I wish to better understand the configuration situation for the outer most valence electrons for the d-block elements, Sc to Zn. I am having a problem understanding how to predict where the two outer most valence electrons must be located for some of the ten d-block elements? For some elements it seems to be clear, such as for the first three Se, Ti and V, where the outer most valence electrons are expected to be in the higher energy 4s sub shell, where they are paired with opposite spin [itex]\uparrow[/itex][itex]\downarrow[/itex] in 4s. But, where are the two outer most electrons predicted to be located for Cr and Cu, given that both have a 4s1 configuration with a single spin up [itex]\uparrow[/itex] electron ? Are both valence electrons in the lower energy 3d for these two elements, or does one electron come from 4s and one from 3d ? Also, what is the current status of Ni ? On Wiki a claim is made, with book reference, that Ni has a [Ar]3d9, 4s1 configuration for ground energy state, yet almost all chemistry texts use [Ar]3d8,4s2 for Ni. So, if the 4s1 is correct based on energy calculations, I have the same question as above, would the two valence electrons for Ni come from 3d, or one from 4s and one 3d ? And, if from 3d, are they the two spin up [itex]\uparrow[/itex] electrons, or one of the [itex]\uparrow[/itex][itex]\downarrow[/itex] pairs found in 3d ? Conversely, if 4s2 is the correct ground energy case for Ni, does that mean the two valence electrons come from 4s as [itex]\uparrow[/itex][itex]\downarrow[/itex], as is found in Sc and Ti and V ? Then, is it possible that for Ni the quantum situation is that BOTH [Ar]3d9,4s1 AND [Ar]3d8,4s2 are quantum electron configuration possibilities, and if yes, how would quantum theory explain this ? Which raises a final question, has the electron configuration been experimentally tested for each of the five stable Ni isotopes separately using ultra-pure samples of Ni-64, Ni-62, Ni-61, Ni-60, Ni-58, all of which are available commercially for sale ? I realize no difference is expected in electron configuration between the five stable isotopes of Ni, but, have the experiments been conducted on each isotope in ultra-pure state separately to rule out the possibility that some difference may be present? I ask because it is known that superconductivity effects can differ between isotopes of the same element, could such isotope effects be present in some of the d-block elements for electron configuration?...just asking. Thanks for any help with the confusing d-block situation for electron configuration.