Fermium (Fm)
Fermium (symbol Fm) is the chemical element with atomic number 100: an actinide, solid at 0 °C, radioactive, made artificially, with an atomic mass of 257 u. Discovered in 1952 (Albert Ghiorso et al.).
Fermium, named after physicist Enrico Fermi, was found along with einsteinium in the debris of the first H-bomb. It is the heaviest element that can be made by bombarding nuclei with neutrons in a reactor; beyond it, particle accelerators are needed. No more than a few billionths of a gram have ever been produced. Its most stable isotope, fermium-257, has a half-life of about 100 days.
Key facts
- Atomic number
- 100
- Symbol
- Fm
- French name
- Fermium
- Atomic mass
- 257 u
- Molar mass
- [257] g/mol
- Family
- Actinides
- Position
- period 7, f-block
- State at 0 °C
- Solid
- Melting point
- 1,526.9 °C
- Electronegativity (Pauling)
- 1.3
- First ionization energy
- 627 kJ/mol
- Electron configuration
- [Rn] 5f¹² 7s²
- Radioactive
- yes
- Discovery
- 1952, Albert Ghiorso et al.
Origin of the name
In honor of the physicist Enrico Fermi.
What does fermium look like?
Never seen: no one has ever gathered enough to make a piece of metal. It is assumed to be silvery.
Where is fermium found?
Absent from nature. It exists only as isolated atoms in the lab.
How is fermium produced?
It forms in trace amounts in high-flux reactors such as HFIR, through a long series of neutron captures starting from curium. Output is counted in picograms (trillionths of a gram).
Top producers
- United States (Oak Ridge)
What is fermium used for?
- Basic research only
- Studying the spontaneous fission of heavy nuclei
In everyday life
- None — only vanishingly small traces exist, in the lab
Role in living things
No biological role; never studied in living organisms.
Hazards of fermium
Radioactive, but present in such tiny amounts that it poses no danger outside the lab.
Cosmic origin
- Human-made: Does not exist (or barely) in nature: synthesized in a reactor or accelerator.
History of fermium
Identified in 1952 by Albert Ghiorso's team in the dust from the “Ivy Mike” test and in coral from the irradiated atoll, it was classified as a military secret until 1955. To be able to publish without revealing its military origin, the researchers also produced it in the lab.
Did you know?
- Fermium-258 splits apart spontaneously in about 0.4 milliseconds: it blocks the chain of neutron captures, a barrier known as the “fermium gap.”
- Enrico Fermi died in November 1954, before the element was publicly named in his honor.
- Even neutron star explosions may not be enough to cross the fermium gap: no one knows whether heavier elements exist in nature.
Isotopes of fermium
No stable isotopes; fermium is the heaviest element that can be made by neutron capture, whether in a reactor or in a nuclear explosion.
- fermium-255: radioactive, half-life 20.1 h. The first isotope identified, in 1952, in the debris of the “Ivy Mike” test.
- fermium-257: radioactive, half-life 101 days. The longest-lived, and the heaviest isotope produced in a reactor.
- fermium-258: radioactive, half-life 370 µs. Its lightning-fast fission blocks the making of heavier elements by neutron capture.
Related elements
Same family (actinides): Actinium (Ac), Thorium (Th), Protactinium (Pa), Uranium (U), Neptunium (Np), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).