ElementsWhat is fermium?
Fermium is a radioactive element first found, like einsteinium, in the debris of the 1952 hydrogen bomb test. It is the heaviest element that can be made in a reactor.
What is fermium used for?
Research only.
Where the name comes from
In honour of the physicist Enrico Fermi.
Key facts
- Symbol
- Fm
- Atomic number
- 100
- Atomic mass
- 257.09511 u
- Family
- Actinide
- Period
- 7
- Group
- none (f-block)
- Block
- f
- State at room temperature
- Solid
- Melting point
- 1,527 °C (1,800 K, 2,780 °F)
- Boiling point
- not measured
- Density
- not measured
- Electrons per shell
- 2, 8, 18, 32, 30, 8, 2
- Electron configuration
- [Rn]5f12 7s2
- Common oxidation states
- +3
- Electronegativity (Pauling)
- 1.3
- First ionisation energy
- 6.5 eV
- Electron affinity
- no value
- Atomic radius (van der Waals)
- no value
- Discovered
- 1952
Place in the periodic table
Fermium is element number 100. It sits in period 7 (row 7) of the periodic table, in the separate row of actinides drawn below the main table, and belongs to the f-block. Just before it comes einsteinium (99), and just after it comes mendelevium (101).
Family: Actinides.
Physical properties
At room temperature fermium is a solid. Its melting point is about 1,527 °C. Its boiling point has not been measured reliably.
Inside the atom
An atom of fermium has 100 protons in its nucleus and, when neutral, 100 electrons around it. The electrons are arranged in 7 shells: 2, 8, 18, 32, 30, 8, 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is [Rn]5f12 7s2.
Its electronegativity on the Pauling scale is 1.3. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 6.5 electronvolts (about 627 kilojoules per mole). One mole of fermium weighs 257.09511 grams, so a single gram contains about 2.34 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains fermium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.