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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$\begingroup$ More stability. $\endgroup$– FawadDec 2, 2017 at 10:08
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$\begingroup$ Don't answers to respective question explain it? chemistry.stackexchange.com/questions/2469/… chemistry.stackexchange.com/questions/151/… $\endgroup$– MithoronDec 2, 2017 at 13:21
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$\begingroup$ physics.stackexchange.com/questions/103718/… $\endgroup$– GertDec 2, 2017 at 14:58
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5$\begingroup$ The main conceptual problem is that you're assuming that simple principles lead to simple systems. They do not. The atom is a complex system based on very simple quantum mechanical and physical rules. In other words, the Aufbau principle is way too simple to capture the complexities of what you're trying to describe. $\endgroup$– ZheDec 3, 2017 at 18:14
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2 Answers
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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.
answered Dec 3, 2017 at 17:18
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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.
answered Dec 19, 2017 at 9:14