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The electron configuration for $\ce{Fe^2+}$ in my coursebook is given as: $$ 1\mathrm s^2\, 2\mathrm s^2\, 2\mathrm p^6\, 3\mathrm s^2\, 3\mathrm p^6\,3\mathrm d^6$$

But shouldn't the $4\mathrm s$ subshell take precedent over the $3\mathrm d$ subshell according to Aufbaus principle?Aufbaus principle

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  • $\begingroup$ For formatting, See here and here. For a more detailed MathJax guide, look here, minor other details $\endgroup$ Sep 12 '20 at 18:27
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    $\begingroup$ Try writing the configuration of $\ce{Fe}$ and then remove two electrons from the outermost shell (higher n-value is farther) $\endgroup$ Sep 12 '20 at 18:28
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    $\begingroup$ Remember releasing of electrons is not the reverse order of their filling, as orbital energies change. $\endgroup$
    – Poutnik
    Sep 12 '20 at 20:05
  • $\begingroup$ I would like to suggest this question as another dupe target, but if I vote to close it will be Golden-Mjölnir’d (dupehammered with a golden badge). $\endgroup$
    – Jan
    Sep 17 '20 at 7:52
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You might want to compare iron(II) not with neutral iron, but with neutral chromium.

Both iron(II) and neutral chromium have 24 electrons, but there the similarity ends:

$\ce{Fe^{2+}}: [\text{Ar}]3d^6$

$\ce{Cr^{0}}: [\text{Ar}]3d^54s^1$

Think of the Bohr model. According to this model the energy level of electrons depends only on the shell number $n$, so we would expect 24 electrons to follow the $[\text{Ar}]3d^6$ configuration. In real life that happens exactly only for single-electron atoms where there are only electron-nucleus interactions. When there are electron-electron interactions they could fill the shells not in order, like the chromium atom described above.

But in a multielectron atom, if you add more nuclear charge you make the electron-nucleus interaction stronger, and the Bohr-model configuration becomes more favorable. One might suppose that dozens of added protons might be needed to get chromium's $[\text{Ar}]3d^54s^1$ to the Bohr-predicted $[\text{Ar}]3d^6$. Instead, the energies of the two configurations are so closely spaced that two extra protons, converting neutral chromium to ferrous iron, is enough.

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    $\begingroup$ Could someone also mention a concrete experimental way of determining this configuration perhaps spectroscopically? Electron configurations look like fairy tales now. How come these single electron wave functions were applied on complex ions of Fe and Co? $\endgroup$
    – M. Farooq
    Sep 13 '20 at 4:53
  • $\begingroup$ Oscar, That is a very hand waving answer by that person. He just scattered the answer with fancy names of X-ray techniques. Electron configurations "existed" well before XPS and other plethora of X-ray techniques were discovered. $\endgroup$
    – M. Farooq
    Sep 13 '20 at 13:49

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