Fm · Element 100 · Actinide
The heaviest element that can be made in weighable amounts by neutron bombardment.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where fermium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All fermium atoms have the same number of protons, 100, and differ only in their number of neutrons: those variants are its isotopes. None of them is stable. The longest-lived known isotope, Fm-257, has 157 neutrons and a half-life of 100.5 days.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Fm-251 | 151 | — | 5.30 hr | ε/β⁺ (98.2%), α (1.8%) |
| Fm-252 | 152 | — | 25.39 hr | α |
| Fm-253 | 153 | — | 3.00 days | ε (88%), α (12%) |
| Fm-255 | 155 | — | 20.07 hr | α |
| Fm-257 longest-lived | 157 | — | 100.5 days | α |
Here the classroom shortcut, rounded atomic mass (257) minus atomic number, gives the same answer: 157.
Decay modes
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 245 pm — that is 0.245 nm, so about 2.04 million of them side by side would span a millimetre.
Thermal range
Fermium melts at 1800 K; its boiling point has not been measured.
The story
The heaviest element that can be made in weighable amounts by neutron bombardment.
Fermium was discovered in late 1952 through analysis of debris from the first hydrogen bomb test, marking a dramatic moment when the destructive power of nuclear weapons accidentally advanced the frontiers of human knowledge.
The discovery occurred during analysis of the "Ivy Mike" thermonuclear test debris:
The discovery involved scientists from multiple prestigious institutions:
Identifying fermium required great analytical capabilities:
The discovery remained classified for three years due to nuclear weapons security:
The element was named to honor Enrico Fermi:
Scientists quickly worked to synthesize fermium under controlled conditions:
The fermium discovery represented multiple scientific breakthroughs:
The discovery had far-reaching implications:
Applications
Fermium represents the frontier of nuclear science, serving as a crucial milestone in humanity's quest to understand the limits of matter and the fundamental forces that bind atomic nuclei.
Fermium plays a critical role in advancing superheavy element science:
Despite working with only atoms at a time, fermium research provides unique insights:
Fermium serves as an exceptional teaching tool for understanding:
Working with fermium drives innovation in advanced technology:
Fermium research addresses fundamental questions about:
Fermium research contributes to understanding:
Fermium research opens pathways to:
Fermium has no commercial, industrial, or practical applications. Its existence is limited to the advanced nuclear research laboratories, where samples range from single accelerator-made atoms to about a billion atoms of reactor-made fermium.
Limitations: Fermium research is constrained by:
Where it comes from
0 mg/kg of Earth's crust · more abundant than 0% of elements
Fermium does not exist anywhere in nature and must be created through the most sophisticated nuclear technology available to humanity. It represents a high point of artificial element synthesis.
Fermium was first created in the debris of thermonuclear weapons tests:
High-Flux Reactor Production: Current synthesis requires the world's most powerful neutron sources:
Creating fermium presents serious technical obstacles:
Reactor-scale fermium (picograms of fermium-257) comes from only a few high-flux reactors:
Isolating fermium requires special measures:
Fermium research faces severe time pressure:
Scientific Achievement: Creating fermium represents one of humanity's greatest technological achievements, requiring the coordination of the world's advanced nuclear facilities and expertise.
Handling
Fermium is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
Fermium is radioactive, but it is so scarce that its longest-lived isotope, fermium-257, has been made only in picogram amounts (trillionths of a gram), so it poses no hazard outside a few research laboratories. It is made by intense neutron irradiation of lighter elements such as plutonium in a high-flux reactor, and fermium-257 has a half-life of about 100 days.
Quick answers
Fermium (symbol Fm) is element 100 on the periodic table, an actinide in period 7. The heaviest element that can be made in weighable amounts by neutron bombardment. At room temperature it is expected to be a solid, and it is radioactive.
[Rn] 5f¹² 7s², giving 7 occupied shells with the distribution 2, 8, 18, 32, 30, 8, 2.No sample of fermium has ever been condensed, so its melting point is calculated rather than measured: it is predicted to melt at 1800 K (1526.9 °C). Its boiling point has not been measured.
Fermium has no stable isotope, so it has no standard atomic weight. The figure quoted, 257, is the mass number of its longest-lived known isotope.
Fermium has no stable isotope. Its longest-lived known isotope, Fm-257, has 157 neutrons (mass number 257 minus atomic number 100) and a half-life of 100.5 days. The classroom shortcut, rounded atomic mass (257) minus atomic number, gives the same 157.
Fermium is predicted to have a density of about 9.7 g/cm³, though no sample large enough to weigh has ever been made. Water is 1.0 g/cm³ for comparison.
Fermium has a Pauling electronegativity of 1.3. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Fermium was discovered in 1952 by Ghiorso et al.. Origin of the name: Enrico Fermi.
Fermium does not occur naturally on Earth in any meaningful quantity — it is made in a reactor or an accelerator.
Yes. Fermium has no stable isotope — every one of its nuclei decays. It does not occur in usable quantities in nature and must be synthesised.