Isotopes of fermium

Fermium (100Fm) 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 255Fm in 1952. 250Fm was independently synthesized shortly after the discovery of 255Fm. There are 20 known radioisotopes ranging in atomic mass from 241Fm to 260Fm (260Fm is unconfirmed), and 4 nuclear isomers, 247mFm, 250mFm, 251mFm, and 253mFm. The longest-lived isotope is 257Fm with a half-life of 100.5 days, and the longest-lived isomer is 247mFm with a half-life of 5.1 seconds.

List of isotopes

 * rowspan=2|241Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 141
 * rowspan=2|241.07421(32)#
 * rowspan=2|730(60) μs
 * SF
 * (various)
 * rowspan=2|5/2+#
 * α (<14%)
 * 237Cf
 * 242Fm
 * style="text-align:right" | 100
 * style="text-align:right" | 142
 * 242.07343(43)#
 * <4 μs
 * SF
 * (various)
 * 0+
 * rowspan=3|243Fm
 * rowspan=3 style="text-align:right" | 100
 * rowspan=3 style="text-align:right" | 143
 * rowspan=3|243.07447(23)#
 * rowspan=3|231(9) ms
 * α (91%)
 * 239Cf
 * rowspan=3|(7/2−)
 * SF (9%)
 * (various)
 * β+ (<10%)
 * 243Es
 * rowspan=2|244Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 144
 * rowspan=2|244.07404(22)#
 * rowspan=2|3.12(8) ms
 * SF
 * (various)
 * rowspan=2|0+
 * α (<1%)
 * 240Cf
 * rowspan=3|245Fm
 * rowspan=3 style="text-align:right" | 100
 * rowspan=3 style="text-align:right" | 145
 * rowspan=3|245.07535(21)#
 * rowspan=3|5.5(7) s
 * α (88.5%)
 * 241Cf
 * rowspan=3|(1/2+)
 * β+ (11.5%)
 * 245Es
 * SF (<0.3%)
 * (various)
 * rowspan=3|246Fm
 * rowspan=3 style="text-align:right" | 100
 * rowspan=3 style="text-align:right" | 146
 * rowspan=3|246.075350(17)
 * rowspan=3|1.54(4) s
 * α (93.2%)
 * 242Cf
 * rowspan=3|0+
 * SF (6.8%)
 * (various)
 * β+ (<1.3%)
 * 246Es
 * rowspan=2|247Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 147
 * rowspan=2|247.07695(12)#
 * rowspan=2|31(1) s
 * α (64%)
 * 243Cf
 * rowspan=2|(7/2+)
 * β+ (36%)
 * 247Es
 * rowspan=3 style="text-indent:1em" | 247mFm
 * rowspan=3 colspan="3" style="text-indent:2em" | 45(7) keV
 * rowspan=3| 5.1(2) s
 * α (88%)
 * 243Cf
 * rowspan=3|(1/2+)
 * IT (12%)
 * 247Fm
 * β+?
 * 247Es
 * rowspan=3|248Fm
 * rowspan=3 style="text-align:right" | 100
 * rowspan=3 style="text-align:right" | 148
 * rowspan=3|248.077186(9)
 * rowspan=3|35.1(8) s
 * α (93%)
 * 244Cf
 * rowspan=3|0+
 * β+ (7%)
 * 248Es
 * SF (.10%)
 * (various)
 * rowspan=2|249Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 149
 * rowspan=2|249.078928(7)
 * rowspan=2|1.6(1) min
 * β+ (84.4%)
 * 249Es
 * rowspan=2|(7/2+)
 * α (15.6%)
 * 245Cf
 * rowspan=3|250Fm
 * rowspan=3 style="text-align:right" | 100
 * rowspan=3 style="text-align:right" | 150
 * rowspan=3|250.079521(9)
 * rowspan=3|30.4(15) min
 * α (90%)
 * 246Cf
 * rowspan=3|0+
 * EC (10%)
 * 250Es
 * SF (6.9×10−3%)
 * (various)
 * style="text-indent:1em" | 250mFm
 * colspan="3" style="text-indent:2em" | 1199.2(10) keV
 * 1.92(5) s
 * IT
 * 250Fm
 * (8−)
 * rowspan=2|251Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 151
 * rowspan=2|251.081540(16)
 * rowspan=2|5.30(8) h
 * β+ (98.2%)
 * 251Es
 * rowspan=2|(9/2−)
 * α (1.8%)
 * 247Cf
 * style="text-indent:1em" | 251mFm
 * colspan="3" style="text-indent:2em" | 200.09(11) keV
 * 21.1(16) μs
 * IT
 * 251Fm
 * (5/2+)
 * rowspan=2|252Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 152
 * rowspan=2|252.082467(6)
 * rowspan=2|25.39(4) h
 * α (99.99%)
 * 248Cf
 * rowspan=2|0+
 * SF (.0023%)
 * (various)
 * rowspan=2|253Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 153
 * rowspan=2|253.085185(4)
 * rowspan=2|3.00(12) d
 * EC (88%)
 * 253Es
 * rowspan=2|(1/2)+
 * α (12%)
 * 249Cf
 * style="text-indent:1em" | 253mFm
 * colspan="3" style="text-indent:2em" | ~351 keV
 * 0.56(6) μs
 * IT
 * 253Fm
 * (11/2-)
 * rowspan=2|254Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 154
 * rowspan=2|254.0868544(30)
 * rowspan=2|3.240(2) h
 * α (99.94%)
 * 250Cf
 * rowspan=2|0+
 * SF (.0592%)
 * (various)
 * rowspan=2|255Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 155
 * rowspan=2|255.089964(5)
 * rowspan=2|20.07(7) h
 * α
 * 251Cf
 * rowspan=2|7/2+
 * SF (2.4×10−5%)
 * (various)
 * rowspan=2|256Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 156
 * rowspan=2|256.091774(8)
 * rowspan=2|157.6(13) min
 * SF (91.9%)
 * (various)
 * rowspan=2|0+
 * α (8.1%)
 * 252Cf
 * rowspan=2|257Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 157
 * rowspan=2|257.095106(7)
 * rowspan=2|100.5(2) d
 * α (99.79%)
 * 253Cf
 * rowspan=2|(9/2+)
 * SF (.21%)
 * (various)
 * 258Fm
 * style="text-align:right" | 100
 * style="text-align:right" | 158
 * 258.09708(22)#
 * 370(14) μs
 * SF
 * (various)
 * 0+
 * 259Fm
 * style="text-align:right" | 100
 * style="text-align:right" | 159
 * 259.1006(3)#
 * 1.5(3) s
 * SF
 * (various)
 * 3/2+#
 * 260Fm
 * style="text-align:right" | 100
 * style="text-align:right" | 160
 * 260.10281(55)#
 * 4 ms
 * SF
 * (various)
 * 0+
 * rowspan=2|256.091774(8)
 * rowspan=2|157.6(13) min
 * SF (91.9%)
 * (various)
 * rowspan=2|0+
 * α (8.1%)
 * 252Cf
 * rowspan=2|257Fm
 * rowspan=2 style="text-align:right" | 100
 * rowspan=2 style="text-align:right" | 157
 * rowspan=2|257.095106(7)
 * rowspan=2|100.5(2) d
 * α (99.79%)
 * 253Cf
 * rowspan=2|(9/2+)
 * SF (.21%)
 * (various)
 * 258Fm
 * style="text-align:right" | 100
 * style="text-align:right" | 158
 * 258.09708(22)#
 * 370(14) μs
 * SF
 * (various)
 * 0+
 * 259Fm
 * style="text-align:right" | 100
 * style="text-align:right" | 159
 * 259.1006(3)#
 * 1.5(3) s
 * SF
 * (various)
 * 3/2+#
 * 260Fm
 * style="text-align:right" | 100
 * style="text-align:right" | 160
 * 260.10281(55)#
 * 4 ms
 * SF
 * (various)
 * 0+
 * style="text-align:right" | 160
 * 260.10281(55)#
 * 4 ms
 * SF
 * (various)
 * 0+

Chronology of isotope discovery
260Fm was not confirmed in 1997.