Sigma and Pi Bonds: Hybridization Explained! INSTRUCTOR: Hey, guys. Had a little request for a discussion of sigma and pi bonds, almost certainly as they are with respect to carbon. So here's what I need you to know. Sigma bonds between a carbon and anything else is the first bond that carbon makes with any other atom. The first one is always sigma. Always. If the carbon makes multiple bonds with an atom, like a carbon carbon double bond, or a carbon carbon triple bond, or something, a carbon nitrogen triple-- I don't care what it is. Any second or third bond it makes with an atom, those are going to be pi bonds. Check this out. Here is a picture of a compound that I just invented. I [? even ?] know the name of it. But the deal is that every first bond is a sigma bond. So here's a bond between carbon and hydrogen. The first one is a sigma bond. This one is the first one. It's a sigma bond. That one there, the first one, is a sigma bond. That's a sigma bond. That's a sigma bond. That's the first one. That's the first one. That's the first one. One of these is a sigma bond because one of them is the first bond. And that's a sigma bond. Every single bond carbon makes with something else is going to be a sigma bond. But every second or third bond is a pi bond. See? There is a bond there. One of those bonds is a pi bond. The first one is sigma. The second one's pi. Here, there are two extra bonds. There's a second and third bond. One of them's a pi bond. And the other one's a pi bond. Every first one is sigma. And every second or third is pi. So it's easy for you to tell whether it's a sigma or pi bond. But what does that even mean is the real question. Well, here's the deal when it comes down to electron configurations. Sigma bonds are always made from hybridized orbitals, whereas the pi bonds are made from leftover p orbitals. And we decide what the hybridization of the carbon is based off of how many multiple bonds that it makes. Check it out. Let's do the three examples. The three examples we're going to do are a carbon that has only single bonds, a carbon that has a double bond, and a carbon that has a triple bond. We're going to do all three of those. Here are carbon atoms with only single bonds. So sigma sigma sigma sigma. These are all sigma bonds. They are made from hybridized orbitals. So when we hybridize the normal carbon atom, we need to have four openings for hybridized orbitals. When this carbon, a carbon that has only single bonds hybridizes, the 1s gets left alone. But the 2s and the p's have to combine. In fact, we need 1, 2, 3, 4 sigma bonds. 1, 2, 3, 4. That requires an s, the 2s, and three p's. So we end up with four sp 3 hybrid orbitals. We take our electrons, spread them out, and boy, isn't it just convenient that we now have room for 1, 2, 3, 4 bonds to be made. This is the hybridized electron configuration for a carbon that has only single bonds. 1, 2, 3, 4 hybridized orbitals, all sigma bonds. When a carbon makes a double bond, however, it makes 1, 2, 3 sigma bonds, and one pi bond. The pi bond is the second bond. This is just an example molecule. It could be any molecule that has a double bond in it, like the one I showed you before. 1, 2, 3 hybridized orbitals are needed. And one p orbital needs to be left over. Remember our rule? Pi bonds are left over p orbitals. So when this hybridizes, you end up with a 1s orbital, 1, 2, 3 sigma bonds, and to hybridize into three equivalent orbitals, we need 1s and 2p orbitals. And we need to leave one of the 2p orbitals alone. Spread out your electrons, 1, 2, 3. I'm not going to follow Aufbau Principle or Hund's Rule or whatever that is. But the point here is I could still make four bonds. 1, 2, 3 sigma, or single bonds, and one opening for a double bond to be made. Any carbon that has a single double bond has this electron configuration for the hybridized carbon. Finally, what happens when a carbon makes a triple bond? Or, really, if it makes two double bonds. These are the same case in that this carbon has a sigma bond there, and a sigma bond here. But it has two pi bonds because the second and the third both count as pi bonds. Here you have a sigma and a sigma. This second one counts as a pi, and this second one counts as a pi. So in both cases, you have two pi bonds being made. You should know by now, that means you have 2p orbitals left over. Your 1s never gets touched, just like before. And that means you have two single bonds being made. You need to hybridize the s with one of the p's. 1, 2, 3, 4 electrons. You can make two single bonds and two double bonds. Look, guys. We really just came up with the sigma pi orbital hybridization rule stuff to explain why carbon bonds the way that it does, why it has the angles that it does, and why, to be honest, why single bonds can rotate but double bonds can't. I haven't explained the geometry, but just trust me on that one. When a carbon has only single bonds, they're all sigmas. And you need four sp 3 hybrid orbitals to do it. When you have a double bond, you need three sp 2 hybrid orbitals for the three single bonds, and one p orbital left over for the double bond, or pi bond. Finally, for the triple bond, you need two single sp-- two single bonds-- sp hybrid orbitals for the sigma bonds. And you need two left over p orbitals for the two pi bonds. It's all here. Ain't no fictions. Best of luck.