I have always understood electron configurations to reflect order of fill, that is, the 4s electrons are represented prior to the 3d electrons. Scandium's electron configuration should be [Ar]4s2 3d1. However, numerous web pages, supported by major universitites, indicate that is not a hard-and-fast rule, that electron configurations can be written in shell order, so scandium can just as properly be shown as [Ar]3d1 4s2. An example: http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Electron-Configurations-563.html Has the convention changed?
I know the Pauli's exclusion principle, but I do not understand why.
I was taught about Hund's rule , but I do not know why it holds still now.
Methinks that the core problem lies in the mathematical content of the atomic orbital wave functions if they are valid assumption. I think that the 4S atomic orbital wave function for N protons nucleus must differ from the same 4S atomic orbital wave function for N+1 protons nucleus, and in the same way the energy levels that correspond to the p orbitals for N and N+1 protons nuclei are different.
First of all, I am suspicious about the current theory of atomic orbital.
I am waiting for someone to deliver the numerical or analytical solution of ab initio calculaiton for many electron atom.