Superactinides are the undiscovered chemical elements from atomic numbers 121 (unbiunium) until 153 (unpenttrium), at which the 5g and 6f electron shells are filled up. The superactinide series is predicted to follow the transactinide series and sits below it on the extended periodic table of the elements. The theoretical existence of the series was proposed by Glenn T. Seaborg, winner of the 1951 Nobel Prize in Chemistry.
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No elements in this region have been synthesized or discovered in nature.[1]
The first element of the g-block may have atomic number 121, and thus would have the systematic name unbiunium. Elements in this region are likely to be highly unstable with respect to radioactive decay, and have extremely short half lives, although element 126 is hypothesized to be within an island of stability that is resistant to fission but not to alpha decay. It is not clear how many elements beyond the expected island of stability are physically possible, or even if the superactindes are complete.
According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially-filled g-orbitals. However, spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number.[2]
| 121 Ubu |
122 Ubb |
123 Ubt |
124 Ubq |
125 Ubp |
126 Ubh |
127 Ubs |
128 Ubo |
129 Ube |
130 Utn |
131 Utu |
132 Utb |
133 Utt |
134 Utq |
135 Utp |
136 Uth |
137 Uts |
138 Uto |
139 Ute |
140 Uqn |
141 Uqu |
142 Uqb |
143 Uqt |
144 Uqq |
145 Uqp |
146 Uqh |
147 Uqs |
148 Uqo |
149 Uqe |
150 Upn |
151 Upu |
152 Upb |
153 Upt |
All of these hypothetical undiscovered elements are named by the International Union of Pure and Applied Chemistry (IUPAC) systematic element name standard which creates a generic name for use until the element has been discovered, confirmed, and an official name approved.
As of April 2011[update], synthesis has been attempted for only unbibium, unbiquadium and unbihexium. (Z = 122, 124 and 126)
The positioning of the g-block in the table (to the left of the f-block, to the right, or in between) is speculative. The positions shown in the table above correspond to the assumption that the Madelung rule will continue to hold for higher atomic numbers; this assumption may or may not be true. At the currently-undiscovered element 120, the orbitals 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 5f, 6s, 6p, 6d, 7s and 7p, and 8s are assumed to be filled, with the remaining orbitals unfilled. For the superactinides, it is expected that the 5g shell will be filled first, then the 6f shell. However, after approximately element 120, the proximity of the electron shells makes placement in a simple table problematic.
J. Huheey: Anorganische Chemie, 2. Auflage, 1995