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So I was reviewing chemistry lesson and I am curious when I saw the electron configuration in the VIB group:

For $\ce{_24Cr}$ we write the electron configuration $\ce{_18[Ar] 4s^1 5d^5}$ instead of $\ce{_18[Ar] 4s^2 5d^4}$,

For $\ce{_42Mo}$ we write $\ce{_36[Kr] 5s^1 4d^5}$ instead of $\ce{_36[Kr] 5s^2 4d^4}$,

But why for Tungsten $\ce{_74W}$ we do write $\ce{_54[Xe] 6s^2 4f^14 5d^4}$ and not $\ce{_54[Xe] 6s^1 4f^14 5d^5}$?

Is it because after the $\ce{f}$ subshell the rule is not valid anymore?

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marked as duplicate by Mithoron, a-cyclohexane-molecule, A.K., Jon Custer, Todd Minehardt Jul 24 '18 at 23:43

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    $\begingroup$ Chemistry 'rules' are more often suggestions. The more electrons, the more interactions there are. Remember that the 'shells' we use are solutions to the single-electron Schrodinger equation, not the full 74-electron problem. Perhaps what should be surprising is that they work even half as well as they do. $\endgroup$ – Jon Custer Jul 24 '18 at 14:03
  • $\begingroup$ chemistry.stackexchange.com/questions/41304/… $\endgroup$ – Mithoron Jul 24 '18 at 18:03