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OCR physical chemistry a-level - electron configuration??

I was doing a past paper and I did a question:
Write the electron configuration of -
chromium as the element:
chromium in the +3 oxidation state:
The answers are - as an element: 1s2 2s2 2p6 3s2 3p6 3d5 4s1
+3 oxidation state: 1s2 2s2 2p6 3s2 3p6 3d3

I don't understand why in the element one the 4s1 sub-shell is filled, and that too only one? not 4s2
and then in the other one why is there no 4s subshell filled?
Cam someone explain this please. Thanks
Reply 1
For the element it is better to have 3d5 4s1, becasue if it was 4s2 it would have a higher energy due to pairing up the 2 electrons in the 4s orbital, and also, the half filled d orbital is favourable. For the ion you have to move the 4s electron into the 3d, then take 3 away to get the 3+ charge, hence why it is 3d3. Hope this helps xx
Reply 2
Original post by buci98
For the element it is better to have 3d5 4s1, becasue if it was 4s2 it would have a higher energy due to pairing up the 2 electrons in the 4s orbital, and also, the half filled d orbital is favourable. For the ion you have to move the 4s electron into the 3d, then take 3 away to get the 3+ charge, hence why it is 3d3. Hope this helps xx


Thank you so much
So is this a general rule or something because I haven't come across this in revision so i'm not really sure how i'd know in the exam xx
Reply 3
The only ones with 4s1 are Cr and Cu, the rest have 4s2.
For the ions it is a general rule, easiest way of doing it is by looking at the group number and taking away the electrons for the oxidation state.
For example, Copper is in group 11, Cu(II) is 11-2 d9.
Reply 4
Original post by buci98
The only ones with 4s1 are Cr and Cu, the rest have 4s2.
For the ions it is a general rule, easiest way of doing it is by looking at the group number and taking away the electrons for the oxidation state.
For example, Copper is in group 11, Cu(II) is 11-2 d9.


Ohh! I understand now. Thank you so much for your help!! xx

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