Fermium (Fm): Electron Configuration, Properties and Uses
Fermium (Fm): Electron Configuration, Properties and Uses
Electron configuration [Rn] 7s2 5f12 · f-block, actinide series · period 7 · Ar [257]
Fermium data
- Atomic number
- 100
- Relative atomic mass
- [257]
- Group
- – (actinide series)
- Period
- 7
- Block
- f
- Category
- Actinide
- Electron configuration
- [Rn] 7s2 5f12
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f12
- Electrons per shell
- 2, 8, 18, 32, 30, 8, 2 (K L M N O P Q)
- Valence electrons
- 2 (in the outer shell)
- Electronegativity (Pauling)
- 1.3
- First ionisation energy
- 6.5 eV
- Atomic radius (van der Waals)
- no data
- State at room temperature
- Solid
- Melting point
- 1527 °C
- Boiling point
- no data
- Density
- no data
- Common oxidation states
- +3
- Discovered
- 1952
Source: PubChem (NIH, accessed 13 Nov 2026); atomic mass: IUPAC standard atomic weight.
Electron shell diagram
K 2 · L 8 · M 18 · N 32 · O 30 · P 8 · Q 2
Orbital diagram (outer electrons)
About fermium
With atomic number 100, fermium (Fm) has 100 protons in its nucleus and 100 electrons as a neutral atom. It belongs to the actinide series in period 7 (f-block), usually shown below the main table. Fermium was first discovered in 1952. It is named after Enrico Fermi. Fermium is a solid at room temperature, melting at 1527 °C.
Fermium electron configuration
The electron configuration of fermium is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f12, or in short [Rn] 7s2 5f12. Its electrons occupy 7 shells: K = 2, L = 8, M = 18, N = 32, O = 30, P = 8, Q = 2. As an actinide, fermium fills the 5f subshell. Its outer 7s subshell holds 2 electrons; valence electrons for this series are not assigned by the usual group rule. When it forms cations, electrons leave the outermost subshell first: Fm3+ = [Rn] 5f11.
Filling order: 1s2(2) → 2s2(4) → 2p6(10) → 3s2(12) → 3p6(18) → 4s2(20) → 3d10(30) → 4p6(36) → 5s2(38) → 4d10(48) → 5p6(54) → 6s2(56) → 4f14(70) → 5d10(80) → 6p6(86) → 7s2(88) → 5f12(100)
Uses of fermium
Fermium is used for research only; like einsteinium, it was first found in the debris of the 1952 hydrogen bomb test.
Safety
Fermium is radioactive. Using, storing or transporting radioactive material requires a licence from the national regulator (BAPETEN in Indonesia). Always read the safety data sheet (SDS) before working with a chemical.
Other actinides
Frequently asked questions
What is the electron configuration of fermium?
The electron configuration of fermium (Fm, atomic number 100) is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f12, or [Rn] 7s2 5f12 in short. By shell: 2, 8, 18, 32, 30, 8, 2.
How many valence electrons does fermium have?
Fermium has 2 electrons in its outer shell. For lanthanides and actinides, valence electrons are not assigned by the usual group rule.
Which group and period is fermium in?
Fermium is in the actinide series, period 7, f-block.
What is the atomic mass of fermium?
Fermium has no stable isotope, so the mass number of its longest-lived isotope, [257], is given.
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