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?