Under normal conditions, fermium exists in solution as the Fm3+ ion, which has a hydration number of 16.9 and an acid dissociation constant of 1.6×10−4 (pKa = 3.8).
Additional analysis and further experimentation showed the produced mendelevium isotope to have mass 256 and to decay by electron capture to fermium-256 with a half-life of 1.5 h.
There are 20 isotopes of fermium listed in NUBASE 2016, with atomic weights of 241 to 260, of which 257Fm is the longest-lived with a half-life of 100.5 days.
Therefore, most fermium is produced on Earth in scientific laboratories, high-power nuclear reactors, or in nuclear weapons tests, and is present only within a few months from the time of the synthesis.