in this video we're going to talk about a vespa theory and what it has to do with molecular geometry perhaps you've seen this word in your textbook it stands for valence shell electron pair repulsion and the basic idea is that we can predict the shape of the molecule based on the fact that electrons repel each other so electrons they want to be as far apart as possible so with that idea in mind we can predict the shapes of certain molecules now the first type of geometry that you need to be familiar with is the linear molecular geometry and 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 one straight line and the angle of a straight line is 180 degrees so that's the bond angle of a molecule with a linear geometry now there's some other examples for example uh carbon dioxide is another molecule with a linear geometry it looks like this looks like one straight line so any molecule where you have an atom at the center and two other atoms on the sides is a linear molecule this is the generic structure of a linear molecule now there's another example of a linear molecule and it's 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 the whole thing is an ion now the middle one has three lone pairs and the other two also has three with the geometry it's straight it's a linear molecular geometry now the next type of molecular geometry that you need to know is the trigonal planar structure when you hear the word try what do you think of tri represents stream and planar means it's flat as a paper a good example of the structure is bh3 so you have born at the center and the three hydrogen atoms are going to be spaced as far apart as possible so it looks like this now a full circle represents an angle of 360 degrees and if you divide that by three you can get the bond angle between hydrogen atoms and 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 is uh cocl2 so in this structure carbon has a double bond to an oxygen and it's attached to two chlorine atoms so anytime you have an atom at the center surrounded by three things and if the central atom doesn't have any lone pairs then what you have is a trigonal planar structure with a bond angle of approximately 120. now the next structure that we need to talk about is the tetrahedral molecular structure so when you hear the prefix tetra what do you think of tetra is equivalent to four so therefore in this structure we're going to have an atom surrounded by four other atoms and that's the tetrahedral structure with a bond angle of about 109.5 so methane fits this example so in methane we have a carbon in the middle surrounded by four hydrogen atoms now this is not a two dimensional structure it's a three dimensional structure because if you take three sixty divided by four you get ninety and it's not 90 is actually 109.5 based on the way these atoms arrange themselves in three dimensional space another 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 you just have different atoms the bond angle is still approximately 109.5 now the next structure that you need to know if you have a test coming up is the trigono 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 at 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 with the trigonal planar structure i'm just gonna draw this right next to it so you can see the difference the trigonal planar structure doesn't have any lone pairs on the central atom it's simply surrounded by three other atoms the trigonal pyramidal structure has three atoms attached to the central atom plus a lone pair so it has four things a good example of the trigonal pyramidal structure is nh3 ammonia in this structure nitrogen has one lone pair and it's attached to three hydrogen atoms another example is a phd so if you notice something i just want to point something out 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 ones that have a trigonal planar structure tend to be in group three but not always though so 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 that's another quick way to identify which one is going to be trigonal planar which one's going to be a trigonal pyramidal if there's no double bonds involved because we did have the example where it was like cocl2 but the carbon attached 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 a pattern i've seen now going back to the trigonal pyramidal structure there's one other thing i need to mention and that is the bond angle i'm going to use ammonia as an example so the bond angle for ammonia make sure you know this because it's a common test question it's about 107 degrees and so that's it for the trigonal pyramidal structure just want to mention that before i forgot it now the next geometry you need to be familiar with is a bent molecular geometry and a good example for this one is water water has a bent shape oxygen contains two lone pairs and those two lone pairs causes the hydrogens to be in a bench structure now perhaps you've seen water drawn like this and this is a common mistake so you don't want to do it but rather the lone pairs causes the hydrogens to be bent with respect to 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 and sulfur also has a lone pair it doesn't have two lone pairs in the case of oxygen but it has one move here and the bond angle is less than 120 notice that this is similar to a trigonal planar structure in a sense 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 on the sulfur atom and that's why the bond angle is similar to a trigonal planar structure water is somewhat similar to a tetrahedral structure in the tetrahedra structure there's four things or four atoms attached to the center atom in the case of oxygen it has four things two atoms and two lone pairs that's why the bond angle is close to that of a tetrahedral structure which is supposed to be 109.5 but in case of water it's actually 104.5 now i want to put certain molecules together so for a tetrahedral structure in the case of methane it has four groups four atoms attached to it the bond angle is 109.5 now if we replace one of those atoms for lone pair as in the case of ammonia we're going to get the trigonal pyramidal structure and because we still have four things in this case three atoms and a lone pair the angle is going to be close to 109.5 but it's a little bit less if you subtract this by 2.5 you're going to get 107 degrees and that's the bond angle of ammonia with its trigonal pyramidal molecular structure the next one 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 bend and we said this is tetrahedral and ammonia is trigonal pyramidal now let's draw the trigonal planar structure let's use bh3 as an example so boron is attached to three atoms so 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 lump here so sulfur dioxide is attached to the center atoms attached to two atoms and a lone 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 really a little bit less than 120 but it's close to it though you