Apologies for the long post this is going to be, but at the moment I am doing A-level chemistry and physics, and I am learning about electron orbitals and quantum physics at the same time, so I have reached a state of confusion. (At a higher level than that required for the A-level; I'm not having any difficulty accepting fact as fact, it's just that I want to try and get a more complete understanding.) I'm only 16, so please try and keep any explanations as simple as possible; certainly pre-university! (This description of events is going to be time shifted greatly, but I'm going to try and recount it in the most logical order I can.) So firstly about the 3d and 4s sub shells. My chemistry teacher has told me rather simply that the 4s sub shell has a lower energy level than the 3d sub shell, primarily, so that the 4s sub shell is filled before the 3d sub shell. However once the 4s sub shell is filled, the 4s and 3d sub shells 'swap' in terms of energy levels, so that the 4s sub shell has more energy than the 3d sub shell. So my first question is why exactly is this? I asked my physics teacher why this is, and his immediate answer was 'because their wave functions change' or something along those lines (I presume he was talking about the electrons in those sub shells, but I really have no idea). My chemistry teacher then described the example of the vanadium atom, where you have a full 4s sub shell, and 3 electrons in the the 3d sub shell. She told us that when you create a V+ ion, the electron is taken from the 4s sub shell because it is the sub shell with the highest energy. So I asked then about what happens when you add another electron to recreate an atom. Does it go back into the 4s shell because that's how it was before, or does it go into the 3d sub shell because then it will be in a lower energy state? She didn't know the answer, so that's my second question, along with why whatever happens actually happens. Also what happens to the 4s sub shell when it only has one electron in it? Does it drop back below the 3d sub shell, or stay where it is? Also what happens when you have a V2+ ion? Today, our physics teacher decided to make the lesson one big aside, in order to try and explain what we're learning about in chemistry at the moment. He explained the Pauli exclusion principle so that it made total sense, and it explained most of what I'd learnt in chemistry, but didn't quite explain the questions I had in chemistry. So anyway, I can't quite remember what I asked then, but I'm stuck with the memory of an explanation about how when an electron is added to a sub shell, that sub shell gains energy due to 'interaction energies' between the electrons in the shell, which made sense. However in particular I'm stuck with the idea that this interaction energy is responsible for the 4s sub shell rising above the 3d sub shell in terms of energy level when full. But this goes against firstly what my teacher said initially about changing wave functions, but also against my understanding that 3d sub shell remains below the 4s sub shell in terms of energy, even when the the 3d sub shell is filled. I mean surely (if the interaction energies thing is relevant) the energy of the 3d sub shell with 10 electrons will be more than the energy of the 4s shell with 2 electrons? So basically this is the same question as my first one, but also where do these 'interaction energies' come into things? My physics teacher then threw another spanner in the works (don't get me wrong, he's a brilliant teacher! :D ) by saying that electrons like to go around in pairs. Which seemed to go against everything I'd learnt about electrons; they're both negatively charged; they're both fermions, so they try to avoid being in the same states. Also in chemistry we'd been told that when filling out the diagrams ( http://www.google.co.uk/imgres?imgu...a=X&ei=nt1BUpOPMMXbtAamv4H4Ag&ved=0CEcQ9QEwCQ ) using arrows representing electron spin, that we should make sure all the boxes in one shell are filled with one arrow, before we start filling the boxes with the second arrows. I also read an article that said the electrons have lower energies when not in pairs due to electromagnetic repulsion, so prefer not to be in pairs. So what on earth was my teacher getting at when he said this??!!! The reason I've put this in the physics section rather than the chemistry section is that my physics teacher appeared to have more of an understanding about the changing energy levels, which is the main crux of my confusion. As you can clearly see all my knowledge of this stuff is incredibly disjointed, and I feel like my head will explode if I don't get it cleared up! I may up remembering/coming up with more questions, but I think that's all of them. Thank you so so much for reading all this, I will greatly appreciate if you answer even one of my questions :) Thank you so much for any answers in advance.