[mrlucky0]

In the periodic table, going from Vanadium to Chromium, an electron migrates from the 4s orbital to the 3d orbital so that all the 3d orbitals are half-filled:

[Ar]4s²3d³ --> [Ar]4s¹3d⁵

I understand this occurs because this configuration results in a favorable, lower energy level. Now, my question is: If we go down a period, why does this occur going from Zirconium to Niobium?

[Kr]5s²4d² --> [Kr]5¹4d⁴

Why would filling the the 4d orbital in this way be favorable (in contrast to the above)? The 4d orbital isn't half-filled now. It would make more sense that what occurs for V to Cr occurs going from Molybdenum to Technetium (that is, 1 element after), so that just like the 3d orbitals, the 4d orbitals are half-filled.

Furthermore, if we go down a period again and look at the "corresponding" elements (Ta to W to Re), this electron migration from the 6s orbital to fill the 5d orbital does not occur at all. The same, further down a period looking at Db, Sg, and Bh.

Why is this? Is there some specific reason why promoting an electron from the S orbital to fill the d orbital becomes less favorable?

I apologize if my question does not make sense, please ask me to clarify.

[renge ishyo]

While a precise answer may not be know what is known is that the energy levels in the valence level of an atom are not "static" things...removing or adding electrons to this outer level can cause the various surrounding orbitals (if they are close enough in energy to accomodate changes) to shift in energy afterwards to compensate.

Sometimes even just shining light on an atom can promote an electron from one orbital to another nearby one (as can occur with copper) and this can cause shifts in the valence level electrons along the lines of those that you described. All of this oribital "flexibility", which seems to increase as you move higher up in the periodic table, screws up our "filling" orbital rules. These rules don't hold as well as the energy levels of surrounding orbitals get closer to one another. Towards the bottom of the periodic table with the lanthanides for example almost all of the valences *across the row* stay the same even though extra electrons are still being added to the orbitals as you move across the row.