G-block

The g-block is a hypothetical region in the periodic table of the elements. No elements in this region have yet been discovered in nature or synthesized artificially. The first element of the g-block would have atomic number 121, and 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 the island of stability.

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.