Fermium (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 nuclear fallout from the Ivy Mike H-bomb test) was Fm in 1952. Fm was independently synthesized (to establish priority if the former result had to remain classified) shortly after the discovery of Fm.

There are 20 known radioisotopes ranging in atomic mass from Fm to Fm (Fm is unconfirmed), and 5 nuclear isomers. The longest-lived isotope is Fm with a half-life of 100.5 days, and the longest-lived isomer is Fm with a half-life of 5.1 seconds.

List of isotopes

|-id=Fermium-241 | rowspan=3|241Fm | rowspan=3 style="text-align:right" | 100 | rowspan=3 style="text-align:right" | 141 | rowspan=3|241.07431(32)# | rowspan=3 style="text-align:center" | 2008 | rowspan=3| | SF | (various) | rowspan=3|5/2+# |- | α (<14%) | 237Cf |- | β+ (<12%) | 241Es |-id=Fermium-242 | 242Fm | style="text-align:right" | 100 | style="text-align:right" | 142 | 242.07343(43)# | style="text-align:center" | 1975 | | SF | (various) | 0+ |-id=Fermium-243 | rowspan=2|243Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 143 | rowspan=2|243.07441(14)# | rowspan=2 style="text-align:center" | 1981 | rowspan=2| | α (91%) | 239Cf | rowspan=2|7/2−# |- | SF (9%) | (various) |-id=Fermium-244 | rowspan=3|244Fm | rowspan=3 style="text-align:right" | 100 | rowspan=3 style="text-align:right" | 144 | rowspan=3|244.07404(22)# | rowspan=3 style="text-align:center" | 1967 | rowspan=3| | SF (>97%) | (various) | rowspan=3|0+ |- | β+ (<2%) | 244Es |- | α (<1%) | 240Cf |-id=Fermium-245 | rowspan=3|245Fm | rowspan=3 style="text-align:right" | 100 | rowspan=3 style="text-align:right" | 145 | rowspan=3|245.07535(21)# | rowspan=3 style="text-align:center" | 1967 | rowspan=3| | α (88.5%) | 241Cf | rowspan=3|1/2+# |- | β+ (11.5%) | 245Es |- | SF (<0.3%) | (various) |-id=Fermium-246 | rowspan=3|246Fm | rowspan=3 style="text-align:right" | 100 | rowspan=3 style="text-align:right" | 146 | rowspan=3|246.075353(15) | rowspan=3 style="text-align:center" | 1966 | rowspan=3| | α (93.2%) | 242Cf | rowspan=3|0+ |- | SF (6.8%) | (various) |- | EC (<1.3%) | 246Es |-id=Fermium-247 | rowspan=2|247Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 147 | rowspan=2|247.07694(19)# | rowspan=2 style="text-align:center" | 1967 | rowspan=2| | α (~64%) | 243Cf | rowspan=2|(7/2+) |- | β+? (~36%) | 247Es |-id=Fermium-247m | rowspan=2 style="text-indent:1em" | 247mFm | rowspan=2 colspan="3" style="text-indent:2em" | 49(8) keV | rowspan=2 style="text-align:center" | 1967 | rowspan=2| | α (88%) | 243Cf | rowspan=2|(1/2+) |- | IT? (12%) | 247Fm |-id=Fermium-248 | rowspan=2|248Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 148 | rowspan=2|248.077185(9) | rowspan=2 style="text-align:center" | 1958 | rowspan=2| | α (99.9%) | 244Cf | rowspan=2|0+ |- | SF (0.1%) | (various) |-id=Fermium-248m | rowspan=3 style="text-indent:1em" | 248mFm | rowspan=3 colspan="3" style="text-indent:2em" | 1200(100)# keV | rowspan=3 style="text-align:center" | (2010)Only published in a conference proceeding and not a refereed journal | rowspan=3| | IT? | 248Fm | rowspan=3|6+# |- | α? | 244Cf |- | β+? | 248Es |-id=Fermium-249 | rowspan=2|249Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 149 | rowspan=2|249.078926(7) | rowspan=2 style="text-align:center" | 1959 | rowspan=2| | β+? (67%) | 249Es | rowspan=2|7/2+ |- | α (33%) | 245Cf |-id=Fermium-250 | rowspan=2|250Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 150 | rowspan=2|250.079520(8) | rowspan=2 style="text-align:center" | 1954 | rowspan=2| | α (99.99%) | 246Cf | rowspan=2|0+ |- | SF (6.9×10−3%) | (various) |-id=Fermium-250m | style="text-indent:1em" | 250mFm | colspan="3" style="text-indent:2em" | 1199.2(10) keV | style="text-align:center" | 1971 | | IT | 250Fm | (8−) |-id=Fermium-251 | rowspan=2|251Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 151 | rowspan=2|251.081545(15) | rowspan=2 style="text-align:center" | 1957 | rowspan=2| | β+ (98.20%) | 251Es | rowspan=2|9/2− |- | α (1.80%) | 247Cf |-id=Fermium-251m | style="text-indent:1em" | 251mFm | colspan="3" style="text-indent:2em" | 200.0(1) keV | style="text-align:center" | 2006 | | IT | 251Fm | 5/2+ |-id=Fermium-252 | rowspan=2|252Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 152 | rowspan=2|252.082466(6) | rowspan=2 style="text-align:center" | 1956 | rowspan=2| | α (99.99%) | 248Cf | rowspan=2|0+ |- | SF (0.0023%) | (various) |-id=Fermium-253 | rowspan=2|253Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 153 | rowspan=2|253.0851809(17) | rowspan=2 style="text-align:center" | 1957 | rowspan=2| | EC (88%) | 253Es | rowspan=2|1/2+ |- | α (12%) | 249Cf |-id=Fermium-253m | style="text-indent:1em" | 253mFm | colspan="3" style="text-indent:2em" | 351(6) keV | style="text-align:center" | 2011 | | IT | 253Fm | 11/2−# |-id=Fermium-254 | rowspan=2|254Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 154 | rowspan=2|254.0868524(20) | rowspan=2 style="text-align:center" | 1954 | rowspan=2| | α (99.94%) | 250Cf | rowspan=2|0+ |- | SF (0.0592%) | (various) |-id=Fermium-255 | rowspan=2|255Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 155 | rowspan=2|255.089963(4) | rowspan=2 style="text-align:center" | 1954 | rowspan=2| | α | 251Cf | rowspan=2|7/2+ |- | SF (2.4×10−5%) | (various) |-id=Fermium-256 | rowspan=2|256Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 156 | rowspan=2|256.091772(3) | rowspan=2 style="text-align:center" | 1955 | rowspan=2| | SF (91.9%) | (various) | rowspan=2|0+ |- | α (8.1%) | 252Cf |-id=Fermium-257 | rowspan=2|257FmHeaviest nuclide produced via neutron capture | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 157 | rowspan=2|257.095105(5) | rowspan=2 style="text-align:center" | 1964 | rowspan=2| | α (99.79%) | 253Cf | rowspan=2|9/2+ |- | SF (0.210%) | (various) |-id=Fermium-258 | 258Fm | style="text-align:right" | 100 | style="text-align:right" | 158 | 258.09708(22)# | style="text-align:center" | 1971 | | SF | (various) | 0+ |-id=Fermium-259 | 259Fm | style="text-align:right" | 100 | style="text-align:right" | 159 | 259.10060(30)# | style="text-align:center" | 1980 | | SF | (various) | |-id=Fermium-260 | 260FmDiscovery of this isotope is unconfirmedNot directly synthesized, occurs as decay product of 260Md | style="text-align:right" | 100 | style="text-align:right" | 160 | 260.10281(47)# | style="text-align:center" | (1992) | 1# min | SF | (various) | 0+

References