Fermium
Fermium is a synthetic element with symbol Fm and atomic number 100. It is a member of the actinide series. It is the heaviest element that can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities, although pure fermium metal has not yet been prepared.
Identity
- Atomic mass
- [257] u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +2, +3
- Phase (25 °C)
- Solid
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f12
- Short configuration
- [Rn] 5f12 7s2
- Electrons per shell
- 2, 8, 18, 32, 30, 8, 2
- Electronegativity (Pauling)
- 1.30
- Electron affinity
- 34.0 kJ/mol
- 1st ionization energy
- 627 kJ/mol
Physical properties
- Melting point
- 1526.8 °C (1800 K)
- Boiling point
- —
- Density
- —
- Molar heat capacity
- —
Chemical behavior of fermium
Fermium has twelve outer electrons ([Rn] 5f¹² 7s²) and acts as +3, with +2 accessible in halides. Its electronegativity is low (1.30) and, like einsteinium, it is paramagnetic with face-centered cubic structure. It is the practical limit of chemistry: it has only been obtained in picograms and pure metal has never been prepared. Its nucleus marked the ceiling of neutron capture: the heaviest isotopes fission spontaneously in microseconds.
Reactivity
4 Fm + 3 O₂ → 2 Fm₂O₃
In air it would oxidize to sesquioxide; in practice only traces in carrier matrices have been observed due to extreme atom scarcity.
2 Fm + 6 HCl → 2 FmCl₃ + 3 H₂
In acidic solution it adopts the usual actinide +3 state; +2 appears only as an intermediate state observed in single-phase experiments.
²⁵⁷Fm + n → ²⁵⁸Fm → (spontaneous fission)
Capture of a neutron forms Fm-258, which fissions in 370 microseconds: the barrier that closes the reactor route to heavier elements.
²⁵⁵Es → ²⁵⁵Fm + β
The Fm-255 usual in the laboratory is born from Es-255 beta decay: the easiest route to obtain useful fermium quantities.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Fermium oxide | Fm₂O₃ | Predicted; only traces observed |
| Fermium chloride | FmCl₃ | Microscale chemistry |
| Fermium cyclopentadienide | Fm(C₅H₅)₃ | Volatilization for gas chromatography |
| Fermium oxalate | Fm₂(C₂O₄)₃ | Einsteinium separation |
| Fm-257 target | ²⁵⁷Fm | Neutron capture barrier |
Electron shell distribution
View in the atom builderIsotopes
No fermium isotope is stable. Fermium-257 (²⁵⁷Fm, 100.5 days) is the heaviest accessible by neutron capture; fermium-255 (²⁵⁵Fm, 20.07 hours) is the usual one in experiments, obtained from Es-255 decay. Fermium-258 fissions spontaneously with a half-life of 370 microseconds, closing the reactor chain.
Occurrence and production
It does not exist in nature: all fermium is born in Oak Ridge's reactor, mixed with einsteinium and californium among fission products and neutron capture. It is the last element isolable in measurable quantities (picograms); from it on, spontaneous fission prevents accumulating anything else in a reactor.
History and origin of the name
Ghiorso and colleagues identified it in 1952 in the debris of the Ivy Mike thermonuclear test, as the heaviest element present in those remains. They named it after Enrico Fermi, father of the first nuclear reactor, in 1954. Its discovery established the practical limit of reactor synthesis.
Applications and relevance of fermium
Uses of the element
Fermium has no practical applications: it serves to study the limits of neutron capture and decay currents toward superheavy elements.
Uses of its compounds
Its volatile compounds (such as cyclopentadienide) developed single-atom gas chromatography, the technique that allowed studying mendelevium chemistry and, later, rutherfordium.
Health, safety and environment
Biological role
It has no known biological function and is extremely radiotoxic. Work is done in trace quantities with carrier matrix containment.
Safety
It is handled only in Oak Ridge and Dubna, in shielded cells. Spontaneous fission of its heavy isotopes generates neutrons requiring specific shielding.
Environment
There are no spills: experiments are conducted with traces remaining in solid matrices treated as radioactive waste and vitrified.
Frequently asked questions about Fermium
Why is fermium-257 the heaviest isotope that can be produced by neutron capture?
Fermium-257 is the last isotope that can be formed through successive neutron capture in a reactor. From it on, heavier isotopes such as fermium-258 disintegrate by spontaneous fission within microseconds, preventing accumulation. This barrier explains why fermium marks the practical limit of neutron capture synthesis and why heavier elements require accelerators.
Which fermium isotope is most used in research?
Although fermium-257 is the most stable, with a half-life of 100.5 days, most studies are conducted with fermium-255, which has a half-life of 20 hours. This is because fermium-255 can be easily isolated as a decay product of einsteinium-255, facilitating its obtainment in sufficient quantities for experiments.
What is the difference between fermium and einsteinium in research?
Einsteinium is produced in quantities on the order of milligrams per year and can be studied in the form of compounds and solutions. Fermium, on the other hand, is only obtained in picogram quantities and pure metal has never been prepared. Therefore, einsteinium allows more detailed chemical studies, while fermium is limited to nuclear and tracer experiments.
Why is fermium considered the last element obtainable in macroscopic quantities?
Fermium-257 is the heaviest isotope that can be produced in measurable quantities by neutron capture in a reactor. From it on, spontaneous fission prevents accumulating heavier isotopes. Although individual atoms of heavier elements have been synthesized in accelerators, never in macroscopic quantities. Therefore, fermium marks the practical limit of superheavy element chemistry.
Why does fermium-258 decay so fast?
Fermium-258 has a half-life of only 370 microseconds and disintegrates almost instantaneously by spontaneous fission. This happens because its nucleus, with 158 neutrons, is too unstable to remain bound. Electrostatic repulsion between the 100 protons exceeds the nuclear forces keeping the nucleus cohesive, provoking its immediate division into lighter fragments.
Why can't fermium form heavier isotopes by neutron capture?
When fermium-257 captures a neutron and becomes fermium-258, this disintegrates by spontaneous fission in less than a millisecond. No fermium isotope disintegrates by beta emission toward mendelevium, which would prevent continuing the chain. This interruption of the neutron capture chain explains why heavier elements cannot be produced this way in a conventional reactor.
What relationship does fermium have with the r process in astrophysics?
The r process (rapid neutron capture) is the mechanism by which heavy elements form in supernova explosions and neutron star mergers. In that process, nuclei capture neutrons so fast they have no time to disintegrate. Fermium-258 and its neighboring isotopes, which fission instantaneously under laboratory conditions, can form briefly in that extreme environment before disintegrating.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.