Drawing Molecular Orbitals

Here are the 7 rules you need to know about how to draw Molecular Orbitals. 

Concept: The 7 Rules of Drawing Molecular Orbitals

Video Transcript

Now that we know how to draw atomic orbital I want to take things one step further and show you guys how atomic orbitals can be turned into molecular orbitals. So, guys here are seven rules for drawing molecular orbitals, and just a heads up, you're not going to find these rules anywhere because I made most of them up, try to look at all the different types of molecular orbitals you might have to arrange and figuring out rules that would work for all of them. Now, some of them are going to be so straightforward in that you're not going to need to look all seven rules but I just put the seven rules there just in case you get a really complicated one, you don't know where to start, you could follow these rules and always know what to do. So, let's go ahead and start with rule number one, the simplest rule, which says that the number of total molecular orbital energy States should be equal to the total number of atomic orbitals. So, if you have three atomic orbitals you should have three molecular orbitals of different energy pretty straightforward.

Next, one orbital of your molecular orbitals should never change phases. So, what that means is that we're going to see that as these molecular orbitals increase in energy some of the orbitals will start to flip, but one orbital should always stay the same, so that could be any orbital you want but I prefer that to be the first one because it just makes an easy reference to always see, look at the first one say that one is not going to change phases as energy increases, the last orbital however must do the opposite, it must always be changing phases with each increasing energy level. So, you must always be flipping it back and forth, 4, the number of nodes in your molecular orbitals must always begin at 0. So, your first molecular orbital should have 0 nodes and then increase with, increase by one with each increasing energy level, so the more energy levels you have you would just increase the number of nodes by one each time until you get to the very last energy level state, 5, your nodes should be as symmetrical as possible, sometimes you're trying to like fit three nodes into a molecular orbital and you say why can't I just put the three nodes on this side and then 0 nodes on the other side, that's not the way it works, what we want to do is you want to space out your nodes as symmetrically as possible, when in doubt, sometimes when a molecular orbital gets complicated like for example if you're doing eight atomic orbitals and you're trying to turn it into molecular orbitals, you're going to get a lot of orbitals there and when in doubt you can actually draw a sine wave from a fake atom 0 to a fake atom n plus 1, I'll show you how to do this, and this helps you to balance out your nodes evenly because you're using a sine wave system to balance out your nodes but we'll do that, we'll probably do that for a more complicated example, 6, if a node passes through an orbital. So, let's say that a node of your sine wave passes directly through an orbital you must delete that orbital, okay? So, that orbital should not exist because by definition, if it's a node electrons cannot exist there. So, no electrons should be in that orbital, and then finally once you have everything drawn, fill the molecular orbitals according to the rules of electron configuration, which would be Aufbau principle you have to build up, Pauli exclusion you can only put two electrons in each orbital and hund's rule you have to fill, or equal energy orbitals one at a time symmetrically, cool? So, let's go ahead and try to do the next following example.

Example: MO of 1,3-butadiene


Problem: Propose reasonable molecular orbitals for the following conjugated atomic orbitals.