The explanations around ionization energies are based on the 4s electrons having the higher energy, and so being removed first. We say that the first ionization energies do not change much across the transition series, because each additional 3d electron more or less screens the 4s electrons from the extra proton in the nucleus. When discussing ionization energies for these elements, you talk in terms of the 4s electrons as the outer electrons being shielded from the nucleus by the inner 3d levels. Those statements are directly opposed to each other and cannot both be right. So the 4s orbital must have a higher energy than the 3d orbitals. The electrons lost first will come from the highest energy level, furthest from the influence of the nucleus. We know that the 4s electrons are lost first during ionization.We say that the 4s orbitals have a lower energy than the 3d, and so the 4s orbitals are filled first.This last bit about the formation of the ions is clearly unsatisfactory. The 4s electrons are lost first followed by one of the 3d electrons. To write the electronic structure for Fe 3 +: KĬonsider the electronic structure of neutral iron and iron (III). But if you refer back to the energies of the orbitals, you will see that the next lowest energy orbital is the 4s - so that fills first. Similar confusion occurs at higher levels, with so much overlap between the energy levels that the 4f orbitals do not fill until after the 6s, for example.Įverything is straightforward up to this point, but the 3-level orbitals are not all full - the 3d levels have not been used yet. This means that the 4s orbital which will fill first, followed by all the 3d orbitals and then the 4p orbitals. The oddity is the position of the 3d orbitals, which are shown at a slightly higher level than the 4s. The diagram (not to scale) summarizes the energies of the orbitals up to the 4p level. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible (Hunds rules). The aufbau principle explains how electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones.
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