I have a question that my teacher can't answer. :( I get that with the electronic configurations for atoms, an s shell fills before a d shell but also empties before a d shell. Okay. So the last couple of subshells for iron are 4s2 3d6; Fe2+ would be 4s0 3d6 and Fe3+ would be 4s0 3d5. Am I right so far? My question is -- what happens if you add one electron to Fe3+? Which subshell does it go in? Do you get 4s0 3d6 (which violates the "s shell first" rule), or 4s1 3d5 (which gives you two different subshell configurations for Fe2+, depending on what it started as)?