Molecular Orbital Theory

Now, before we can talk about molecular orbital theory, let's have a quick recap on electron orbital diagrams. So we're gonna say we call that electrons are distributed one S 22, S, 22 P and so on. Within within orbital's using what we call the off about principles, remember that electron orbital themselves show electrons as residing within atomic orbital's. Now, with this whole thing of electron orbital diagrams and electrons, we're gonna say we have the poly exclusion principle and Hunt's role under the poly exclusion principle and orbital can hold a maximum of two electrons that have to have opposite spins. One points up in one points down, remember this affects our spin quantum number of um Sebas Hunt's rule says that same energy orbital's also known as degenerate orbital's our first half filled before being totally filled. Right? So we for pr but as we go up up up and then come back around down, down, down if necessary. So again, before we can learn about molecular orbital theory, let's just recap electron orbital diagrams

Provide the electron orbital diagram for nitrogen atom, nitrogen has an atomic number of seven. And since we're dealing with nitrogen atom we're dealing with it's neutral form which means it also has seven electrons. Following the off Bob principle, we fill up one s completely before moving on to to us, filling to us completely before moving on to two p. So we'd say one up one down. Following the Pauli exclusion principle, electrons in the same orbital can must have opposite spins. So that's two so far. One up one down. So so far we've drawn four electrons, we gotta get to seven. So we need three more Now. These next atomic orbital are all two p orbital's so they're all degenerate. They have the same energy. So following Hunt's rule, we have to have Phil So this would be electron five, six and seven. So this would represent the electron orbital diagram for the nitrogen atom.

Now when we talk about molecular orbital theory, we're talking about more than one atom, kind of combining their electrons together. So we're gonna stay here. When Adams pool their electron's the electron orbital diagrams are shown vertically? We're going to say, even though they're shown vertically, we're gonna use the same three principles to draw these vertical electron orbital diagrams. So here it says film the electron orbital diagrams for two oxygen atoms that are combining their electrons. First of all, oxygen has an atomic number of eight. Oxygen's electron configuration is one S two, two, S 2, two P 4. So we could think of this as being one oxygen atom here for this column and one oxygen atom here for this column. And all we're gonna do here is we're going to fill in their electron orbital diagrams vertically. So one s. 2 means that we have one up down, one up, 1 down to s to one up one down, one up one down and then two people orbital's are all same energies are degenerate. So we follow huns rule, we need to fill in four electrons. So up up up, come back around, down. Same here. Up, up up, come back around down. So this is an illustration of us prepping the pulling of electrons for the two oxygen atoms here. We're still seeing them as two separate electron orbital diagrams. We haven't pulled them together yet. That's what's going to happen here in this space. So click on to the next video. And let's see what happens when we start to pull together, electrons found in atomic orbital's Do they get a new description? What's going to happen?

from electron orbital diagrams we transition to molecular orbital diagrams. They show chemical bonding as the combining of valence electrons from atomic orbital's of elements into what we call molecular orbital's. Now, molecular orbital are just a set of orbital's created from the combining of electrons between two elements. Now, if we take a look at this example it says fill in the molecular orbital diagram for when two oxygen atoms combined their valence electrons. So if we take a look here, we saw that we had our vertical electron orbital diagrams which were our atomic orbital's. So this portion here and this portion here. Yeah, those electrons are pulled together into what we call our molecular orbital's here. Mhm. Now they follow the same three principles. They follow off about principle as we start filling lower energy orbital's first and moving up. We also follow huns rule where the german orbital's orbits with the same manager our first half build before being totally filled. We also follow the poly exclusion principle where two electrons in an orbital must have opposite spins. All right, so we're just gonna pull them together and to do that. We're going to follow these rules. So if we take a look here at these rules, it says if it is not given determine the number of valence electrons for both elements two. We're going to construct the molecular orbital diagram based on the location of the valence electrons, Period one elements would start out with one s. Period two elements would start out with two S and period three elements would start out with three s. Remember your period is the row in which the element is found on the periodic table and then finally we follow three principles and fill in the molecular orbital is based on increasing energy. So as we move on, the energy increases here. I've already given us the number of valence electrons for the oxygen atoms. So, remember we'd say that oxygen is one S 22, us 22 P four. But we're only looking at the valence electrons are only looking at these electrons here. It has six valence electrons because it's in group six a All right, so we're gonna start filling in. We go one up 1 down, one up 1 down. We've already filled in. Or we can because in total we have four atomic orbital electrons and we just filled those four orbital um four electron orbital electrons into these two molecular orbital's. Now next we're going to have four here and we're gonna have four here. For a total of eight electrons found within atomic orbital's. So now we're gonna do is fill them into these molecular orbital's. So up up following Hunt's rule down down. So we've used four electrons so far meaning we have four left. So up up. So that's six. So we have two more electrons up pop. So this would represent the molecular orbital diagram when we're talking about our two oxygen atoms. This is how they would fill in their valence electrons into these given molecular orbital's. So as you go further and further into molecular orbital theory, we'll see more and more of these molecular orbital is being used.

Now when combining the valence electrons between elements, there are two types of molecular orbital is involved. We have first are bonding molecular orbital, this is just regions where we have high electron density between elements that promotes bond formation. Now if we have bond formation then we have the opposite of that bond destruction or bond prevention. This has to do with our anti bonding molecular orbital here. It's designated with a star. We're going to say this is a region of low electron density. Also known as a note that prevents bond formation. What we need to realize here is that filled bonding molecular orbital are going to increase stability and filled anti bonding molecular orbital will decrease stability. So you have these two forces at work, One trying to form the bond, the other one's trying to prevent it. Now here we're going to see in terms of a molecular orbital diagram, we're gonna say electrons are de localized so they're spread out throughout the molecule and we're gonna stay here valence electrons from atomic orbital are combined into molecular orbital's. These are discussions that we talked about when first looking at molecular orbital theory. So if we're talking about hydrogen and helium, their valence electrons are found in one s orbital's. So here we would say we have our one s atomic orbital R one S atomic orbital. And here we're going to say, let's say we're looking at here um a helium. So healing would have yeah Electrons found in this atomic orbital and the other one. Those could be distributed into the molecular orbital's here, we have our what we call our bonding molecular orbital that's designated by the sigma sign or signal one. S. And remember if we see a star that means that we have an anti bonding molecular orbital. So this would be sigma star one S, which basically says we're dealing with an anti bonding molecular orbital. And remember we would distribute the electrons found within our atomic orbital's into these molecular orbital's so click under the next video and we'll see how exactly this would work in terms of hydrogen and helium.

in this example. We need to construct the molecular orbital diagram for the dye helium cat ion, which is H E two plus. All right, so the way we do this is we're going to determine the number of valence electrons for both elements here. We're dealing with two helium. Remember he liam's have atomic numbers of two. We also say that helium, they each have two valence electrons. So they meet two times four on two times two Which is four valence electrons. But then here we're going to say this plus one charge means we've lost an electron. So in total we have three valence electrons for the dichotomy in castile. What we do next is we construct the molecular orbital diagram based on the location of the valence electrons. If we're dealing with period one elements like we are here then we start out with one s if it's period two elements it's to us and if it's period three it's three us Step three, we follow the three principles sold off Bob principle, Pauli exclusion principle and hunts rule and fill in the molecular orbital from bottom to the top. If we come back up here to our image here, we're dealing with three valence electrons. So we'd actually erase one of these and we have three total valence electrons which would then distribute into these molecular orbital's. So we started by filling out the lowest energy one according to off ball principle. So go one up one down and then we still have one more electron left that we need to fill in and we go here in the anti bonding molecular orbital. So this is what we would say are bonding molecular orbital diagram would resemble it will look like this. Okay. Where we would fill in totally be bonding molecular orbital that is sigma one S. And have half filled my anti bonding molecular orbital, which is sigma star one S.