# This question regarding quantum number

What is the correct set of quantum numbers for the eighth electron that fills the orbitals in an atom of oxygen?

A. n = 2, l = 1, ml = –1, ms = –1/2
B. n = 2, l = 1, ml = +1, ms = –1/2
C. n = 2, l = 1, ml = +1, ms = +1/2
D. n = 2, l = 0, ml = –1, ms = +1/2
E. n = 1, l = 1, ml = +1, ms = –1/2

I chose B, because it is in the 2nd shell, p subshell, on the right orbital and has a negative spin. But they said the answer is A. This website sometimes has errors so could someone tell me if I am right?

Personally, I don't quite like the way the question is phrased - how does one define "the eighth electron in an atom of oxygen"? It only makes sense if you make reference to the ground state of the +1 oxygen ion.

The answer by the site seems correct to me. Have you heard of Hund's rules?

• TT0
I see, I realised I was thinking about the eighth electron on the 2nd shell and not overall, thanks!

I see, I realised I was thinking about the eighth electron on the 2nd shell and not overall, thanks!
My bad - I was actually the one thinking about the eighth electron on the second shell!
So in that case, your answer is correct, and the site is wrong, because Hund's second rule is to maximise L, and so the 4th electron in the 2p shell should be in the ml = +1 orbital.

• TT0
Borek
Mentor
Does ml value really matter? I can be wrong, but these are degenerate orbitals (with the same energy), so the electron doesn't care what is the ml value. When there is an external magnetic field, it can make things different, but in an isolated atom question doesn't make much sense to me.

• TT0
DrClaude
Mentor
Does ml value really matter? I can be wrong, but these are degenerate orbitals (with the same energy), so the electron doesn't care what is the ml value. When there is an external magnetic field, it can make things different, but in an isolated atom question doesn't make much sense to me.
Indeed, the question doesn't make any sense.

• TT0 and Borek
Yes, taken on its own, the question doesn't make much sense. I strongly suspect the question is trying (in some way) to specifically target Hund's rules. Using Hund's rules, one would systematically "place" the electrons one-by-one to determine the J and S values for the atom. So, the "eighth electron" technically refers to the "last" electron to be "placed" in this scheme.

• TT0
DrClaude
Mentor
Using Hund's rules, one would systematically "place" the electrons one-by-one
That's the Aufbau principle.

So, the "eighth electron" technically refers to the "last" electron to be "placed" in this scheme.
Yes, but as Borek said, the ##m_l## and ##m_s## states being of equal energy, there is no "order" in which to put in the electron.

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Borek
Mentor
Somehow I suspect someone was confusing a convention (when drawing orbitals we always fill squares from the left) with the "real" thing, assumed this order matters and extended the idea to ml values. Pure speculation on my side, but I have seen similar thinking on several occasions So is A still the correct answer?

DrClaude
Mentor
So is A still the correct answer?
As far as I am concerned, there is no correct answer, but it can't be D.