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Chromium ($\mathrm{3d^5\,4s^1}$) and copper ($\mathrm{3d^{10}\,4s^1}$) are fairly well-known exceptions to the aufbau principle, often attributed to the stability of the half- or fully-filled 3d subshell (as discussed in this prior question).

However, what experimental evidence is there for this? Is there a spectroscopic method (for example) which can be used to probe electronic configurations?

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  • $\begingroup$ @ToddMinehardt, There is an article in the Journal of Chemical Education, The Full Story of the Electron Configurations of the Transition Elements, by W. H. Eugen Schwarz. He has theoretical explanations but no experimental discussion. I don't fully understand article either. It is based on "d-orbital" collapse! $\endgroup$
    – AChem
    Nov 26, 2021 at 1:38
  • $\begingroup$ Atomic spectroscopy (both emission and absorption) would give you this answer. Any method that involves spin-orbit coupling as well. I didn't pot this as a full answer because I am not sure $\endgroup$
    – Elie H
    Dec 3, 2021 at 0:12

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X-ray Photoelectron Spectroscopy (XPS) can be used to analyze 3d and 4s orbital energies of these elements. This unique behavior comes from the Aufbau principle, which explains why one electron moves to the d-orbital to fill half of the shell or total of the shell.

I used XPS for my previous research with elemental analysis (i.e. ratio of elements and which oxidation state they are in). I am not an expert in this device but comparing metallic copper's 3d shell binding energy with a different element (let's say Zn) would show that their B.E.'s are pretty close. To the best of my knowledge, lower atomic numbered elements in the 3$^{rd}$ row would not show any 3d peak due to very low B.E. (this is the outermost shell).

I don't know if that is the answer but XPS is a strong candidate for demonstrating this phenomena experimentally.

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    $\begingroup$ Thanks, I have also used XPS but I am not an expert. However, I do not think that XPS can be used for determine electron configurations. $\endgroup$
    – AChem
    Dec 4, 2021 at 21:41
  • $\begingroup$ I believe this is the correct answer, I will try and get an reference/proof of this soon. We have discussed this question in my lab lol. $\endgroup$
    – Elie H
    Dec 5, 2021 at 15:21

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