Fermium

ACTINOID · GROUP None · PERIOD 7
100
Fm
Fermium
257

Atomic Data

Atomic Number100
SymbolFm
Atomic Weight257 u
Density (STP)N/A
Melting Point1526.85 °C (1800 K)
Boiling PointN/A °C (None K)
Electronegativity1.3 (Pauling)
Electron Config.1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f12 6s2 6p6 7s2
Oxidation States+2, +3
Phase at STPSolid
CategoryActinoid
Period / Group7 / None
CAS Number7440-72-4

Electron Configuration

[Rn] 5f12 7s2

Shell n Subshell Electrons Cumulative
K11s22
L22s24
L22p610
M33s212
M33p618
M33d1028
N44s230
N44p636
N44d1046
N44f1460
O55s262
O55p668
O55d1078
O55f1290
P66s292
P66p698
Q77s2100
Total 100 100

Isotopes of Fermium

Fermium is monoisotopic: ²⁵⁷Fm is its only naturally occurring stable isotope, accounting for 100% of all natural Fermium.

Isotope Symbol Protons Neutrons Abundance Stability
Fermium-257²⁵⁷Fm100157traceStable

Abundance & Occurrence

Fermium is present in Earth's crust at approximately trace amounts by mass and at approximately trace amounts by mass throughout the universe.

Earth's Crust (ppm by mass)

Fermium
None ppm
Silicon (ref.)
277,000 ppm
Oxygen (ref.)
461,000 ppm

Universe (ppm by mass)

Fermium
None ppm
Helium (ref.)
230,000 ppm
Hydrogen (ref.)
739,000 ppm

Discovery & History

1952
Albert Ghiorso et al. (Lawrence Berkeley): Fermium was discovered alongside einsteinium in the Ivy Mike hydrogen bomb debris; element 100 resulted from uranium undergoing rapid multiple neutron capture during the thermonuclear explosion, producing isotopes far heavier than any previously observed.
1955
Joint US laboratory team: The element was named in honour of Enrico Fermi, architect of the first nuclear reactor and a founder of nuclear physics, shortly after his death; fermium's discovery demonstrated that nuclei heavier than californium could exist.
1971
Oak Ridge National Laboratory: Fm-257 (t½ = 100.5 days), the longest-lived fermium isotope, was produced in milligram quantities in the HFIR reactor; fermium cannot be produced in visible amounts and represents the heaviest element ever weighable in any macroscopic sense.

Read more about the discovery of the periodic table of elements →

Safety & Handling

  • Alpha radiation: Fermium-257 (t½ = 100.5 days, alpha emitter) is highly radiotoxic; its production in microgram quantities still requires handling in dedicated alpha-rated glove boxes with continuous air monitoring.
  • Cannot be produced in macroscopic amounts: Fermium cannot be produced in visible quantities under any practical conditions; all work involves sub-microgram samples at the boundary of nuclear and radiochemistry.
  • Research facility context: Fermium is handled only at a handful of research reactors (Oak Ridge HFIR) and nuclear chemistry laboratories; all operations are conducted by trained radiochemists under strict radiation protection regimes.
  • Regulatory controls: All fermium work requires national nuclear regulatory authority licensing and detailed material accountancy.

Real-World Uses

  • Nuclear structure and actinide chemistry research: Fermium isotopes (Fm-255, Fm-257) are produced in nuclear reactors and studied using rapid gas-phase and solvent-extraction chemistry to determine oxidation states, ionic radii, and chemical bonding in the heaviest accessible actinides.
  • Limits of nuclear stability: Fermium-257 is the heaviest nuclide accessible by neutron capture from uranium, marking an experimental boundary beyond which the neutron capture synthesis route for making heavier elements becomes impractical due to very short half-lives.
  • No commercial applications: Fermium is produced only in picogram quantities in specialised research reactors; its short half-lives (longest stable isotope Fm-257 has t½ = 100.5 d) preclude any practical use outside fundamental nuclear science.

Downloadable Resources

Free periodic table reference sheets for classrooms, study sessions, and laboratory use.

Colour PDF Black & White PDF

Frequently Asked Questions

What is fermium used for?

Fermium has no practical applications. It can only be produced in nanogram quantities in the highest-flux nuclear reactors over months of irradiation, and its longest-lived isotope (Fm-257) has a half-life of only 100.5 days. Fermium is used purely for scientific research: studying its nuclear and chemical properties and as a stepping stone to understanding heavier elements.

How was fermium discovered?

Fermium was discovered in the debris of the Ivy Mike hydrogen bomb test in November 1952, alongside einsteinium. Scientists analysing coral samples collected after the test found uranium had captured multiple neutrons to create elements up to atomic number 100. The discovery was classified until 1955. Fermium was named in honour of Enrico Fermi, the physicist who built the first artificial nuclear reactor and made seminal contributions to nuclear physics.

Is fermium the heaviest element that can be produced in a reactor?

Yes. Fermium-257 is the heaviest nuclide that can be produced through successive neutron capture and beta decay in a nuclear reactor. Beyond fermium, the neutron-rich isotopes that would form are so short-lived that they decay before they can capture another neutron to proceed further up the periodic table. Elements heavier than fermium must be synthesised by direct nuclear reactions in particle accelerators, which produce them only one or a few atoms at a time.

What is the longest-lived isotope of fermium?

The longest-lived isotope of fermium is fermium-257, with a half-life of 100.5 days. It decays by alpha emission and spontaneous fission. Fermium-253 (half-life 3 days) and fermium-255 (half-life 20.1 hours) are other isotopes that have been studied. The short half-lives of all fermium isotopes mean that any sample decays rapidly, limiting experiments to a brief window after production.