VSEPR Theory - Basic Introduction

In this video we are going to talk about the vespa

theory

and what it has to do with molecular geometry. Maybe you have seen this word in your textbook, it means repulsion of electron pairs in the valence shell and the

basic

idea is that we can predict the shape of the molecule based on the fact that the electrons repel each other, so the electrons want to be as far apart as possible, so with that idea in mind we can predict the shapes of certain molecules. Now it's the first type of geometry you should be familiar with. con is the linear molecular geometry and the molecules that have this type of shape for example are becl2 which looks like this this is beryllium chloride as you can see it looks like a straight line and the angle of a straight line is 180 degrees, so that's the link. angle of a molecule with a linear geometry now there are some other examples for example carbon dioxide is another molecule with a linear geometry it looks like a straight line so any molecule where you have one atom in the center and two other atoms on the sides are a linear molecule, this is the generic structure of a linear molecule.

Now there is another example of a linear molecule and it is quite different from the first two. This is the triiodide ion and this Lewis structure looks like this. You have three iodide ions, I mean. atoms everything is an ion now the middle one has three lone pairs and the other two also have three with the geometry is straight it is a linear molecular geometry now the next type of molecular geometry you need to know is the trigonal planar structure when you hear the word test , what do you think tri represents current and planar means flat like a paper?

A good example of the structure is bh3, so you are born in the center and the three hydrogen atoms will be spaced as far apart as possible. possible, then it looks like this, now a full circle represents a 360 degree angle and if you divide it by three you can get the bond angle between the hydrogen atoms, so the bond angle for a trigonal planar structure is 120, which is 360 divided by 3. Some other examples of a trigonal planar structure are uh cocl2, so in this structure the carbon has a double bond with an oxygen and is bonded to two chlorine atoms, so every time that you have an atom in the center surrounded by three things and if the central atom doesn't.

If you don't have lone pairs, then what you have is a trigonal planar structure with a bond angle of about 120. Now, the next structure we need to talk about is the tetrahedral molecular structure, so when you hear the prefix tetra, what are you thinking about? tetra is equivalent to four, so in this structure we are going to have one atom surrounded by four other atoms and that is the tetrahedral structure with a bond angle of approximately 109.5, so methane fits this example, so in methane we have a carbon in the middle. surrounded by four hydrogen atoms, this is not a two-dimensional structure, it is a three-dimensional structure because if you take three sixties divided by four you get ninety and it's not 90, it's actually 109.5 based on the way these atoms are arranged in the three-dimensional space.

An example of a tetrahedral structure is silicon tetrafluoride, like carbon. Silicon is surrounded by four atoms, in this case four fluorine atoms instead of four hydrogen atoms, but the structure, the geometry is very similar, it just has different atoms, the bond angle is still approximately 109.5. Now the next structure you need to know if you have a test coming up is the trigone pyramidal structure. Once again we hear the prefix tri, so there has to be three of something. Now this is different from the trigonal planar structure. The trigonal pyramidal structure has an atom in the center with a lone pair now that atom is still surrounded by three other atoms so it looks like this that's the trigonal pyramidal structure now just to compare it to the trigonal planar structure I'm going to draw this right next to it. side so you can see the difference the trigonal planar structure has no lone pairs on the central atom it is simply surrounded by three other atoms the trigonal pyramidal structure has three atoms attached to the central atom plus one lone pair so it has four things a good example of the trigonal pyramidal structure is ammonia NH3 in this structure the nitrogen has a lone pair and is bonded to three hydrogen atoms.

Another example is a PhD so if you notice anything I just want to point something out that the elements that have the trigonal planar structure tend to be in group five like nh3 ph3 and ash3 when hydrogen is like the only other atom attached to it and the that have a trigonal planar structure tend to be in group three, but not always, bh3 is an example um another example includes uh alcl3 so these elements are in group 3a and these elements tend to be in group 5a, so which is another quick way to identify which will be trigonal planar which will be trigonal pyramidal if there are no double bonds involved because we had the example where it was like cocl2 but the carbon bonded to an oxygen had a double bond if there are no double bonds for the most part these elements will be in group 3a and these will be in group 5a at least that's just one pattern I've seen now Going back to the trigonal pyramidal structure, there's another thing I should mention and that is the bond angle.

I'm going to use ammonia as an example, so make sure you know this because it's a common test question that this is about. 107 degrees and that's it for the trigonal pyramidal structure. I just want to mention that before I forgot, now the next geometry that you should be familiar with is a folded molecular geometry and a good example for this is water. Water has a bent shape. Oxygen. It contains two lone pairs and those two lone pairs make the hydrogens in a bench structure. You may have seen water drawn this way and this is a common mistake so you don't want to do it but the lone pairs cause the Hydrogens have to bend around each other and the bond angle for water is 104.5 degrees.

Another example of a bench structure is the sulfur dioxide molecule in this structure. Sulfur has two oxygen atoms, one has a double bond and the other has a single bond. Sulfur also has a lone pair, it doesn't have two lone pairs in the case of oxygen, but here it has one motion and the bond angle is less than 120. Note that this is similar to a trigonal planar structure in that it has two. atoms and a lone pair a trigonal planar structure has three lone pairs with a bond angle of about 120. so so2 has three things two atoms and a lone pair in the sulfur atom and that is why the bond angle is similar to a trigonal planar structure water is somewhat similar to a tetrahedral structure in the structure of tetrahedra there are four things or four atoms attached to the central atom in the case of oxygen it has four things two atoms and two lone pairs that is why the bond angle is close to that of a tetrahedral structure which is supposed to be 109.5 but in the case of water it is actually 104.5 now I want to put together certain molecules so that for a tetrahedral structure in the case of methane it has four groups, four atoms united, the bond angle is 109.5 now if we replace one of those atoms for a lone pair, as in the case of ammonia, we will get the trigonal pyramidal structure and since we still have four things, in this case three atoms and a lone pair, the angle will be close to 109.5, but it is a little less, if you subtract this by 2.5 you will get 107 degrees and that is the bond angle of ammonia with its trigonal pyramidal molecular structure.

Next is water, which looks like this. It has two lone pairs instead of one, but it still has four electron groups two three four, so the bond angle for this is 104.5 degrees, but the molecular structure is curved and we said it's tetrahedral and ammonia It is trigonal pyramidal. Now let's draw the trigonal plane structure. Let's use bh3 as an example, so boron is bonded to three atoms, therefore the bond angle is uh 120 degrees, now similar to bh3, we have so2, so one of the atoms in this structure is replaced with a bulge here, so the sulfur dioxide is attached to the central atoms attached to two atoms and a solitary one. pair, so you still have three things around it, which means the bond angle is close to 120. but it's not exactly 120, so it's actually a little less than 120, but it's close to it, although