What is the Correct Electron Configuration for Fe(2+)?

[yecko]

Homework Statement

Electron configuration of Fe(2+)

Homework Equations

spdf configuration

The Attempt at a Solution

1s2 2s2 2p6 3s2 3p6 3d3 4s2

But from (https://www.thestudentroom.co.uk/showthread.php?t=1232420),
it says 1s2 2s2 2p6 3s2 3p6 3d6.
isn't electrons fill up 4s shell first, before filling up 3d shell, for more stable arrangement?
or have Cr structure of "4s1 3d5"?
thanks

 

[kuruman]

You may find this useful. As I understand it, the energy levels are not fixed and static like pigeonholes if you take electron-electron repulsion into account.
http://ericscerri.blogspot.com/2012/06/trouble-with-using-aufbau-to-find.html

 

[Ygggdrasil]

As a rule of thumb, when transition metals form ions, the 4s electrons are lost first: https://www.chemguide.co.uk/atoms/properties/3d4sproblem.html

I'm not exactly sure why this is, but perhaps someone with a better knowledge of quantum mechanics could provide an explanation.

 

[Borek]

This is a can of worms. My bet is that the exact configuration can depend on external factors - that is, whether electrons are technically on s or d may depend on the compound, its symmetry and field strength. I would go with Ygg's rule of thumb, in general ground configuration and - in many cases - exact configuration of ions is a purely academic problem.

 

[blue_leaf77]

But from (https://www.thestudentroom.co.uk/showthread.php?t=1232420),
it says 1s2 2s2 2p6 3s2 3p6 3d6.

If you have access to quantum chemistry package I would suggest that you try restricted Hartree-Fock calculation. Usually it predicts the correct configuration (i.e. it yields the lowest single configuration energy) as that used as general rule of thumb.

 

[HAYAO]

This is such a good question, and the answer is not very simple. I am thinking that this is somewhat technical enough to not be in a homework section.

Aufbau principle is not very reliable (Edit: they only work for very light atoms). Combination of electron-nuclear interaction and electron-electron interaction, sometimes cause reverse in the orbital energies.