When a balloon is rubbed on a wool sweater, the balloon builds up a negative electrical charge. When a glass rod is rubbed on silk, the rod builds up a positive electrical charge. Explain why the charged balloon will be attracted to the charged glass rod.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.