 Hello, this is Professor Steven Nesheba, and I want to tell you a little bit about this molecule ethene. You can see it's carbon-carbon with a double bond, and the first thing I just want to show you is that it's configured this way when it has hydrogens on it, but when we add a couple of methyl groups, this carbon and this carbon sp3s, onto the ends, then it can be configured in this, what we call the trans configuration, and it can also be configured in this cis configuration. So we have trans and cis, which are just sort of the orientation of these extra groups off the carbons that are involved in the double bond. Second thing to mention to you is is the fact that it's a planar molecule, which you can see this way, and the reason is planar, is that the we have a pair of electrons here in this pi subsystem molecular orbital, and it's oriented in a way that it enforces that that planarity. And that's also true in the other these trans, as you can see here, it's a kind of a similar story, as with just the ethene, and it's the same story if we go to the cis molecule, which I'm just going to pull up like this. So in all three cases, it's the molecular orbital, the pi-cis subsystem molecular orbital, a pair of electrons in this orbital that enforce the planarity of the molecule, and it's really in the end it's that planarity that causes there to be a stiffness that causes there to be a in this case a cis, or a trans, or or in this case just the just the ethene itself.