order of electron filling/emptying shells?

[Mdude]

Can somebody please resolve the following (paradox) in my mind:

When filling subshells, 3d fills before 5s, 6s before 4f, and 3d before 4p,

HOWEVER

the configuration of an iron (III) ion is: [Ar] 3d^5.

It seems a paradox to me because if you would fill, say, 4s BEFORE 3d, wouldn't you empty 3d BEFORE 4s? I realize that there is added stability to having every d subshell half-filled, so does that mean this case is one of the exceptions?

An example: what would be the correct configuration of a Zr^+1 ion? [Kr]5s^2 or [Kr]5s^1 4p^1 ?

Thanks so much for your help!

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[cwfergus]

Can somebody please resolve the following (paradox) in my mind:

When filling subshells, 3d fills before 5s, 6s before 4f, and 3d before 4p,

HOWEVER

the configuration of an iron (III) ion is: [Ar] 3d^5.

It seems a paradox to me because if you would fill, say, 4s BEFORE 3d, wouldn't you empty 3d BEFORE 4s? I realize that there is added stability to having every d subshell half-filled, so does that mean this case is one of the exceptions?

An example: what would be the correct configuration of a Zr^+1 ion? [Kr]5s^2 or [Kr]5s^1 4p^1 ?

Thanks so much for your help!

So first, im havinng trouble trying to understand waht you're saying. Yes, Iron itself has a configuration of : 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6
Now considering it has a +3 charge, we must take away 3. You always take the one with the highest principal quantum number, so that takes care of the 4s^2 and 1 of the 3d^6

so that is correct.

And for your second one, Zr has 40 protons, with the +1, we must make 39 electrons, so use the rules and see waht it is.

[Kr] 5s^1 4d^2

Zr by itself would just be [Kr] 5s^2 4d^2
but you take away an electron from the highest, dont worry about angular quantum or magnetic, worry about principal with ions

 

[Quantum Chem]

Iron fills in that fashion to stabilize itself more. Each d orbital is capable of holding 10 electrons, so it has five subshells. By moving two of the electrons from the 4s shell to the 3d they are able to align in the same direction as the other 3d orbitals, ie all up or all down. This just happens to lower the energy due to the decrease in electron-electron repulsion in the 4s shell (1 up, 1 down).

This may not help you know when this rule applies, but this happens the most on metals that are able to put 5 electrons in that shell, ie the family that Iron is in.
May not be a great explanation, but I tried.