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VSEPR Model - Concept
M.Ed., Columbia Teachers College
Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.
The VSEPR model stands for Valence Shell Electron Pair Repulsion model. The VSEPR model is a model which predicts the geometrical shapes of molecules based on the repulsion between their lone pairs. Types of VSEPR structures include linear, trigonal planar and tetrahedral.
Alright so we're going to talk about VSEPR Model these are the shapes that covalent compounds take the form of when they come together. So we've drawn those dot diagrams, now we're going to take them and make them 3 dimensional models and actually, what they actually look like in space and we're going to call that VSEPR, Valence, Shell, Electron Pair Repulsion. Okay so one of the first shapes we're going to talk about is the linear shape, so I'm going to take this linear as carbon dioxide and carbonmoxide. So let's take a look at those and see those and see how they line up. So carbonmonoxie, sorry we're going to talk about carbon dioxide. It looks like this, and we want these guys, these oxygens to be as far apart as possible. We know these electrons are repulsed by each other, meaning that there're going to be as far apart each other as they can. So they're actually going to create a linear form, this angle is going to be 180 degrees as far apart from each other as it possibly can get a linear shape.
Another type of linear that you might see is carbon monoxide, which is 2 atoms. Anytime you see atoms no matter what they are, they're going to have a linear shape. So carbon monoxide is triple bonded and no matter what these are going to be straight across from each other an angle of 180. So these 2 types of molecules are linear molecules, they're straight across, something that's close to linear but not quite is a Bent molecule. Let's take water for example, water I'll just draw it like this, water on a Lewis Dot Structure looks like this right. We have lone pairs of electrons and 2 hydrogens bound to it. Well actually we want to have these lone pair of electrons, these un bonded pairs want to have as much room to play around as possible. So the bonded ones are a little bit more restricted so they're going to push these guys down so what it really looks like is this, you've probably seen this before. So these guys create, have as much room as they can to like move around and play and these guys are kind of pushed down. This is what we call a bent molecule.
The difference between linear and bent are these lone pairs notice the linear doesn't have the lone pairs in the central atom but the bent however does which pushes that down making it bent. The angle between this is 104.5 okay another type of bent molecule you'll see is something like ozone. Ozone is O3 it looks, it's a resonated model I'll just do one of them, it looks like that. Notice the difference between this guy is this has 2 lone pairs of electrons this guy only has one. It doesn't matter the electrons are still going to want as much room as possible making these guys push down making it look like this also a bent molecule. These lone pair of electron here and here the bent molecule different than the linear molecule, they want, they're actually going to make these guys closer to each other. Okay so that is the bent molecule.
The third type is the trigonal planar, these guys have a central atom and 3 electrons around them. The other one just had 2, this one has 3 making, hence the trigonal planar. So let's do the sulphur trioxide. We have sulphur and this also is resonated, I'll just draw one of the resonated structures it looks like this. Sorry I'll draw the lone pair of electrons around there also to make it better. And these guys are going to want to be, these are in one plane hence trigonal planar they're in one plane. They are as far apart from each other as possible, this is going to be 120 degree angle between the different oxygen. They're actually going to be equally spaced throughout, that's an example of trigonal planar. There's 3 atoms around the central atom hence the trigonal.
Something very similar to a trigonal planar is trigonal pyramidal. Trigonal pyramidal, ammonia is a good example of trigonal pyramidal. So we have our nitrogen, I'm just going to draw this regular know they're the same thing, between this one and this one the major difference is the lone pair of electrons. This one doesn't have them, this one does, the same exact idea is going to happen, these guys are going to go crazy and they want to have a lot of space. They're going to push those hydrogens down into a pyramid shape hence pyramidal. Okay it's going to kind of sit on it and this is, the bond that'll go between these hydrogens is 107.3 okay it's my favorite radio station actually. So this is the trigonal pyramidal notice the massive differences of lone pair of electrons, this one doesn't have them, this one does.
The last thing you're probably going to see in class is going to be tetrahedral. Tetra don't forget is a prefix for 4, so if the central atom was 4, 4 atoms around it. So we have carbon with 4 hydrogens around it. These guys are going to be equally spaced out the angle is going to be 109.5 and I actually have a 3D model show you how it looks like because you think they're not in the same plane like this picture describes, they actually look like this okay. So we have our carbon in the middle let's say and we have the hydrogen surrounding it in equally spaced area. Okay so no matter how I turn it the hydrogens are going to be, it's going to look like the identical shape, okay it's not actually on the same plane as it actually depicts here. So these are the 5 main shapes that you're going to see again they're called the VSPER Models Valence, Shell, Electron Pair Repulsion theory and this is what covalent compound looks like in 3D.
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