Hi. I have read some QM and am trying to use it to understand why the noble gas configuration is the most desirable for an atom. It is my understanding that an anti-symmetric spacial wavefunction has a lower energy, since the electrons tend to be further apart. This means that the atom will have lower energy when the spins are parallel, because you have a symmetric spin state and hence an anti-symmetric spacial wavefunction. I think this is the basis for Hund's first rule? So say we are on the first row of the periodic table and we gradually fill the 2p subshell. With nitrogen we have 3 electrons in the 2p subshell, so it's possible for them to all have parallel spins (so long as they are in different orbitals). By my reasoning above (or equivalently (I think) Hund's first rule), this will be the arrangement that happens in nature since it will help minimize the energy of the atom. If we now go to oxygen with 4 electrons in the 2p subshell, two of the electrons are forced to be in the same orbital (symmetric spacial wavefunction), so they will have to be in the anti-symmetric spin configuration. The electrons will be closer together on average, so will have more energy. Does this mean that an oxygen atom has more energy than nitrogen atom? Similarly, fluorine has more energy than oxygen, and neon has more energy than fluorine? I was hoping I would get the exact opposite of this! Is this analysis correct? If so, why (if not to minimize energy) is a full outer shell more desirable for an atom? If the above is incorrect, where am I going wrong? Thanks for any help!