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Below is a copy of the method I am using to determine electron configurations.

Question:

Write the full and short electron configuration for tellurium.

My final answer was:

Full Configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p4

Short: $[\ce{Kr}]~\ce{5s^2 4d^10 5p^4}$

However, checking with google, it seems the answer is actually: $[\ce{Kr}]~\ce{4d^10 5s^2 5p^4}$

Is the silly "follow the yellow brick road" thing faulty? Or is it that order doesn't matter in this situation?

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    $\begingroup$ related, but not a duplicate chemistry.stackexchange.com/questions/7942/… $\endgroup$
    – user4076
    Jan 22, 2014 at 10:01
  • $\begingroup$ The order does not matter. Preferably, it should be arranged by the principal quantum number. $\endgroup$
    – Tan Yong Boon
    Feb 25, 2018 at 5:21

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I haven't done that in decades (or so), but I'd bet that you did right!

The result from google only lists the final outcome, not the order in which the orbitals are filled. Except for some examples $\ce{La}$, $\ce{Ac}$, etc., the Aufbau principle is reliable. So: Follow the yellow brick road :)

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    $\begingroup$ Thank you. I've found this image and it has cleared some misunderstandings that I originally had. I thought that all the level three orbitals (spd) had to be filled before a level four could be filled. However, with your explanation, I can see now why 4s must be filled before 3d can be filled. $\endgroup$
    – Dave Chen
    Jan 22, 2014 at 10:03
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The Aufbau principal is flawed because there are many instances in which the electrons within a given sublevel (say 5s) are not all at close enough energy for that sublevel to fill completely without SOME electrons being more stable at higher "average energy" sublevels first. So what ends up happening is that the 5s starts filling before the 4d if there are not enough electrons to put some in both, but if there are enough to put some in both, some go into the 4d before some go into the 5s.

Certain numbers in the d sublevels are more stable than others, particularly having one electron in each orbital (5 total).

In the case of Tellurium, the 5s sublevel may have a lower average energy than the 4d, but the 4d starts and finishes filling before the 5s as long as there are enough electrons to put some in both sublevels.

http://www.rsc.org/eic/2013/11/aufbau-electron-configuration

Current electron configuration tables show the 4d lower than the 5s when there are electrons in both sublevels, but when there is one electron in the 5s and none in the 4d the 4d may not be shown.

http://periodictable.com/Properties/A/ElectronConfigurationString.al.html

Overall, there are 2 issues. One is that 20 out of 70 natural elements don't follow the Aufbau pattern. Some deviate a lot.

The other is that the Aufbau pattern itself is wrong because xs sublevels should really be placed higher than (x-1)d sublevels, ex. 4d comes before 5s in terms of the order that they tend to fill when there are enough electrons to put some in both.

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