# Isotopes of fermium

Fermium ($_{100}$Fm) is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be discovered (in fallout from nuclear testing) was $^{255}$Fm in 1952. $^{250}$Fm was independently synthesized shortly after the discovery of $^{255}$Fm. There are 20 known radioisotopes ranging in atomic mass from $^{241}$Fm to $^{260}$Fm ($^{260}$Fm is unconfirmed), and 4 nuclear isomers, $^{247m}$Fm, $^{250m}$Fm, $^{251m}$Fm, and $^{253m}$Fm. The longest-lived isotope is $^{257}$Fm with a half-life of 100.5 days, and the longest-lived isomer is $^{247m}$Fm with a half-life of 5.1 seconds.

## List of isotopes

| Nuclide | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity |
| Spin and parity |||
| $^{241}$Fm | 100 | 141 | 241.07431(32)# | 730(60) μs | SF? | (various) | 5/2+# |
| $^{241}$Fm | 100 | 141 | 241.07431(32)# | 730(60) μs | α (<14%) | $^{237}$Cf | 5/2+# |
| $^{241}$Fm | 100 | 141 | 241.07431(32)# | 730(60) μs | β$^+$ (<12%) | $^{241}$Es | 5/2+# |
| $^{242}$Fm | 100 | 142 | 242.07343(43)# | 0.8(2) ms | SF | (various) | 0+ |
| $^{242}$Fm | 100 | 142 | 242.07343(43)# | 0.8(2) ms | α? | $^{238}$Cf | 0+ |
| $^{243}$Fm | 100 | 143 | 243.07441(14)# | 231(9) ms | α (91%) | $^{239}$Cf | (7/2−) |
| $^{243}$Fm | 100 | 143 | 243.07441(14)# | 231(9) ms | SF (9%) | (various) | (7/2−) |
| $^{243}$Fm | 100 | 143 | 243.07441(14)# | 231(9) ms | β$^+$? | $^{243}$Es | (7/2−) |
| $^{244}$Fm | 100 | 144 | 244.07404(22)# | 3.12(8) ms | SF (>97%) | (various) | 0+ |
| $^{244}$Fm | 100 | 144 | 244.07404(22)# | 3.12(8) ms | β$^+$ (<2%) | $^{244}$Es | 0+ |
| $^{244}$Fm | 100 | 144 | 244.07404(22)# | 3.12(8) ms | α (<1%) | $^{240}$Cf | 0+ |
| $^{245}$Fm | 100 | 145 | 245.07535(21)# | 4.2(13) s | α (88.5%) | $^{241}$Cf | (1/2+) |
| $^{245}$Fm | 100 | 145 | 245.07535(21)# | 4.2(13) s | β$^+$ (11.5%) | $^{245}$Es | (1/2+) |
| $^{245}$Fm | 100 | 145 | 245.07535(21)# | 4.2(13) s | SF (<0.3%) | (various) | (1/2+) |
| $^{246}$Fm | 100 | 146 | 246.075350(17) | 1.54(4) s | α (93.2%) | $^{242}$Cf | 0+ |
| $^{246}$Fm | 100 | 146 | 246.075350(17) | 1.54(4) s | SF (6.8%) | (various) | 0+ |
| $^{246}$Fm | 100 | 146 | 246.075350(17) | 1.54(4) s | EC (<1.3%) | $^{246}$Es | 0+ |
| $^{247}$Fm | 100 | 147 | 247.07694(19)# | 31(1) s | α (~64%) | $^{243}$Cf | (7/2+) |
| $^{247}$Fm | 100 | 147 | 247.07694(19)# | 31(1) s | β$^+$? (~36%) | $^{247}$Es | (7/2+) |
| $^{247m}$Fm | 49(8) keV | 49(8) keV | 49(8) keV | 5.1(2) s | α (88%) | $^{243}$Cf | (1/2+) |
| $^{247m}$Fm | 49(8) keV | 49(8) keV | 49(8) keV | 5.1(2) s | IT? | $^{247}$Fm | (1/2+) |
| $^{247m}$Fm | 49(8) keV | 49(8) keV | 49(8) keV | 5.1(2) s | β$^+$? | $^{247}$Es | (1/2+) |
| $^{248}$Fm | 100 | 148 | 248.0771855(91) | 34.5(12) s | α (99.9%) | $^{244}$Cf | 0+ |
| $^{248}$Fm | 100 | 148 | 248.0771855(91) | 34.5(12) s | SF (0.1%) | (various) | 0+ |
| $^{248}$Fm | 100 | 148 | 248.0771855(91) | 34.5(12) s | β$^+$? | $^{248}$Es | 0+ |
| $^{248m}$Fm | 1200(100)# keV | 1200(100)# keV | 1200(100)# keV | 10.1(6) ms | IT? | $^{248}$Fm | 6+# |
| $^{248m}$Fm | 1200(100)# keV | 1200(100)# keV | 1200(100)# keV | 10.1(6) ms | α? | $^{244}$Cf | 6+# |
| $^{248m}$Fm | 1200(100)# keV | 1200(100)# keV | 1200(100)# keV | 10.1(6) ms | β$^+$? | $^{248}$Es | 6+# |
| $^{249}$Fm | 100 | 149 | 249.0789260(67) | 1.6(1) min | β$^+$? (67%) | $^{249}$Es | 7/2+ |
| $^{249}$Fm | 100 | 149 | 249.0789260(67) | 1.6(1) min | α (33%) | $^{245}$Cf | 7/2+ |
| $^{250}$Fm | 100 | 150 | 250.0795198(85) | 31.0(11) min | α (99.99%) | $^{246}$Cf | 0+ |
| $^{250}$Fm | 100 | 150 | 250.0795198(85) | 31.0(11) min | SF (6.9×10$^{−3}$%) | (various) | 0+ |
| $^{250}$Fm | 100 | 150 | 250.0795198(85) | 31.0(11) min | EC? | $^{250}$Es | 0+ |
| $^{250m}$Fm | 1199.2(10) keV | 1199.2(10) keV | 1199.2(10) keV | 1.92(5) s | IT | $^{250}$Fm | (8−) |
| $^{250m}$Fm | 1199.2(10) keV | 1199.2(10) keV | 1199.2(10) keV | 1.92(5) s | α? | $^{246}$Cf | (8−) |
| $^{250m}$Fm | 1199.2(10) keV | 1199.2(10) keV | 1199.2(10) keV | 1.92(5) s | β$^+$? | $^{250}$Es | (8−) |
| $^{250m}$Fm | 1199.2(10) keV | 1199.2(10) keV | 1199.2(10) keV | 1.92(5) s | SF? | (various) | (8−) |
| $^{251}$Fm | 100 | 151 | 251.081545(15) | 5.30(8) h | β$^+$ (98.20%) | $^{251}$Es | 9/2− |
| $^{251}$Fm | 100 | 151 | 251.081545(15) | 5.30(8) h | α (1.80%) | $^{247}$Cf | 9/2− |
| $^{251m}$Fm | 200.0(1) keV | 200.0(1) keV | 200.0(1) keV | 21.8(8) μs | IT | $^{251}$Fm | 5/2+ |
| $^{252}$Fm | 100 | 152 | 252.0824660(56) | 25.39(4) h | α (99.99%) | $^{248}$Cf | 0+ |
| $^{252}$Fm | 100 | 152 | 252.0824660(56) | 25.39(4) h | SF (.0023%) | (various) | 0+ |
| $^{253}$Fm | 100 | 153 | 253.0851809(16) | 3.00(12) d | EC (88%) | $^{253}$Es | 1/2+ |
| $^{253}$Fm | 100 | 153 | 253.0851809(16) | 3.00(12) d | α (12%) | $^{249}$Cf | 1/2+ |
| $^{253m}$Fm | 351(6) keV | 351(6) keV | 351(6) keV | 0.56(6) μs | IT | $^{253}$Fm | 11/2-# |
| $^{254}$Fm | 100 | 154 | 254.0868524(20) | 3.240(2) h | α (99.94%) | $^{250}$Cf | 0+ |
| $^{254}$Fm | 100 | 154 | 254.0868524(20) | 3.240(2) h | SF (0.0592%) | (various) | 0+ |
| $^{255}$Fm | 100 | 155 | 255.0899635(42) | 20.07(7) h | α | $^{251}$Cf | 7/2+ |
| $^{255}$Fm | 100 | 155 | 255.0899635(42) | 20.07(7) h | SF (2.4×10$^{−5}$%) | (various) | 7/2+ |
| $^{256}$Fm | 100 | 156 | 256.0917717(32) | 157.1(13) min | SF (91.9%) | (various) | 0+ |
| $^{256}$Fm | 100 | 156 | 256.0917717(32) | 157.1(13) min | α (8.1%) | $^{252}$Cf | 0+ |
| $^{257}$Fm | 100 | 157 | 257.0951054(47) | 100.5(2) d | α (99.79%) | $^{253}$Cf | 9/2+ |
| $^{257}$Fm | 100 | 157 | 257.0951054(47) | 100.5(2) d | SF (.210%) | (various) | 9/2+ |
| $^{258}$Fm | 100 | 158 | 258.09708(22)# | 370(14) μs | SF | (various) | 0+ |
| $^{258}$Fm | 100 | 158 | 258.09708(22)# | 370(14) μs | α? | $^{254}$Cf | 0+ |
| $^{259}$Fm | 100 | 159 | 259.10060(30)# | 1.5(2) s | SF | (various) | 3/2+# |
| $^{260}$Fm | 100 | 160 | 260.10281(47)# | 1# min | SF | (various) | 0+ |
| This table header & footer: view ||||||||
* ↑ $^m$Fm – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* ↑ Modes of decay: EC:Electron capture IT:Isomeric transition SF:Spontaneous fission
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* ↑ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ Theoretically capable of β$^+$β$^+$ decay to $^{252}$Cf
* ↑ Heaviest nuclide produced via neutron capture
* ↑ Discovery of this isotope is unconfirmed
* ↑ Not directly synthesized, occurs as decay product of $^{260}$Md

## Chronology of isotope discovery

| Isotope | Discovered | Reaction |
| $^{241}$Fm | 2008 | $^{204}$Pb($^{40}$Ar,3n) |
| $^{242}$Fm | 1975 | $^{204}$Pb($^{40}$Ar,2n), $^{206}$Pb($^{40}$Ar,4n) |
| $^{243}$Fm | 1981 | $^{206}$Pb($^{40}$Ar,3n) |
| $^{244}$Fm | 1967 | $^{233}$U($^{16}$O,5n) |
| $^{245}$Fm | 1967 | $^{233}$U($^{16}$O,4n) |
| $^{246}$Fm | 1966 | $^{235}$U($^{16}$O,5n) |
| $^{247}$Fm | 1967 | $^{239}$Pu($^{12}$C,4n) |
| $^{248}$Fm | 1958 | $^{240}$Pu($^{12}$C,4n) |
| $^{249}$Fm | 1960 | $^{238}$U($^{16}$O,5n) |
| $^{250}$Fm | 1954 | $^{238}$U($^{16}$O,4n) |
| $^{251}$Fm | 1957 | $^{249}$Cf(α,2n) |
| $^{252}$Fm | 1956 | $^{249}$Cf(α,n) |
| $^{253}$Fm | 1957 | $^{252}$Cf(α,3n) |
| $^{254}$Fm | 1954 | Neutron capture |
| $^{255}$Fm | 1954 | Neutron capture |
| $^{256}$Fm | 1955 | Neutron capture |
| $^{257}$Fm | 1964 | Neutron capture |
| $^{258}$Fm | 1971 | $^{257}$Fm(d,p) |
| $^{259}$Fm | 1980 | $^{257}$Fm(t,p) |
| $^{260}$Fm? | 1992? | $^{254}$Es+$^{18}$O, $^{22}$Ne — transfer (EC of $^{260}$Md) |
$^{260}$Fm was not confirmed in 1997.