Which elements in the 3d series have exceptions in their electronic configurations?

Step-by-step solution

In the case of Cr, which has 3d54s1 configuration instead of 3d4 4s2 ; the energy gap between the two sets (3d and 4s) of orbitals is small enough to prevent electron entering the 3d orbitals. Similarly in case of Cu, the configuration is 3d104s1 and not 3d94s2 Hence, these are the exceptions.