Why in electronic configuration 3D comes later

1.	Why in electronic configuration 3D comes later
Answer» cing a second electron into a 3d orbital produces more repulsion than if the next electron went into the 4s orbital. There is not a very BIG GAP between the energies of the 3d and 4s orbitals. The REDUCTION in repulsion more than compensates for the energy NEEDED to do this.

Discussion