Electronic configuration of Copper

[804572119]

Why is the electronic configuration of Copper [Ar]3d104s1 and not [Ar]3d94s2?

[Jamlah Dalie 1K]

There are some atoms that have exceptions when it comes to their electron configuration, in class we were responsible to know the exceptions for Copper and Chromium. Basically the reason why Copper's configuration is [Ar]3d104s1 rather than [Ar]3d94s2 is because this allows Copper to be more stable.

[Alex Nguyen 3I]

Just adding on, copper is more stable with a full 3d orbital than with an incomplete 3d orbital and a full 4s orbital. This is experimentally determined data, and it has something to do with the symmetry of the electrons. Same goes for chromium, it is more stable with a half full 3d orbital (i.e. one unpaired electron in every orbital) than with one less electron and a full 4s orbital.