Atomic number 100

Fermium

Fm
+ phase
− phase

Shells Orbitals 5f 7s

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100 electrons in 7 shells: 2, 8, 18, 32, 30, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Atomic mass: 257.09511 Da (longest-lived isotope)
Electron configuration: [Rn] 5f¹² 7s²
Phase at 25 °C: solid
Melting point: 1527 °C
Electronegativity (Pauling): 1.3
Discovered: December 1952, American scientists from Argonne, Los Alamos and Berkeley

Neighbours in the table

Es Md

The first fermium identified sat in contaminated coral taken from ground zero at Eniwetok atoll. It came from the first hydrogen-bomb test, in 1952, and went to Berkeley for analysis, Los Alamos records. Roughly two months later a new source of radioactivity emerged there. It gave out alpha particles and had a half-life, the time for half to decay, of about one day. It proved to be fermium-255, made when uranium-238 absorbed 17 neutrons, then passed through eight beta decays, each turning a neutron into a proton, in Jefferson Lab’s account.

Jefferson Lab credits a team led by Albert Ghiorso. IUPAC, the International Union of Pure and Applied Chemistry, dates the identification to December 1952 and names scientists from Argonne, Los Alamos and Berkeley. Cold War tensions kept the finding secret on US military orders until 1955, according to Los Alamos, and the name honours the Italian physicist Enrico Fermi.

PubChem gives the mass of its longest-lived isotope, fermium-257, 257.09511 Da. Los Alamos lists oxidation states of +2 and +3, bookkeeping charges that treat every bond as ionic; Jefferson Lab lists +3. Samarium(II) chloride readily reduces Fm³⁺ to a stable Fm²⁺ ion, Los Alamos reports. Its potential for that couple, the ion pair trading one electron, is −1.15 V against the normal hydrogen electrode, the standard reference, close to ytterbium’s.

Where it is found

No fermium occurs naturally in the crust of the Earth, IUPAC states. Every fermium isotope has a short half-life, so whatever the young planet held decayed away long ago, Los Alamos explains. The element did form at Oklo, where a natural fission reactor once ran, Los Alamos adds, but none of it remains.

All fermium studied today is made. Lighter elements such as uranium and curium capture neutron after neutron, with beta decays in between, until element 100 appears. Most comes from Oak Ridge National Laboratory’s High Flux Isotope Reactor (HFIR), an 85 MW machine in Tennessee, according to Los Alamos. One HFIR campaign irradiates tens of grams of curium and returns picogram quantities of fermium. Solvent extraction and ion exchange then pull it away from debris, other actinides and lanthanide fission products.

One isotope can be had pure by an easier route. Fermium-255 is the beta-decay daughter of einsteinium-255, whose half-life is 39.8 days, and Los Alamos describes “milking” the fermium from a purified einsteinium stock as it grows in. Every chemical property known for fermium comes from tracer amounts, too few atoms to weigh, tracked by their radioactivity.

Why it matters

Reactor production of new elements stops at fermium. Neutron capture essentially ends at mass number 257, Los Alamos explains, because heavier fermium isotopes undergo spontaneous fission, the nucleus splitting unprompted, almost at once. Los Alamos gives fermium-258 a half-life of just 370 microseconds, fermium-259 1.5 seconds and fermium-260 4 milliseconds; the Atomic Mass Data Center estimates about one minute for fermium-260. Fermium is therefore the heaviest element that neutron bombardment can form, according to IUPAC.

Fermium-257 has a half-life of 100.5 days. In the evaluation of the International Atomic Energy Agency (IAEA), 99.79 per cent of its decays emit an alpha particle and 0.21 per cent are spontaneous fission. The alpha branch leaves californium-253, according to Jefferson Lab. Yearly reactor output of fermium-257 lies in the picogram range, Los Alamos reports.

So little is made, and its isotopes live so briefly, that fermium has no uses beyond basic research, according to Los Alamos. IUPAC notes interest from particle physics but no commercial application. Its isotopes have still settled claims about heavier elements. Chemical identification of fermium-250, a decay product of nobelium, confirmed the discovery of element 102, Los Alamos records. Fermium-253 was among the decay products that confirmed copernicium, in IUPAC’s account.

Properties

Properties of fermium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	100
Atomic mass	257.09511 Da	Da	PubChem, longest-lived isotope
Melting point	1527 °C	°C	Los Alamos: 1527 °C
Electronegativity (Pauling)	1.3
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹²
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Fermium’s hazard is ionising radiation, but none occurs naturally in the Earth’s crust, IUPAC states, and reactors make it only in picogram to nanogram amounts, according to Los Alamos. Haz-Map, in PubChem’s safety record, names fermium-257 as the radioactive isotope of concern and rates its internal toxicity, the harm it does once inside the body, very high. Inhaled, it takes weeks for half of it to clear from the lungs. The element’s hazards are summarised in the PubChem Laboratory Chemical Safety Summary for fermium.

Questions

Why is fermium named after Enrico Fermi?

Fermi had built the first artificial self-sustained nuclear reactor and had died shortly before the name was chosen, Los Alamos notes. Berkeley's claim to have found element 100 first was generally recognised, which gave its team the right to name it. Working independently, a rival team at Stockholm's Nobel Institute for Physics published its own claim to element 100 in May 1954, based on an isotope lasting 30 minutes.

Why was fermium found in hydrogen-bomb debris?

Building element 100 from uranium means packing many neutrons into one nucleus, and a heavy nucleus absorbing neutrons was then thought to be a rare event. Its likelihood rises with the neutron flux, the neutrons crossing an area each second, and nuclear explosions are the strongest neutron sources on Earth, in Los Alamos's explanation. Element 100 was expected in at least ten times smaller yield than einsteinium, so its discovery required more material, and the coral provided it.

Is fermium radioactive?

Yes. Fermium has no stable isotope, according to Jefferson Lab. The Atomic Mass Data Center lists nuclides from fermium-241 to fermium-260, and Los Alamos counts two metastable ones, long-lived excited states of a nucleus. The Data Center's figures give fermium-253 a half-life of 3.00 days, fermium-252 25.39 hours and fermium-255 20.07 hours. Haz-Map rates fermium's internal toxicity as very high.

Has anyone ever seen fermium metal?

No. Neither fermium metal nor a solid pure fermium compound has been prepared, Los Alamos states. Instead Fm(III) is studied as a trace inside a rare earth host crystal, and fermium–rare earth alloys have been measured. A fermium-ytterbium alloy gave the enthalpy of vaporisation, the energy to vaporise the metal. Los Alamos reports it was deduced that the metal prefers a divalent state, each atom giving up two electrons, though modest compression can make it trivalent, giving up three.