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 For example, consider the case of Cr,
which has 3d
5 4s
1 configuration instead of 3d
44s
2; the energy gap
between the two sets (3d and 4s) of orbitals is small enough to prevent
electron entering the 3d orbitals