# This question regarding quantum number

1. Jan 16, 2016

### TT0

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?

2. Jan 16, 2016

### Fightfish

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?

3. Jan 16, 2016

### TT0

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

4. Jan 16, 2016

### Fightfish

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.

5. Jan 16, 2016

### Staff: 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.

6. Jan 16, 2016

### Staff: Mentor

Indeed, the question doesn't make any sense.

7. Jan 17, 2016

### Fightfish

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.

8. Jan 17, 2016

### Staff: Mentor

That's the Aufbau principle.

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.

Last edited: Jan 18, 2016
9. Jan 17, 2016

### Staff: 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

10. Jan 17, 2016

### TT0

So is A still the correct answer?

11. Jan 18, 2016

### Staff: Mentor

As far as I am concerned, there is no correct answer, but it can't be D.