I'm struggling to understand why the element Nickel can be written like so: [Ar]3d84s2, but cannot be written as [Ar]3d10.

I know that the d-orbital can 'house' 10 electrons, and that according to the Aufbau principle, electrons must be 'housed' in the vacant orbital which has the lowest energy, so the first way I presented seems wrong to me.

I'm sure that there's some confusion on my end here, or rather, I'm missing something that might seem obvious, but I really don't get it.

  • $\begingroup$ I believe that the 4s orbital is of lower energy than the 3d orbital, yet where writing out the electron configuration, you put 4s after 3d. You can look at the periodic table as a guide for this, i.e. period 4 column 1 and 2 is 4s and period 4 column 3 to 12 is 3d. $\endgroup$
    – Liam
    Commented Mar 19, 2020 at 17:57
  • 1
    $\begingroup$ Nickel is an element that exists and it has its actual electron configuration. Aufbau, on the other hand, is just a rough guideline with plenty of exceptions. Elements do not care about Aufbau. At best, Aufbau is like parlay in the first Pirates of the Caribbean movie: a rule that is not such a great rule. $\endgroup$
    – Ed V
    Commented Mar 19, 2020 at 17:59

1 Answer 1


The available energy levels are arranged in the following order, going from the lowest level to the highest : $1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,$ etc. So the $4s$ level is filled before the $3d$.

  • 3
    $\begingroup$ Particularly once you throw in $d$ and $f$ electrons, the neat, simple ordering of energy levels is no longer applicable. A check of electron configurations across the transition elements shows that quite clearly. $\endgroup$
    – Jon Custer
    Commented Mar 19, 2020 at 18:12
  • $\begingroup$ Thank you! What's an easy way to keep in mind how the energy levels are arranged? $\endgroup$
    – uriyabsc
    Commented Mar 19, 2020 at 18:44

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