The reason is, and this is the crazy thing, it turns out that many of these atoms are going to prefer to blend their second shell orbitals together, and what that's going to do is it's going to make new hybrid orbitals. So, what that means is that instead of the 2s orbital being lower energy and the p orbitals being higher energy, what winds up happening is that the carbon says, "Hey, if I can just take these 4 different orbitals and combine them, kind of mash them up and make them all a little bit higher energy, but all equal, then I could wind up filling them with bonds. If I could fill them with bonds, then I could become more stable because now I could have all of my orbitals filled." So what winds up happening is, check this out: the 2s orbital goes a little bit higher, and because I'm blending the p orbitals with the s, the p orbitals dip down a little bit. So what winds up happening is that all the orbitals hybridize together and they all wind up being the same energy. They all turn into what we call degenerate orbitals. Degenerate just means that they all have the same energy. Once they're degenerate, what that means is that I'm going to follow Hund's rule. So instead of the 2s just having 2 electrons, now I'm going to follow Hund's rule, which is that one each orbital gets one electron. Does that make sense?
So now, here's the interesting part. Since we blended one of the s orbitals and since we had blended 3 of the p orbitals together and we made these 4 degenerate orbitals, these new orbitals are referred to as sp3. Sp3 because one of them is the s and 3 of them are p's. Does that make sense? Now the reason that I have a 2 in front is that because these are all in the second shell. So this would be 2s and 2p's that are blending together to make sp3. So you could either just call it sp3 or sometimes you'll see it called 2sp3. That just means that it's the orbitals from the second shell that are all coming together. All right. So hopefully, that makes sense so far. Now I'm going to go ahead and show you guys how to apply this and how to use this knowledge to figure out hybridization and molecular geometry of atoms. So let's go ahead and get started.