Why do real gases not behave exactly like ideal gases?

The molecules have (a) volume and (b) attractive forces  

Step-by-step explanation:

At ordinary conditions, the molecules are so far apart that the gases behave almost ideally.

However, if you use high pressure and/or low temperatures, you force the molecules to be close together.

There are two competing effects:

The attractive forces become much stronger at close distances, so the volume is less than that predicted by the Ideal Gas Law. The  volume of the molecules becomes a significant portion of the volume of the container. The molecules have less volume in which to can move around, so the pressure is higher than that predicted by the Ideal Gas Law.                                                                                                                                                                                                                                                                                                                                                                              

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.