now, when it comes to exceptions in ionization energy, one of them, we can say, is when in the same period or, oh, Group 68 elements have a lower ionization energy than Group 58 Element. The explanation is that piece of shell orbital's are most stable when half filled or totally filled with electrons. If we look here, we have nitrogen versus oxygen. Oxygen is more to the right of the periodic table, so you would assume it should have a higher ionization energy. But in fact it doesn't. The reason is here. Nitrogen is already in a stable state. It's P orbital's are half filled, right. We have three electrons here. When I remove one electron from nitrogen, it becomes helium to s to to p two, and now, as a result, my P orbital's are no longer half filled and therefore not as stable for oxygen. On the other hand, here it's almost half filled for the two p orbital's. But we have one too many electrons. Oxygen will be willing to give up its electron to become to us to to p three, and in that way becomes half filled for its P Orbital's and more stable. Since oxygen is freely willing to give up these, this electron less energy is required to remove it. That's why it's ionization. Energy would be lower than nitrogen. So remember P Orbital's are most stable when they're either half filled or totally filled.