MHT CET 2021 Chemistry Solved Paper

The 3d-orbitals of Cr and Cu have half-filled and completely filled electronic configuration respectively which are the most stable electronic configurations. Thus, these both elements do not follow Aufbau principle according to which 4s orbital should be filled first with 2 electrons as it have lower energy than 3d-orbital. So, their electronic configuration are
Cr(Z=24)=1s22s22p63s23p63d54s1
Cu(Z=29)=1s22s22p63s23p63d104s1