Why do chlorine and nitrogen follow the Aufbau principle, but copper does not?

Looks like I have to clear up some misconceptions here...

• The Aufbau Principle merely states that the lowest energy orbital is occupied first. That is usually true. It is the snake-path diagram that people tout as truth, and it is not absolute...
• Atoms don't care about the Aufbau principle. Only humans do. Atoms aren't alive. They can't think. All atoms care about is that they can stay lazy and obtain the lowest energy state with minimal work, however it needs to act to get there.
• Therefore, the Aufbau principle has no bearing on what electron configurations atoms take on.

Next thing is...

• Chlorine "follows" the so-called Aufbau principle, and "so does" nitrogen, in the sense that they both have so-called normal electron configurations.
• Copper "does not", but for a completely unrelated reason.

Copper, with configuration

#[Ne]3d^10 4s^1#,

actually DOES "follow" the Aufbau principle (the snake-path diagram) as a result of its #3d# orbitals being so low in energy (by complete coincidence). It is in your best interest to read through this answer to see that.

Due to the #bb(3d)# orbitals' low energy (in complete ignorance of the Aufbau principle!), copper's resultant electron configuration fills its #3d# orbitals first, and then adds an electron into its #4s# orbital since its #3d# are now full.

They were low enough in energy that even though they rise in energy by gaining electrons, they don't rise above the #4s#.