Posted: May 26, 2016 3:25 pm
zoon wrote:As a complete non-expert, after googling Wikipedia here (redirected from a PhysicsStackExchange question and answer), it looks to me as though the best available rule for predicting which orbitals electrons occupy in the atoms of different elements (the Madelung energy ordering rule) has a number of experimental exceptions, such as copper, chromium and palladium, and nobody yet knows why. The rule seems to have been an experimental rule of thumb in the first place, rather than being derived from theory.
I think spin-matching may be sufficient to explain most of the exceptional cases. Electrons have two spin states: Spin up and spin down. It is most energetically favorable to have all electrons of both spins in a sub-shell. But the next most energetically favorable state is to have all electrons of one spin state in a sub-shell. The Madelung rule for Palladium is weird. There is no way a 10-member sub-shell would stop at just eight electrons. It is far, far more favorable, energetically, to finish filling the d sub-shell and leave the s sub-shell, which is pretty close energetically, empty.