# Electrons and their quantum numbers for the oxygen atom

by fluidistic
Tags: atom, electrons, numbers, oxygen, quantum
 PF Patron P: 3,144 1. The problem statement, all variables and given/known data Write down the electonic configuration for the O atom. Write down the quantum numbers $(n,l,m_l,m_s)$ for each one of the electrons. 2. Relevant equations Pauli exclusion principle. Least energy principle for filling the shells. 3. The attempt at a solution $1s^22s^22p^4$. I have trouble for the electrons on the 2p shell. The quantum numbers of the electrons are: (1,0,0,1/2) (1,0,0,-1/2) (2,0,0,1/2) (2,0,0,-1/2) Here I am not sure: (2,1,0,1/2) (2,1,0,-1/2) And 2 more electrons but I really don't know which quantum numbers to choose. They all have the same energy (degenerated). An external magnetic field would solve some degeneracy (the $m_l$ one, not the $m_s$ one). So... $(2,1,\pm 1 , \pm 1/2)$ and $(2,1,0 , \pm 1/2)$ are all possible for the 4 electrons. However if I apply an external magnetic field, I can discard $m_l=1$, right?
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P: 9,262
The energy levels for a given l are filled with one electron on each m first. The oxygen atom is known to have two unpaired outer electrons with equal spins.

 What are the three rules for filling atomic orbitals by electrons? Answer: Aufbau principle- build from the bottom up, and fill lowest energy levels first Pauli exclusion principle- each orbital may have only two electrons, with opposite spins Hund's rule- each orbital of a level must each have one electron before any have two

ehild
PF Patron
P: 3,144
 Quote by ehild The energy levels for a given l are filled with one electron on each m first. The oxygen atom is known to have two unpaired outer electrons with equal spins. http://wiki.answers.com/Q/What_are_t...#ixzz1f4c6ZDhx ehild
Thank you ehild.
So it means that I have the choice? For example here are my arbitrary choice for the quantum numbers for the 4 electrons on the 2p shell:
1----(2,1,0,-1/2)
2----(2,1,-1,-/1/2)
3----(2,1,1,-1/2)
4----(2,1,0,1/2).
The 2 unpaired electrons are 2 and 3.
However if I apply an external magnetic field the answer would change since I have to apply Aufbau principle, i.e. "fill lowest energy levels first". And in reality I think there's always an external magnetic field (as tiny as it may be) and so it seems that the paired 2p electrons must have their $m_l$ quantum number equal to -1.
So I'm tempted to choose:
1----(2,1,0,-1/2)
2----(2,1,-1,-/1/2)
3----(2,1,-1,1/2)
4----(2,1,0,1/2).
But I have a problem here. Aubfau principle seems to be in contradiction with Hund's rule. One electron should have the $m_l=1$ quantum number according to Hund's rule, while if I do this, I'd give it more energy than if it had $m_l=0$, so I go in counter of Aubfau principle.
So I'm unsure how to answer the problem.

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P: 9,262

## Electrons and their quantum numbers for the oxygen atom

The second configuration is not correct for the free oxygen atom. Do not forget that there is magnetic momentum assigned to spin, too. Two electrons on the same level (with the same m) and only the spins opposite repel each other more than attract because of their opposite spins, and a weak magnetic field does not change it. The effect of the magnetic field appears in the spins of the electrons occupying the m=1 and m=-1 levels.

I know the electron configuration of oxygen from Chemistry. It has two
electrons with paired spins - with m=0, and two unpaired electrons
which take part in covalent bonds -paired with a lone electron of an other atom.
See this: http://chemwiki.ucdavis.edu/Inorgani...#Hund%27s_Rule

ehild
PF Patron
P: 3,144
 Quote by ehild The second configuration is not correct for the free oxygen atom. Do not forget that there is magnetic momentum assigned to spin, too. Two electrons on the same level (with the same m) and only the spins opposite repel each other more than attract because of their opposite spins, and a weak magnetic field does not change it. The effect of the magnetic field appears in the spins of the electrons occupying the m=1 and m=-1 levels. I know the electron configuration of oxygen from Chemistry. It has two electrons with paired spins - with m=0, and two unpaired electrons which take part in covalent bonds -paired with a lone electron of an other atom. See this: http://chemwiki.ucdavis.edu/Inorgani...#Hund%27s_Rule ehild
Ok thank you.
I've read the link you provided, but I don't know where it's written that the 2 paired electrons of the 2p shell have their quantum numbers $m_l=0$.
Also in the link we can see that all lone electrons have their spin always up. Is it a convention or it must be this way, always? (didn't find the part that explain this). If it must always be like this, this would also invalidate my first choice in my last post, namely:
1----(2,1,0,-1/2)
2----(2,1,-1,-/1/2)
3----(2,1,1,-1/2)
4----(2,1,0,1/2).
Because the unpaired electrons 2 and 3 have their spin down.
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P: 9,262
 Quote by fluidistic Ok thank you. I've read the link you provided, but I don't know where it's written that the 2 paired electrons of the 2p shell have their quantum numbers $m_l=0$.
I do not know. To find the energy belonging to a configuration is not that simple. In weak magnetic fields the spin is coupled to the the orbital momentum, and their resultant determines the magnetic potential energy. I do not know enough about this field.

ehild
 PF Patron P: 3,144 Ok thank you for all the information.

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