Electron Configuration Exceptions?

Electron configurations.....

There are indeed several elements whose electron configurations don't agree with what the Aufbau principle suggests. But a deeper issue is that the Aufbau principle itself is not all its cracked up to be. It is a model based on the (n+ℓ) rule, and it works everywhere, except where it doesn't, and that turns out to be for quite a few elements. There are many elements for which the electron configurations according to the Aufbau principle are just plain wrong.

........ [ I know this goes beyond your original question, but the "exceptions" don't mean much if what they are the exceptions of is wrong. ]

The beginning of the problems lie along the first-row transition metals where the convention is to write the 4s sublevel and then the 3d sublevel, when if fact, the 4s should be last in the sequence. The 4s sublevel is ALWAYS higher in energy than the 3d and should be written last in the electron configuration. Yet there are many out-of-date textbooks and many teachers' dusty old notes which give the 4s sublevel followed by the 3d. That is incorrect.

You may have heard some teachers stammering around saying that the 4s is lower in energy than the 3d, and then have to hand-wave some nonsense as to why the electrons are first lost from the 4s sublevel. The electrons are lost from the 4s because they are the highest in energy and the most easy to remove, AND the 4s electrons are written last in the electron configuration of the first row transition metals.

The problem lies with K and Ca and the fact that electrons are found in the 4s sublevel. That's because the 4s for Z=19 and Z=20 is lower than the unoccupied 3d orbitals. But at Z=21 (Sc) the energy of the 3d is less than the 4s and the electrons are found in the 3d before the 4s. (Yes, the atomic number is a factor in this discussion.)

K..... [Ar] 4s1

Ca... [Ar] 4s2

Sc .. [Ar] 3d1, 4s2

Ti ... [Ar] 3d2, 4s2

V .... [Ar] 3d3, 4s2

Cr.... [Ar] 3d5, 4s1

Mn.. [Ar] 3d5, 4s2

Fe... [Ar] 3d6, 4s2

Ni.... [Ar] 3d7, 4s2

Co... [Ar] 3d8, 4s2

Cu... [Ar] 3d10, 4s1

Zn....[Ar] 3d10, 4s2

When one of these transition metals ionizes and actually loses one or more electrons, they will come from the 4s sublevel first, followed by the 3d. The common ions of the first-row transition metals will never have any electrons in the 4s sublevel.

The intricacies of this are addressed in an article in the Journal of Chemical Education (April 2010) by W. H. Eugen Schwarz: The Full Story of the Electron Configurations of the Transition Elements.

Also check out this article: https://www.chemedx.org/article/clarifying-electro...

This diagram, showing the "collapse: of the d-orbitals comes from the article by Schwarz. It shows the energy of the 3d orbitals falling below the energy the 4s at Z-21.

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Yes