research.
Biological role
Fermium has no known biological role. It is toxic due to its radioactivity.
Natural abundance
Fermium can be obtained, in microgram quantities, from the neutron bombardment of plutonium in a nuclear reactor.

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) 2.45 Covalent radius (Å) 1.67
Electron affinity (kJ mol−1) Unknown Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)


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.



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 3
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  257Fm 257.095 - 100.5 d  α 


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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Fermium was discovered in 1953 in the debris of the first thermonuclear explosion which took place on a Pacific atoll on 1 November 1952. In this a uranium-238 bomb was used to provide the heat necessary to trigger a thermonuclear explosion. The uranium-238 had been exposed to such a flux of neutrons that some of its atoms had captured several of them, thereby forming elements of atomic numbers 93 to 100, and among the last of these was an isotope of element 100, fermium-255. News of its discovery was kept secret until 1955.

Meanwhile a group at the Nobel Institute in Stockholm had independently made a few atoms of fermium by bombarding uranium-238 with oxygen nuclei and obtained fermium-250, which has a half-life of 30 minutes.
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Listen to Fermium Podcast
Transcript :

Chemistry in its element: fermium


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

(End promo)

Meera Senthilingham

This week, all rise for element 100. Here's Brian Clegg:

Brian Clegg

The number 100 is a very significant one for human beings. It's partly because our number system is based on ten - so ten tens seems to have a special significance. In years, it's around the maximum lifetime of a human being, making a century more than just a useful division in the historical timeline. But in the natural world, 100 isn't quite so important. There's nothing about being element 100 that makes fermium stand out - the periodic table doesn't attach any significance to base 10. But it's hard not to think that fermium must be special in some way.

Like element 99 (einsteinium), fermium was first made in the hydrogen bomb test on Elugelab Island on the Eniwetok Atoll in the South Pacific. The test bomb exploded on the first of November 1952*, blasting vast quantities of material into the atmosphere that drifted down as fallout. The team from the University of Berkeley at California that tested tonnes of ash and coral debris found around 200 atoms of element 100.

This had been created from uranium 238. Fusion in the hydrogen bomb was triggered by a conventional atomic bomb, and the remnants of that trigger's uranium fuel absorbed a swathe of neutrons, some of which then changed to protons as they underwent beta decay, finally producing fermium 255.

The discoverers aptly named the element after Enrico Fermi, the Italian-born physicist whose work at the University of Chicago was crucial to the development of nuclear explosives. This work took place under the bleachers of a dusty, disused football stadium. The site hadn't been used for three years since the president of Chicago University closed down the football team as a distraction from academic work. In a claustrophobic space beneath the stands was an old squash court. Here, in 1942, Fermi and his team built the world's first manmade nuclear reactor, literally an atomic pile of carbon bricks where materials for the atomic bomb would be produced. Fermi, who won the Nobel Prize in 1938, also worked in quantum mechanics and particle physics, making him an ideal candidate for an elementary name.

The element was almost named centurium, however. In 1953, scientists at the Nobel Institute in Stockholm had produced fermium 250 by bombarding uranium with oxygen nuclei. At the time, the discoveries from the hydrogen bomb were classified, so the Swedes, who tentatively came up with the centurium name for one hundred, could have got in first, had fermium not been rapidly de-classified. It might be no coincidence that the Berkeley team allowed the Nobel Institute's name nobelium for element 102 to continue to be used when the Swedes' claim for discovering that element proved dubious. There could have been a certain amount of guilt for sneaking in fermium under their noses.

Fermium is an actinide, part of the floating bar of elements that is squeezed out from between actinium and lawrencium. Perhaps its greatest claim to fame on the periodic table is that it defines the start of the most obscure of the artificial elements - those above 100 are referred to as the transfermium elements. It is certainly the highest numbered element that has had a practical use identified.

Although not yet deployed, fermium 255 is a strong alpha particle emitter with a half life - the time it takes half the material to undergo nuclear decay - of around 20 hours. In medical radioactive applications this is a good combination, where alpha sources are used in radiotherapy for cancer. This is a convenient half-life as it means the alpha particles - nuclei of helium atoms with two protons and two neutrons - are produced long enough for the source to be deployed, but the waste matter becomes a low level hazard very quickly.

Fermium is usually produced using accelerators like cyclotrons now, although it has a special place in the periodic table as the highest numbered element that can be produced in a nuclear reactor, rather than by smashing atoms together in an accelerator. This is something of a useless capability, however. The fermium produced in reactors seems a good, useable product. It's fermium 257, which has a very practical half life of 100 days. But there's never a chance to use it. Inside a reactor there are plenty of loose neutrons floating about - this is how the chain reaction of the reactor works. Fermium 257 is great at absorbing neutrons and immediately become fermium 258. This has a tiny half life of less than a millisecond. So before you can get your hands on the fermium produced in a reactor it has disappeared.

Like its transfermium colleagues, fermium has only been made in relatively tiny quantities. This means that no one has produced a big enough sample of fermium to be able to see it, though the expectation is that like other similar elements it would be a silvery-grey metal.

Fermium has limited value, but anything numbered 100 inevitably feels a little special. And perhaps fermium is, at least when it's made in a nuclear reactor. You can see fermium as a sneaky element. As we've seen, this is a product that you can make, that should last 100 days before half of it has disappeared, yet in practice it vanishes after milliseconds. Perhaps what makes fermium special is that it's an element with a wicked sense of humour.

Meera Senthilingham

So the element that scientists are trying - and failing - to get their hands on. That was Brian Clegg, with the disappearing properties of fermium. Now next week, and element that we can see, and it's a lanthanide with a diverse range of applications.

Simon Cotton

Lutetium and its compounds have found some applications, the most important of these is the use of the oxide in making catalysts for cracking hydrocarbons in the petrochemical industry. But there are other more specialist uses, such as using the radioactive lutetium-177 isotope in cancer therapy. Lutetium ions were also used to dope gadolinium gallium garnet to make magnetic bubble computer memory that was eventually replaced by modern-day hard drives.

Lutetium triflate has also been found to be a very effective recyclable catalyst for organic synthesis in aqueous systems - it avoids the use of organic solvents giving it green credentials.

Meera Senthilingham

And to find out the chemistry and properties of lutetium that make it so widely applicable, join Simon Cotton in next week's Chemistry in its element. until then, I'm Meera Senthilingham and thank you for listening.


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

(End promo)

*The correct date is 1952, not 1942 as in the podcast audio file
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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.
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