Fermium was identified in material from the first thermonuclear test and can also be produced through neutron irradiation in reactors. It is studied in very small quantities. With no assigned standard atomic weight, descriptions of its nuclear behavior must refer to particular isotopes.
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AN ELEMENT IN CONTEXT
Why compare fragment distributions from different fermium isotopes?
Fermium isotopes in fission studies
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Specify the isotope comparison
Fermium isotopes share proton number while differing in neutron number. Comparing them can therefore reveal changes in nuclear behavior within one elemental identity.
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FmFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
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What makes the isotope comparison scientifically informative?
Atomic identity, reference properties and structure models
INTERACTIVE 3D100 visible sites
An electron censusDrag to rotate · scroll or pinch to zoom
[Rn] 5f12 7s2. Grouping electrons by principal shell connects the neutral-atom configuration to periodic-table patterns.
What this representation means
A three-dimensional counting diagram. Dots are count markers, not measured electron positions; spherical placement and spacing are arbitrary. There are no electron trajectories here. Orbitals describe quantum states.
Fermium was identified in material originating from the 1952 Ivy Mike thermonuclear test, with the chemical identification work extending into 1953. Named for Enrico Fermi, it marked atomic number one hundred without supplying a visible new metal for ordinary examination. Its later research history centers on tiny samples and individual atoms, where nuclear and spectroscopy can reveal properties that a conventional materials laboratory cannot measure.
1952–1953
Production and identification
Test material yields evidence for fermium during subsequent analysis.
2024
Nuclear-radius study
Laser spectroscopy reports trends in charge radii across fermium isotopes.
Fermium studied in laboratories is produced artificially through nuclear reactions and has no mineable natural source. Its availability is far below ordinary materials quantities. The selected and production route determine what can be observed before decay, making a sample’s nuclear history central to the interpretation of any experiment.
Fermium’s neutral-atom configuration places in a nearly filled 5f shell. Researchers study its atoms and through radiation and laser interactions, not through familiar mechanical tests on a metal bar. The absence of macroscopic samples means that experimentally supported atomic properties must be distinguished from inferred bulk behavior.
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
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Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
Not recorded
293.15 K, 101.325 kPa
Melting point
Not recorded
at standard pressure unless noted
Boiling point
Not recorded
101.325 kPa
Density
Not recorded
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity in Pauling
1.3 Pauling
Pauling scale
First ionization energy in kJ/mol
627 kJ/mol
gas-phase neutral atom, ground state
Covalent radius
Not recorded
single-bond covalent radius
Common oxidation states
+3
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust
Not recorded
upper continental crust, mass fraction
Has stable isotopes
Noderived
Discovery year in CE
1952 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 7
Fm 1.3 Pauling · Fermium
Electronegativity across period 7. Exact values.
Ra
0.9
Ac
1.1
Th
1.3
Pa
1.5
U
1.38
Np
1.36
Pu
1.28
Am
1.13
Cm
1.28
Bk
1.3
Cf
1.3
Es
1.3
Fm
1.3
Md
1.3
No
1.3
Lr
1.3
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
