Electron Configurations that dont follow the pattern

[Hana Sigsbee 3B]

when predicting an electron configuration, how would we know if a certain element will follow the pattern or not?

[Izamary Marquez 2H]

Can you explain what you mean by "the pattern"?

[Hannah Alltucker 3L]

I believe the only two elements we really need to know that don't follow the regular pattern are Copper and Chromium. This has something to do with the sD sub-level being most stable at half full or completely full, so it simply grabs an extra electron from the previous sub-level. I know there are more, but these were the only ones I've ever actually used in my previous chem classes as we don't tend to use super obscure elements very often. However, if it is kind of obscure and has a large orbital, it might be worth asking about if we ever do run into one.

[Samudrala_Vaishnavi 3A]

So the elements that don't follow up the "building up principle" rules are: Mo, Cu, Cr, Ag, and Au. This is due to the fact that an electron from one of the orbitals moves to the other orbital to pair with its unpaired electron. If you were to do the electron configuration for Cu for instance it would be [Ar]4s^2 3d^9 or increasing order [Ar] 3d^19 4s^2 using the normal method. But this is the WRONG configuration. This is because one of the electrons from s moves to d to complete d's unpaired electron. SO the CORRECT electron configuration is [Ar] 4s^1 3d^10 or increasing order [Ar] 3d^10 4s^1. This occurs the same with the other exceptions so just find the normal electron configuration and move around the electron count like what I did for Cu.