Isotopes
There are 19 isotopes of fermium listed in N 2003, with atomic weights of 242 to 260, of which 257Fm is the longest-lived with a half-life of 100.5 days. 253Fm has a half-life of 3 days, while 251Fm of 5.3 h, 252Fm of 25.4 h, 254Fm of 3.2 h, 255Fm of 20.1 h, and 256Fm of 2.6 hours. All the remaining ones have half-lives ranging from 30 minutes to less than a millisecond. The neutron-capture product of fermium-257, 258Fm, undergoes spontaneous fission with a half-life of just 370(14) microseconds; 259Fm and 260Fm are also unstable with respect to spontaneous fission (t½ = 1.5(3) s and 4 ms respectively). This means that neutron capture cannot be used to create nuclides with a mass number greater than 257, unless carried out in a nuclear explosion. As 257Fm is an α-emitter, decaying to 253Cf, and no fermium isotopes undergo beta minus decay (which would produce isotopes of the next element, mendelevium), fermium is also the last element that can be prepared by a neutron-capture process.
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