Fermium (Fm) is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be discovered (in fallout from nuclear testing) was 255Fm in 1952. 250Fm was independently synthesized shortly after the discovery of 255Fm. There are 19 known radioisotopes ranging in atomic mass from 242Fm to 260Fm, and 2 nuclear isomers, 250mFm and 251mFm. The longest-lived isotope is 257Fm with a half-life of 100.5 days, and the longest-lived isomer is 250mFm with a half-life of 1.8 seconds.
nuclide symbol |
Z(p) | N(n) | isotopic mass (u) |
half-life | decay mode(s)[1][n 1] |
daughter isotope(s) |
nuclear spin |
---|---|---|---|---|---|---|---|
excitation energy | |||||||
242Fm | 100 | 142 | 242.07343(43)# | 0.8(2) ms | SF | (various) | 0+ |
α (rare) | 238Cf | ||||||
243Fm | 100 | 143 | 243.07435(23)# | 210(60) ms [0.18(+8-4) s] |
α (60%) | 239Cf | 7/2-# |
β+ (40%) | 243Es | ||||||
SF (.57%) | (various) | ||||||
244Fm | 100 | 144 | 244.07408(30)# | 3.3(5) ms | SF (99%) | (various) | 0+ |
α (1%) | 240Cf | ||||||
245Fm | 100 | 145 | 245.07539(30)# | 4.2(13) s | α (95.7%) | 241Cf | 1/2+# |
β+ (4.2%) | 245Es | ||||||
SF (.13%) | (various) | ||||||
246Fm | 100 | 146 | 246.07530(4) | 1.1(2) s | α (85%) | 242Cf | 0+ |
β+ (10%) | 246Es | ||||||
β+, SF (10%) | (various) | ||||||
SF (4.5%) | (various) | ||||||
247Fm | 100 | 147 | 247.07685(15)# | 29(1) s | α (50%) | 243Cf | (7/2+) |
β+ (50%) | 247Es | ||||||
248Fm | 100 | 148 | 248.077195(13) | 36(2) s | α (99%) | 244Cf | 0+ |
β+ (1%) | 248Es | ||||||
SF (.05%) | (various) | ||||||
249Fm | 100 | 149 | 249.07903(11)# | 2.6(7) min | β+ (85%) | 249Es | (7/2+)# |
α (15%) | 245Cf | ||||||
250Fm | 100 | 150 | 250.079521(13) | 30(3) min | α (90%) | 246Cf | 0+ |
EC (10%) | 250Es | ||||||
SF (6×10−4%) | (various) | ||||||
250mFm | 1500(300)# keV | 1.8(1) s | IT | 250Fm | 7,8# | ||
251Fm | 100 | 151 | 251.081575(9) | 5.30(8) h | β+ (98.2%) | 251Es | (9/2-) |
α (1.8%) | 247Cf | ||||||
251mFm | 191(2) keV | 15.2(23) µs | (5/2+) | ||||
252Fm | 100 | 152 | 252.082467(6) | 25.39(4) h | α (99.99%) | 248Cf | 0+ |
SF (.0023%) | (various) | ||||||
β+β+ (rare) | 252Cf | ||||||
253Fm | 100 | 153 | 253.085185(4) | 3.00(12) d | EC (88%) | 253Es | (1/2)+ |
α (12%) | 249Cf | ||||||
254Fm | 100 | 154 | 254.0868542(30) | 3.240(2) h | α (99.94%) | 250Cf | 0+ |
SF (.06%) | (various) | ||||||
255Fm | 100 | 155 | 255.089962(5) | 20.07(7) h | α | 251Cf | 7/2+ |
SF (2×10−5%) | (various) | ||||||
256Fm | 100 | 156 | 256.091773(8) | 157.6(13) min | SF (91.9%) | (various) | 0+ |
α (8.1%) | 252Cf | ||||||
257Fm[n 2] | 100 | 157 | 257.095105(7) | 100.5(2) d | α (99.79%) | 253Cf | (9/2+) |
SF (.21%) | (various) | ||||||
258Fm | 100 | 158 | 258.09708(22)# | 370(14) µs | SF | (various) | 0+ |
259Fm | 100 | 159 | 259.1006(3)# | 1.5(3) s | SF | (various) | 3/2+# |
260Fm | 100 | 160 | 260.10268(54)# | 1# min | 0+ |
Isotopes of einsteinium | Isotopes of fermium | Isotopes of mendelevium |
Index to isotope pages · Table of nuclides |