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 , and it is abbreviated form [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. Same case for many other elements.
Why is that?
The reasoning is to condense the electron configuration. Ar has the same 1s2 2s2 2p6 3s2 3p6 configuration as that snippet in Iron. It's purely to shorten out writing all the configuration. I helped out at a school and they were doing the same thing, so the teacher decided to have them write out both the long version (1s2 2s2 2p6 3s2 3p6 4s2 3d6) and then translate that to the short version (Ar 4s2 3d6).
Thanks for your quick response.
I totally understand the abreviated form of the configuration.
Actually my question was why in the periodic table [Ar] 3d6 4s2 appears instead of [Ar] 4s2 3d6.
There must be a reason for that.
It may just be a mistake in the print. Sometime people/ computers who transpose the information may put it in alpha-numeric order. It's not wrong, per say, but its not ideal.
You can look at the orbitals in two useful ways. The energy of the orbitals and the location of the orbitals. When someone writes [Ar] 4s2 3d6, they are writing in order of energy (orbitals fill in order of energy). When they write [Ar] 3d6 4s2, they are writing in order of location, that is, n=3 orbitals before n=4 orbitals. This is useful because when atoms LOSE electrons, they lose them according to their location. Electrons in the 4s orbital are lost before electrons in the 3d orbital. Also, the last orbitals listed are the valence orbitals.
Electrons are filled in the 4s before the 3d (energy), but they are lost from the 4s before the 3d (location). This is why Fe forms a 2+ ion and also a 3+ ion (when it loses one of its d-orbital electrons).