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I know that to aid the symmetry of singly occupied orbitals one electron jumps from 4s orbital to 3d orbital thus giving Chromium the configuration 3d5 4s1.

But my question is, when we're evaluating the quantum numbers of the last electron, shouldn't we definitively use the farthest electron from the nucleus? Irrespective of which electron is assorted last. Because it's not like a Chromium atom is created by sequentially inserting electrons one after the other voluntarily.

In a perfectly stable Chromium atom, which has the configuration Ar(18) 3d5 4s1, the last or ultimate electron in terms of energy level, orbital and radius, should be the 4s1 electron. Why is it that most references I found tell me to consider the 3d5 electron?

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3d is the highest energy, 0.04 Hartree higher than 4s according to NIST's Atomic Reference Data for Electronic Structure Calculations, Chromium

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  • $\begingroup$ Oh so we calculate the electron with the highest energy? I reckon then the 4s1 electron comes all the way after the five 3d electrons. Right? $\endgroup$ – Aniruddha GS May 20 '16 at 5:35
  • $\begingroup$ @AniruddhaGS I don't understand what you mean by "comes all the way after"? $\endgroup$ – DavePhD May 20 '16 at 10:02
  • $\begingroup$ I mean when we calculate quantum numbers for the last electrons. 4s1 doesn't even come into consideration until we've discussed all five 3d electrons. $\endgroup$ – Aniruddha GS May 20 '16 at 12:34
  • $\begingroup$ Also I should say that the reason why I'm asking this is that in a discussion we were asked to take into consideration the quantum numbers of the last electron of Chromium. I considered 4s1 as the last electron and so did a few others. But most considered 3d5 as the last electron. Nevertheless, which one is the correct consideration? $\endgroup$ – Aniruddha GS May 20 '16 at 12:37
  • $\begingroup$ @AniruddhaGS "last electron" doesn't mean anything to me. See this question about which electrons are written last in a configuration: chemistry.stackexchange.com/questions/27538/… $\endgroup$ – DavePhD May 20 '16 at 15:00

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