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I am teaching my students how to write the electronic configuration of the elements of the periodic table.

Thus, following the rules on how to fill the orbitals, the electronic configuration of iron (for example) is 1s2 2s2 2p6 3s2 3p6 4s2 3d6 , thus, it is abbreviated form is [Ar] 4s2 3d6.

However, I has not been easy to find the explanation on why in any periodic table it is written as [Ar] 3d6 4s2 instead of [Ar] 4s2 3d6. Same case for many other elements.

Why is that?

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    $\begingroup$ I have seen it written both ways. However, the '3d' orbitals in this case are going to be lower in energy than the 4s and therefore it is written in a way that indicates increasing energy from left to right. $\endgroup$ – LordStryker Mar 19 '15 at 13:48
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I would recommend following the convention used by NIST which is to list the subshells in order of principle quantum number (n), and in order of angular momentum quantum number (l) for a given n.

See also ELECTRONIC STRUCTURE OF THE ELEMENTS

The question refers to "rules on how to fill the orbitals", but I don't think there are any rules, just memorization aids.

See The low-lying level structure of atomic lawrencium (Z = 103): energies and absorption rates for an sample of how difficult it is to calculate the ground state configuration of an atom. The only rule is Quantum Electrodynamics.

Also, the configurations are really just designations or leading configurations, with the actually ground state being a mix of configuration. See for example the above lawrencium reference or A critical compilation of energy levels and spectral lines of neutral boron, where it explained "Configuration and term labels have little physical meaning for highly mixed levels". Boron ground state is 95% $1s^22s^22p$ and 4% $1s^22p^3$. Lawrencium is calculated to be 86% $7s^27p$ and 6% $6d7s7p$

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  • $\begingroup$ I guess, from a teaching perspective, listing the sub-shells order of increasing energy. My goal in teaching general chemistry is to help students make connections between quantum theory, the periodic table, and chemical reactivity. I teach my students to write electron configurations using the periodic table as a guide. However, I do tell them about exceptions and why they happen, but I don't want them to be more concerned about them. They can opt to take physical chemistry if they want to delve deeper. $\endgroup$ – Tami M. Mar 19 '15 at 16:35
  • $\begingroup$ @TamiM. so would you have your students switch the order of 3d and 4s when you go from Fe to Fe+ since the 4s becomes higher energy, or do you always go by the neutral atom energy level order? $\endgroup$ – DavePhD Mar 19 '15 at 17:18
  • $\begingroup$ I would probably accept either way. Honestly, I would accept either valid way of writing electron configurations, even for neutral atoms. But, when I teach general chemistry, I do teach them to use the periodic table to write electron configurations. If I were teaching a more advanced course, I would go much more in depth about mixed configurations. As it is, I go into more detail about these things then many general chemistry lecturers (I can't help it, I am a physical chemist). As I said before, I just try to focus on connecting theory to laws and chemical reactivity. $\endgroup$ – Tami M. Mar 19 '15 at 18:53
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In some textbooks that I have read, I came across electron configurations that listed the 3d electrons before the 4s electrons. I believe it is to keep the electrons with the same principal quantum number (n) together. For example $[Ar] 3d^6 4s^2$ in full would be $1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2$. When I teach general chemistry, I opt to list the sub-shells in order of increasing energy instead, since it correlates with the structure of the periodic table.

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