I have been pondering why nickel only exists as the Ni2+ ion in nature. I know that Ni+ and Ni3+ also exist, but unlike iron (Fe), Ni3+ is not nearly as common as Ni2+. So, I have investigated this and arrived at an explanation but haven't quite reached a conclusion yet.
This is how I understand it:
Nickel has an electron configuration of (Ar)4s2_3d8, and iron has (Ar)4s2_3d6.
It's easy to understand the ionization to Fe2+ and Ni2+ because the 4s2 electrons are farther from the nucleus and now easier to remove. It's also easy to comprehend that the ionization energy is higher for iron because the 3d shell in nickel has more electrons, thus shielding more against the nucleus's attractive force.
Ionizing to Fe3+ might be understandable as it might be relatively easy to remove one of iron's only paired electrons in 3d6, and I guess that 3d5 is stable because the shell is half-filled.
But why is it so challenging to ionize nickel's 3d8 to 3d7, 3d6, etc.? Is a shell with 8 electrons already stable? Or why is it so?
I hope to be able to understand this.
Alright, I found the answer. The key is just to calculate the effective nuclear charge (Slaters rule) of the electrons in the 3d shell in both Fe and Ni, giving us a value of 7.55 (28-(18+2.45)) for Ni and 6.25 (26-(18+1.75)) for Fe.
I think the point is that Fe has one d-electron in excess of the half-filled d-shell and easily gives it away in order to minimize the total orbital momentum of this shell in accordance with the well-known Hund’s rule.
Ni already has three d-electrons over half and their reduction to two becomes not as beneficial for reducing the total orbital momentum as in the case of Fe.
Having a valence of 2 is equally beneficial for both Fe and Ni, since it involves s-electrons with zero orbital momentum.
Therefore, the Fe3+ state is more common than Ni3+.
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