Isotopes of fermium (100Fm)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
252Fm synth 25.39 h SF
α 248Cf
253Fm synth 3 d ε 253Es
α 249Cf
255Fm synth 20.07 h SF
α 251Cf
257Fm synth 100.5 d α 253Cf
SF

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

Nuclide
[n 1]
Z N Isotopic mass (Da)
[n 2][n 3]
Half-life
Decay
mode

[n 4]
Daughter
isotope

Spin and
parity
[n 5][n 6]
Excitation energy
241Fm 100 141 241.07421(32)# 730(60) μs SF (various) 5/2+#
α (<14%) 237Cf
242Fm 100 142 242.07343(43)# <4 μs[2] SF (various) 0+
243Fm[3] 100 143 243.07447(23)# 231(9) ms α (91%) 239Cf (7/2−)
SF (9%) (various)
β+ (<10%) 243Es
244Fm[2] 100 144 244.07404(22)# 3.12(8) ms SF (various) 0+
α (<1%) 240Cf
245Fm[4] 100 145 245.07535(21)# 5.5(7) s α (88.5%) 241Cf (1/2+)
β+ (11.5%) 245Es
SF (<0.3%)[3] (various)
246Fm[5] 100 146 246.075350(17) 1.54(4) s α (93.2%) 242Cf 0+
SF (6.8%) (various)
β+ (<1.3%) 246Es
247Fm[6] 100 147 247.07695(12)# 31(1) s α (64%) 243Cf (7/2+)
β+ (36%) 247Es
247mFm 45(7) keV 5.1(2) s α (88%) 243Cf (1/2+)
IT (12%) 247Fm
β+? 247Es
248Fm 100 148 248.077186(9) 35.1(8) s α (93%) 244Cf 0+
β+ (7%) 248Es
SF (.10%) (various)
249Fm 100 149 249.078928(7) 1.6(1) min β+ (84.4%) 249Es (7/2+)
α (15.6%)[7] 245Cf
250Fm 100 150 250.079521(9) 30.4(15) min α (90%) 246Cf 0+
EC (10%) 250Es
SF (6.9×10−3%) (various)
250mFm 1199.2(10) keV 1.92(5) s IT 250Fm (8−)
251Fm 100 151 251.081540(16) 5.30(8) h β+ (98.2%) 251Es (9/2−)
α (1.8%) 247Cf
251mFm 200.09(11) keV 21.1(16) μs IT 251Fm (5/2+)
252Fm 100 152 252.082467(6) 25.39(4) h α (99.99%) 248Cf 0+
SF (.0023%) (various)
253Fm 100 153 253.085185(4) 3.00(12) d EC (88%) 253Es (1/2)+
α (12%) 249Cf
253mFm[8] ~351 keV 0.56(6) μs IT 253Fm (11/2-)
254Fm 100 154 254.0868544(30) 3.240(2) h α (99.94%) 250Cf 0+
SF (.0592%) (various)
255Fm 100 155 255.089964(5) 20.07(7) h α 251Cf 7/2+
SF (2.4×10−5%) (various)
256Fm 100 156 256.091774(8) 157.6(13) min SF (91.9%) (various) 0+
α (8.1%) 252Cf
257Fm[n 7] 100 157 257.095106(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[n 8][n 9] 100 160 260.10281(55)# 4 ms SF (various) 0+
This table header & footer:
  1. mFm  Excited nuclear isomer.
  2. ()  Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. #  Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. Modes of decay:
    EC:Electron capture
    IT:Isomeric transition
    SF:Spontaneous fission
  5. () spin value  Indicates spin with weak assignment arguments.
  6. #  Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  7. Heaviest nuclide produced via neutron capture
  8. Discovery of this isotope is unconfirmed
  9. Not directly synthesized, occurs as decay product of 260Md

Chronology of isotope discovery

IsotopeDiscoveredReaction
241Fm2008204Pb(40Ar,3n)
242Fm1975204Pb(40Ar,2n), 206Pb(40Ar,4n)
243Fm1981206Pb(40Ar,3n)
244Fm1967233U(16O,5n)
245Fm1967233U(16O,4n)
246Fm1966235U(16O,5n)
247Fm1967239Pu(12C,4n)
248Fm1958240Pu(12C,4n)
249Fm1960238U(16O,5n)
250Fm1954238U(16O,4n)
251Fm1957249Cf(α,2n)
252Fm1956249Cf(α,n)
253Fm1957252Cf(α,3n)
254Fm1954Neutron capture
255Fm1954Neutron capture
256Fm1955Neutron capture
257Fm1964Neutron capture
258Fm1971257Fm(d,p)
259Fm1980257Fm(t,p)
260Fm?1992?254Es+18O, 22Ne — transfer (EC of 260Md)

260Fm was not confirmed in 1997.

References

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