1. The problem statement, all variables and given/known data Write down the electonic configuration for the O atom. Write down the quantum numbers [itex](n,l,m_l,m_s)[/itex] 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 [itex]1s^22s^22p^4[/itex]. 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 [itex]m_l[/itex] one, not the [itex]m_s[/itex] one). So... [itex](2,1,\pm 1 , \pm 1/2)[/itex] and [itex](2,1,0 , \pm 1/2)[/itex] are all possible for the 4 electrons. However if I apply an external magnetic field, I can discard [itex]m_l=1[/itex], right?