The (n–1) stands for the inner d orbitals which may have one to ten
electrons and the outermost ns orbital may have one or two electrons However, this generalisation has several exceptions because of very
little energy difference between (n-1)d and ns orbitals Furthermore,
half and completely filled sets of orbitals are relatively more stable A
consequence of this factor is reflected in the electronic configurations
of Cr and Cu in the 3d series