It doesn't. The (n−1)d orbitals, PARTICULARLY in the last-column transition metals, are consistently lower in energy than the ns orbitals... and as such, we should be writing 3d104s2, 4d105s2, and 5d106s2.
See how the 3d orbitals are lower in energy than the 4s for the first-row transition metals here:
![Data from Appendix B.9 of Inorganic Chemistry by Miessler, Fischer, and Tarr]()
Data from Appendix B.9 of Inorganic Chemistry by Miessler, Fischer, and Tarr
And you can further see how the Aufbau principle fails for the heavier transition metals, in that the (n−1)d and ns orbital potential energies criss-cross rather unpredictably:
![Data from Appendix B.9 of Inorganic Chemistry by Miessler, Fischer, and Tarr]()
Data from Appendix B.9 of Inorganic Chemistry by Miessler, Fischer, and Tarr
![Data from Appendix B.9 of Inorganic Chemistry by Miessler, Fischer, and Tarr]()
Data from Appendix B.9 of Inorganic Chemistry by Miessler, Fischer, and Tarr
