At the far reaches of the actinide series, this heavy element is made only in extreme neutron-rich environments or in specialized reactors, and its variants help scientists probe nuclear structure and decay pathways. Studying them requires careful production, rapid separation, and sensitive detection.
There are 20 Fermium Isotopes, ranging from Fm-241 to Fm-260. For each isotope you’ll find below Atomic mass (u),Half-life (s),Primary decay modes, organized so you can compare mass, stability and decay at a glance — see the detailed list you’ll find below.
How are fermium isotopes produced and studied?
Most are created by neutron capture in reactors or by heavy-ion reactions in accelerators, then separated chemically or physically and identified with decay spectroscopy; their short half-lives mean experiments rely on quick transfer systems and fast detectors to measure properties before the nuclei decay.
Which fermium isotopes are most useful to researchers?
Researchers focus on isotopes with longer half-lives for detailed measurements and on short-lived species to study rapid decay modes; the compiled list below gives exact half-lives and decay channels so you can pick isotopes suited to synthesis, spectroscopy, or theoretical comparison.
Fermium Isotopes
| Isotope | Atomic mass (u) | Half-life (s) | Primary decay modes |
|---|---|---|---|
| Fm-241 | 241.07 | 0.00075 | Spontaneous fission |
| Fm-242 | 242.07 | 0.0000008 | Spontaneous fission |
| Fm-243 | 243.07 | 0.21 | Alpha decay, Spontaneous fission |
| Fm-244 | 244.07 | 0.0036 | Spontaneous fission, Alpha decay |
| Fm-245 | 245.08 | 4.2 | Alpha decay, Electron capture |
| Fm-246 | 246.08 | 1.54 | Alpha decay, Spontaneous fission |
| Fm-247 | 247.08 | 35 | Alpha decay, Electron capture |
| Fm-248 | 248.09 | 36 | Alpha decay, Spontaneous fission |
| Fm-249 | 249.09 | 156 | Alpha decay, Electron capture |
| Fm-250 | 250.1 | 1,800 (30 min) | Alpha decay, Electron capture |
| Fm-251 | 251.1 | 19,260 (5.35 h) | Electron capture, Alpha decay |
| Fm-252 | 252.1 | 91,620 (25.45 h) | Alpha decay, Spontaneous fission |
| Fm-253 | 253.1 | 259,200 (3 d) | Electron capture, Alpha decay |
| Fm-254 | 254.1 | 11,664 (3.24 h) | Alpha decay, Spontaneous fission |
| Fm-255 | 255.11 | 72,360 (20.1 h) | Alpha decay |
| Fm-256 | 256.11 | 9,432 (2.62 h) | Spontaneous fission, Alpha decay |
| Fm-257 | 257.12 | 8,683,200 (100.5 d) | Alpha decay, Spontaneous fission |
| Fm-258 | 258.12 | 0.00037 | Spontaneous fission |
| Fm-259 | 259.13 | 0.0015 | Spontaneous fission |
| Fm-260 | 260.13 | 0.004 | Spontaneous fission |
Images and Descriptions
Fm-241
The lightest known fermium isotope, observed in 2006. It is extremely unstable, decaying almost instantly via spontaneous fission. Its fleeting existence is confirmed through heavy-ion fusion reactions, offering insights into the limits of nuclear stability.
Fm-242
Among the shortest-lived fermium isotopes, Fm-242 decays in less than a microsecond, almost exclusively by spontaneous fission. It is produced in particle accelerators by fusing lighter nuclei, helping scientists study the rapid fission process in heavy elements.
Fm-243
This isotope has a very short half-life of about one-fifth of a second. It was first synthesized at the Lawrence Berkeley National Laboratory. It primarily undergoes alpha decay, transforming into a californium isotope, but also has a minor spontaneous fission branch.
Fm-244
Dominated by spontaneous fission, this isotope has a half-life of only a few milliseconds. It’s synthesized in heavy-ion fusion reactions, such as bombarding plutonium targets with carbon ions. Its rapid fission makes it a subject of study for nuclear structure.
Fm-245
With a half-life of several seconds, Fm-245 is primarily an alpha emitter. It is produced by bombarding uranium or plutonium with heavy ions. Its properties help researchers map the decay characteristics of neutron-deficient heavy nuclei.
Fm-246
This isotope decays mainly through alpha emission with a half-life of over a second. It was one of the earlier fermium isotopes to be synthesized in a cyclotron. Its decay chain provides valuable data for understanding nuclear shell effects in heavy elements.
Fm-247
Fm-247 has a half-life of about 35 seconds and decays mainly by emitting an alpha particle. It can be created through fusion-evaporation reactions in a particle accelerator. Its properties contribute to the broader understanding of alpha decay systematics.
Fm-248
With a half-life of 36 seconds, this isotope decays almost exclusively via alpha emission. It is synthesized by bombarding plutonium targets with carbon ions. Studying its decay helps refine models of nuclear stability for transuranic elements.
Fm-249
Fm-249 has a half-life of 2.6 minutes, decaying mostly by alpha emission. It is typically produced as the decay product of nobelium-253. Its relatively longer life allows for more detailed study of its decay properties compared to lighter isotopes.
Fm-250
This isotope has a half-hour half-life, making it more stable than lighter fermium isotopes. It decays via alpha emission or electron capture. It can be produced by bombarding californium-249 with neutrons or alpha particles, linking it to other heavy element production chains.
Fm-251
With a half-life over five hours, Fm-251 primarily decays by capturing an electron, turning into einsteinium-251. It is synthesized in cyclotrons and provides an important data point for nuclear models predicting decay modes in this region of the nuclide chart.
Fm-252
Fm-252 has a half-life of just over a day and is a strong alpha emitter. It is produced through neutron capture in high-flux reactors or cyclotrons. Its decay product, californium-248, is useful in the synthesis of other superheavy elements.
Fm-253
This isotope has a half-life of three days, decaying primarily by electron capture into einsteinium-253. It can be produced from the beta decay of einsteinium-253’s short-lived isomer or through neutron irradiation of californium, making it accessible for chemical studies.
Fm-254
First discovered in the debris of the 1952 ‘Ivy Mike’ nuclear test, this isotope has a 3.24-hour half-life. It was formed by the rapid capture of multiple neutrons by uranium. Its discovery was a landmark event in the history of transuranic elements.
Fm-255
Also discovered in the ‘Ivy Mike’ test debris, Fm-255 is an alpha emitter with a 20.1-hour half-life. It can be produced in milligram quantities in high-flux reactors, which allows for detailed studies of fermium’s chemical and physical properties.
Fm-256
Fm-256 decays predominantly by spontaneous fission, a key characteristic of heavier fermium isotopes. With a 2.62-hour half-life, it can be created by neutron capture from Fm-255. Its high fission rate is important for understanding nuclear stability limits.
Fm-257
This is the longest-lived isotope of fermium, with a half-life of 100.5 days. It’s produced in specialized high-flux reactors and can be made in nanogram to microgram amounts. Its relative stability makes it crucial for studying the chemistry of fermium.
Fm-258
Fm-258 is extraordinarily unstable, decaying in just 370 microseconds almost entirely by spontaneous fission. This abrupt drop in stability compared to Fm-257 is a significant feature in nuclear physics, marking a boundary known as the ‘fermium sea of instability’.
Fm-259
With a 1.5-millisecond half-life, this isotope also decays exclusively by spontaneous fission. It was first synthesized by bombarding einsteinium-254 with tritium ions. Its properties confirm the trend of increasing fission instability for neutron-rich heavy nuclei.
Fm-260
Currently the heaviest confirmed fermium isotope, Fm-260 was identified in the 1990s as a decay product of a heavier element. It has a half-life of about 4 milliseconds and vanishes through spontaneous fission, representing the outer edge of our experimental knowledge of fermium.
