Isotopes of fermium

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| | 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)# | | 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| | 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| | 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| | 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| | 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| | 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)&nbsp;keV | 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| | 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)#&nbsp;keV | 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| | 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| | 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)&nbsp;keV | | 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| | 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)&nbsp;keV | | 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| | 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| | 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)&nbsp;keV | | 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| | 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| | 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| | SF (91.9%) | (various) | rowspan=2|0+ |- | α (8.1%) | ²⁵²Cf |-id=Fermium-257 | rowspan=2|²⁵⁷Fm | rowspan=2 style="text-align:right" | 100 | rowspan=2 style="text-align:right" | 157 | rowspan=2|257.095105(5) | 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)# | | SF | (various) | 0+ |-id=Fermium-259 | ²⁵⁹Fm | style="text-align:right" | 100 | style="text-align:right" | 159 | 259.10060(30)# | | SF | (various) | |-id=Fermium-260 | ²⁶⁰Fm | style="text-align:right" | 100 | style="text-align:right" | 160 | 260.10281(47)# | 1#&nbsp;min | SF | (various) | 0+

Chronology of isotope discovery

²⁶⁰Fm was not confirmed in 1997.

References