hund's rule and electron configuration

[wantVCUdental]

Very confused with the exceptions to the rule.
in KBB, for Iron with 26e, the 3d is filled...
the answer is: 1s2, 2s2, 2p6, 3s2, 3p6, 3d6, 4s2
What i don't understand is why is it not 3d8? why is 4s involved?
thanks

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[slm266]

How does the question read exactly and which book/edition is it? Does it say anything about ground/excited state?

No clue otherwise.

 

[americanpierg]

Very confused with the exceptions to the rule.
in KBB, for Iron with 26e, the 3d is filled...
the answer is: 1s2, 2s2, 2p6, 3s2, 3p6, 3d6, 4s2
What i don't understand is why is it not 3d8? why is 4s involved?
thanks

This isn't an exception to Hund's rules. Exceptions to Hund's rules are outside the grasp of anyone not in graduate studies in chemistry or physics. This is a totally different topic. To determine which fills first, orbitals s=0 p=1 d=2 f=3, etc. Add up the number of the shell and the correct number corresponding to the orbital, the lower number will fill up first and if they add up to the same, the lower shell number will take priority. This is true most of the time UNLESS filling a near-complete d orbital first creates a more stable configuration, such as Cr famly and Cu family. Sacraficng one electron to make only a half filled S orbital while creating a stable 5 electron or 10 electron D orbital is more stable than a full S orbital with only 4 or 9 electrons in the D orbital. This also occurs in a similar fashion with f orbitals, but you won't need to know that for the DAT.

4s= 4
3d= 5

Cr= [Ar] 4s1 3d5
Cu= [Ar] 4s1 3d10

 

[wantVCUdental]

oops, just noticed that 4s is lower than 3d.
Thanks,
so i won't have to know any exception to the rules 1s, 2s, 2p, 3s, 3p, 4s, 3d... etc?

 

[hausee]

Just mem. the following pic and you are good until 7f! 0.0

 

[americanpierg]

oops, just noticed that 4s is lower than 3d.
Thanks,
so i won't have to know any exception to the rules 1s, 2s, 2p, 3s, 3p, 4s, 3d... etc?

Yes, the lower shell # plus the orbital # will take priority in filling first. You will have to know that the chromium and copper families will complete their D orbitals before their S orbitals. I believe this popped up in destroyer too.

 

[wantVCUdental]

Yes, the lower shell # plus the orbital # will take priority in filling first. You will have to know that the chromium and copper families will complete their D orbitals before their S orbitals. I believe this popped up in destroyer too.

This is why i ask whether i have to know the exception, because it popped up on destroyer 🙁. Me no like destroyer.