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From what I have observed in the periodic table is that many transition elements do not follow the Aufbau principle. And it's not just the elements like $\ce{Cu}$ and $\ce{Cr}$ that differ from predicted configurations because half-filled and completely filled d-orbitals are more stable, their are many odd exceptions like Nb ($\mathrm{[Kr]}4d^4\,5s^1$), Pd ($\mathrm{[Kr]}4d^{10}\,5s^0$). What reasons lead to such configurations?

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It's basically because you can't just add up electronic energies using a shell-by-shell, electron-by-electron approach. As soon as you have two or more electrons in close quarters, they interact with each other in ways that cannot be described by simple electrostatic laws because of the quantum mechanical nature of the electrons. The comments (specifically Mithoron's) refer to other questions whose answers explain this much better than I can. Please look at those.

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This is the first answer I am answering after joining with this forum, I read all the answers in the above and I try to add few more details rather them.

The aufbau principle states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels

As the present you can understand there are much of deviations now, that's why after that

*Pauli's exclusion principle

*Hund's rule of maximum multiplicity

such principles were generated. But as you say there were such deviations in some element's electron configuration(eg; chromium, copper, niobium, molybdenum, ruthenium, rhodium, silver, platinum) in chemistry, they have made reasons for them.As the @Fawad said they are for the more stability. Most of the d orbitals shows these deviations because of

*s orbitals has more geometrical stability than d orbitals

*d orbitals has shared 5 orbitals, but s has only 1 orbital, so it has much numeric stability

They are in two energy levels but due to such reasons the d orbitals has exceed the energy than the 4s. So when we are understanding the periodic table we should give more attention for the stability also.

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