# Order of Principal Quantum Number N

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#1
Preparing for a-levels, I learnt that the orbitals available for the first 4 principal quantum numbers are: 1s,2s,2p,3s,3p,3d,4s,4p,4d,4f

Only recently did I learnt that the principal quantum numbers do not go in order, they go as: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,6s,4f (and so on)

Is there any explanation to why the numbers are not in order?
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8 months ago
#2
I believe it is something to do with their energy level and which one is occupied first because of that.

Pigster
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8 months ago
#3
(Original post by NimitMistry13)
Preparing for a-levels, I learnt that the orbitals available for the first 4 principal quantum numbers are: 1s,2s,2p,3s,3p,3d,4s,4p,4d,4f

Only recently did I learnt that the principal quantum numbers do not go in order, they go as: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,6s,4f (and so on)

Is there any explanation to why the numbers are not in order?
You are not being as precise with your language as you should be. I expect you are referring to the fill order not being the same as the shell order.

The 3d vs 4s fill order is not simple and most teachers and exam writers do not understand it, so why should a Y11?

Which spec will you be starting (if you know)? Some boards (sensibly) stop at Kr, so you won't even need to know about the f-subshell.
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#4
(Original post by Pigster)
You are not being as precise with your language as you should be. I expect you are referring to the fill order not being the same as the shell order.

The 3d vs 4s fill order is not simple and most teachers and exam writers do not understand it, so why should a Y11?

Which spec will you be starting (if you know)? Some boards (sensibly) stop at Kr, so you won't even need to know about the f-subshell.
Yuup, I was wondering why the fill order is not in numerical order (like the shells)
Even if teachers and examiners can't understand it, that's just more reason for me to work hard on it. And besides, I'd rather learn something than watch TV all day
Oh and i'll be doing OCR A-level Chemistry (H420)
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8 months ago
#5
(Original post by NimitMistry13)
Yuup, I was wondering why the fill order is not in numerical order (like the shells)
Even if teachers and examiners can't understand it, that's just more reason for me to work hard on it. And besides, I'd rather learn something than watch TV all day
Oh and i'll be doing OCR A-level Chemistry (H420)
Without naming names, one of the authors of one of the text books, who writes some of the papers, believes that the sloppy Aufbau is correct and has written information and questions about fill order that is factually incorrect.

Closer to the truth can be found here: http://ericscerri.blogspot.com/2012/...u-to-find.html

The thing about the sloppy Aufbau is that, up to Kr (which is all that OCR A goes to) is that it works*, i.e. it is able to predict the filled orbitals (although it gets the actual order they fill wrong) which is what will be examined (they just ask what the e- configuration of the atoms or ions, not how they got there). So why does it matter if the filling order is wrong? Ultimately, it doesn't. As in ultimately you'll get the mark if what you write down is correct.

I teach the right way and point out that the text book method also ends up with the same answer, but filling in a different order. The text book gives the impression that it is simple. The right way is not simple, but actually the rule isn't any more tricky that the sloppy version, if taught well.

*assuming you are happy with a fudge of an explanation about Cr and Cu. You'll probably be told that Cr (which an the face of it should be 3d4 4s2) can borrow a 4s e- and then have a half-filled 3d subshell which gives it some magical stability. But if there were some magical stability, why wouldn't V join that party (it is 3d3 4s2 and could be 3d5, if it could borrow)
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