the half-lives of the isotopes of unbiunium from 290Ubu to 339Ubu suggested that those from 290Ubu to 303Ubu would not be bound and would decay through proton emission, those from 304Ubu through 314Ubu would undergo alpha decay, and those from 315Ubu to 339Ubu would undergo spontaneous fission. Only the isotopes from 309Ubu to 314Ubu would have long enough alpha-decay lifetimes to be detected in laboratories, starting decay chains terminating in spontaneous fission at moscovium, tennessine, or ununennium. This would present a grave problem for experiments aiming at synthesizing isotopes of unbiunium if true, because the isotopes whose alpha decay could be observed could not be reached by any presently usable combination of target and projectile. Calculations in 2016 and 2017 by the same authors on elements 123 and 125 suggest a less bleak outcome, with alpha decay chains from the more reachable nuclides 300–307Ubt passing through unbiunium and leading down to bohrium or nihonium. It has also been suggested that cluster decay might be a significant decay mode in competition with alpha decay and spontaneous fission in the region past Z = 120, which would pose yet another hurdle for experimental identification of these nuclides. Predicted chemistry Unbiunium is predicted to be the first element of an unprecedentedly long transition series, called the superactinides in analogy to the earlier actinides. While its behavior is not likely to be very distinct from lanthanum and actinium, it is likely to pose a limit to the applicability of the periodic law; after element 121, the 5g, 6f, 7d, and 8p1/2 orbitals are expected to fill up together due to their very close energies, and around the elements in the late 150s and 160s, the 9s, 9p1/2, and 8p3/2 subshells join in, so that the chemistry of the elements just beyond 121 and 122 (the last for which complete calculations have been conducted) is expected to be so similar that their position in the periodic table would be purely a formal matter. Based on the Aufbau principle, one would expect the 5g subshell to begin filling at the unbiunium atom. However, while lanthanum does have significant 4f involvement in its chemistry, it does not yet have a 4f electron in its ground-state gas-phase configuration; a greater delay occurs for 5f, where neither actinium nor thorium atoms have a 5f electron although 5f contributes to their chemistry. It is predicted that a similar situation of