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VSEPR and Molecular Geometry

Added Dec 04, 2018 •

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This video gives definitions, examples, and rules for drawing Lewis structures and identifying molecular geometry and electronic geometry. Watch the video and answer the 5 questions.

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in this video will use Vsepr theory to practice identifying the major molecular geometries including Bond angles the Vsepr model is a simple yet powerful tool to predict the shape of molecules so we'll look at the valence shell electrons these are the electrons on the outside of the atom and they come in general in pairs since electrons are negative and they're on the outside of the atom each atom has a cloud around it and that clouds will repel other atoms when they come close together from molecules we also have that repulsion between atoms so here we have a central atom in purple and we're going to add other atoms to it to make a molecule so we had one and then when we add another member that we the negative cloud of electrons around each atom so when we add another one they push away or try to put them close to each other can see if they move away they spread out to be as far as way as they can be but still Bond it and we could adding atoms to this and you can see that the shape changes now that we have an idea about Vesper let's put it to work and go through each of the major molecular shapes we use our chart help us organize our ideas and build our men as we learn it's often assume that you'll know that if we have two atoms it's going to be linear what you should know though it really doesn't matter if we have single bonds double bonds or triple. It's still going to have a linear killer geometry and that goes for all of the structures will look at today the type of bond single double triple doesn't influence the molecular geometry back to our chart is organized by steric number by the number of lone pair electrons the steric number is the number of things attached to that Central atom that can be atoms or it can be lone pair electrons also called bonded electron pairs we use the along with the number of Lone pairs to figure out the molecular geometry of the molecule when we have a steric number of two with no lone pairs that means there's two atoms attached to the central atom in there are no long pairs we have a molecular geometry example we'll start with the Lewis structure and you can see that Central carbon atom there are no lone pairs we can see there's two oxygen atoms and we're not concerned about the lone pairs on the oxygen atoms the only worry about the central atom so the purple that's our Central atom and will add the two oxygens each one is double bonded oxygen the other they spread out to be as far away from each other as possible and we have a linear geometry and the bond angle is 180 degrees next we move to trigonal planar where we have three atoms and no lone pairs bonded to a central atom has a trigonal planar molecular shape in the Lewis structure you can see three atoms and no lone pairs bonded to that Central Boron atom here we have the central atom the board and then we'll add flooring Adams there's one to spread out three to form the trigonal planar molecular geometry and the bond angles of 120 degrees and there are no lone pairs so this is the trigonal planar molecular geometry when we have a steric number 3 with two atoms and one lone pair we have a bent molecular geometry lone pairs sometimes called nonbonding electron pairs are extremely important influence the shape of molecules let's look at SO2 we have our two atoms bonded at Central sulfur and then we have a lone pair so that makes this a bent molecule but you can't tell this by looking just at the lowest we need Vsepr theory to help us visualize how these atoms are going to spread out the two atoms and the lump are push the Adams away as well to figure out the molecular geometry let's see what that looks like we have our Central sulfur atom and we'll put two oxygen atoms on that they're double bonded and next will add the lone pair and watch what happens when we do that the lone pair it pushes down the two oxygen atoms and now we have a bent molecular geometry it's important to note that these lone pairs have their own orbital here and they do occupy space and that's what leads them to repel and push the oxygen atoms away before we move on let's live listen to our minds with some practice the first thing is we'll draw the Lewis structures I've done that for you to speed things along at this point you want to try to visualize what these structures might look like in three dimensions is those Adams spread out and put way from each other then find the steric number and then count all the atoms and lone pairs on the central atom and then remember or look up the molecular geometry pause and give this a try for BF3 we have the central Boron atom and then we have three atoms attached to it with no lone pairs on that Central Boron you can imagine the fluorine atoms will spread out to be as far away from each other as possible might looks like this since we have a steric number of 3 and no lone pairs that makes this trigonal planar printer which doesn't even show up in our table we only have two atoms so it's going to be linear for no also called an 02 - we have two atoms attached to the central nitrogen there and then we have a lone pair so we have a steric number of 3 and you can imagine that those two atoms on the outside the oxygens are going to be down by that lone pair on the nitrogen that'll give us a bent molecular geometry okay so we'll look at three more important molecular geometries and then do some practice when we have 4 atoms bonded at Central atom no lone pairs we have a tetrahedral molecular geometry CH4 methane is often given as an example of the tetrahedral structure we have four hydrogen atoms attached to that Central carbon and no lone pairs so here's our Central carbon atom we had one two three and finally the fourth hydrogen atom and they spread out into this tetrahedral geometry we see the bond angle of 1 of 9 points we look at this they're all as far away from each other as they can possibly be we'll still being bonded to that Central carbon for the trigonal pyramidal we have a steric number of 4 that's three atoms and one lone pair attached to that Central atom NH3 ammonia is a good example of our trigonal pyramidal molecular geometry we have that Central nitrogen there with three atoms attached and then the lone pair and you can imagine that there's hydrogen to spread out but then that lone pair that's going to push them down to give it the trigonal pyramidal molecular geometry that would look something like this three atoms which also and then we add are lone pair which pushes them down to give us this trigonal pyramidal molecular geometry so we've looked at 4 we have the three atoms in the one lone pair is the trigonal pyramidal molecular geometry next up we need to look at what happens when we have two lone Pairs and two atoms with two lone Pairs and two atoms steric number for we have molecular geometry good example is water in the Lewis structure you can see you have two atoms and two lone pairs that makes this a bent molecular geometry we had our two hydrogen atoms they spread out and then we'll add her to Lone pairs 1/2 and this is the bent molecular geometry for water you might note that the bond angle is 109.5 if you put the bond angle up for water and be a little bit. so this is a general model that doesn't give us the specific Bond angle for specific molecules just a general idea of what the bond angle will be for a bent molecular geometry time for some more practice that's the core Stone of learning I've given you these Lewis structures and you need to find the molecular geometries try to Envision how the atoms and lone pairs are spreading out before you go to the table to check the molecular geometry so pause try to figure out the shape of these molecules for the ones that you had problems with go back earlier in the video and look at that particular molecular figure out where the confusion lies two more molecular geometries that we want to be aware of are the trigonal bipyramidal and the octahedral so when we have 5 atoms attached to that Central Atom 1 2 3 there's are trigonal for 5 now its trigonal bipyramidal this is the shake that we have so we have a steric number of 5 with 5 atoms attached no lone pairs if we add another atom to this we end up with what's called octahedral This is the octahedral shape and you can see all of the atoms they're spreading out to be as far away from each other as they can pause we get in the coordinates with the Vsepr theory we've been talking about know that we can have a v atoms attached to the central atom there for a trigonal bipyramidal but we could replace one of the atoms with a lone pair and that would give us the different geometry that's a little bit beyond the scope of what we want to do here but do understand there are other geometries for steric number five and six when you have lone pairs in this video we've looked at the major molecular geometry the Vsepr model you should have an intuitive sense now of how lone Pairs and atoms interact with each other and repel spreading out to give a molecule its shape using the Lewis structure looking at the steric number and then just be thinking about how these atoms spread out you should go to figure out these major molecular geometries this is Doctor be with the Vsepr model and thanks for watching

Uploaded by Debra Hood.

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