Complex sentence: According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially filled g-orbitals, but spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number.
Split into simpler sentences: According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially filled g-orbitals. Spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number.