Hey guys, I'm trying to teach myself organic chemistry so this might be a really stupid question but I'm completely baffled by the molecule NH2. So far I've learned about concepts like the octet rule, hybridization and Lewis structure. But I can't figure out where the extra electron for NH2 comes from.
My understanding is that Nitrogen has 5 valence electrons. When bonding with two hydrogen molecules, it gains 2 which leaves 1 left to fill the second energy shell. My understanding is that, due to hunds rule, the px, py and pz orbitals will have one electron each before bonding. So let's say that two hydrogen molecules bond with px and py. That should leave pz unfilled with only one electron. But my book magically shows two and seems to think that labeling it a lone pair has solved the problem of where this extra electron has magically come from. It doesn't even try to explain it, it just moves on.
In researching this, I saw that apparently NH3 is a molecule that exists too. Now this actually makes sense since the 3rd hydrogen atom will fill in the pz orbital and now the octet rule is satisfied. But now I'm even more confused. Can nitrogen just magically choose when it feels like being filled by 2 or 3 hydrogen atoms? I guess I could probably figure out how this occurs if I could just figure out where the extra electron comes from for the pz oribital in NH2? For NH3, I know it comes from a hydrogen atom but for NH2, it seems to come from thin air. Why doesn’t Nitrogen just fill all 3 remaining electron locations with electrons from thin air, call them lone pairs and then act inert like neon so that it doesn't even react with hydrogen anymore. Especially since it can apparently pick and choose when it wants to do this with NH2 and NH3. I'm so confused.