Most of the isotope data on this site has been obtained from the National Nuclear Data Center.
Naturally Occurring Isotopes
Fermium has no naturally occurring isotopes.
Known Isotopes
| Mass Number | Half-life | Decay Mode | Branching Percentage |
| 241 | 0.73 milliseconds | Spontaneous Fission | > 78.00% |
| Alpha Decay | < 14.00% | ||
| Electron Capture | < 12.00% | ||
| 242 | < 4 microseconds | Spontaneous Fission | <= 100.00% |
| 243 | 231 milliseconds | Alpha Decay | 91.00% |
| Spontaneous Fission | 9.00% | ||
| Electron Capture | < 10.00% | ||
| 244 | 3.12 milliseconds | Alpha Decay | < 1.00% |
| Spontaneous Fission | > 97.00% | ||
| Electron Capture | < 2.00% | ||
| 245 | 4.2 seconds | Alpha Decay | <= 100.00% |
| 246 | 1.54 seconds | Alpha Decay | 93.20% |
| Spontaneous Fission | 6.80% | ||
| Electron Capture | <= 1.30% | ||
| 247 | 31 seconds | Alpha Decay | >= 84.00% |
| Electron Capture | <= 16.00% | ||
| 247m | 5.1 seconds | Alpha Decay | 84.00% |
| 248 | 36 seconds | Alpha Decay | 93.00% |
| Electron Capture | 7.00% | ||
| Spontaneous Fission | 0.10% | ||
| 249 | 2.6 minutes | Electron Capture | 67.00% |
| Alpha Decay | 33.00% | ||
| 250 | 30 minutes | Alpha Decay | > 90.00% |
| Electron Capture | < 10.00% | ||
| Spontaneous Fission | 6.9×10-3 % | ||
| 250m | 1.93 seconds | Isomeric Transition | 100.00% |
| 251 | 5.30 hours | Electron Capture | 98.20% |
| Alpha Decay | 1.80% | ||
| 252 | 25.39 hours | Spontaneous Fission | 2.3×10-3 % |
| Alpha Decay | 100.00% | ||
| 253 | 3.00 days | Electron Capture | 88.00% |
| Alpha Decay | 12.00% | ||
| 254 | 3.240 hours | Alpha Decay | 99.94% |
| Spontaneous Fission | 0.06% | ||
| 255 | 20.07 hours | Alpha Decay | 100.00% |
| Spontaneous Fission | 2.4×10-5 % | ||
| 256 | 157.6 minutes | Spontaneous Fission | 91.90% |
| Alpha Decay | 8.10% | ||
| 257 | 100.5 days | Alpha Decay | 99.79% |
| Spontaneous Fission | 0.21% | ||
| 258 | 370 microseconds | Spontaneous Fission | <= 100.00% |
| 259 | 1.5 seconds | Spontaneous Fission | 100.00% |
| 260 | ≈ 4 milliseconds | Spontaneous Fission | 100.00% |
This page is maintained by Steve Gagnon.