General Chemistry

Transcript of the video and more practice problems below

We are going to talk about the VSEPR model or VSEPR theory which is used to predict the geometry of molecules. VSEPR stands for valence shell electron-pair repulsion and it is a pretty long name which sometimes makes student think that this is a very complicated subject but I’ll try to convince you in this video that it is not so bad.

Valence shell electron-pair repulsion-the key word here is the repulsion. As long as we understand what this repulsion refers to and we are able to apply this principle, we should be able to predict the geometry of many molecules. In order to understand the principle, let’s start with something that is not about the molecules and atoms, just a simple concept and can be anything. If we have any central unit whether this is an atom or anything else, what the VSEPR says is that, we need to be able to predict how the other atoms and units connected to it are going to be arranged in space considering the fact that, for example, in this case, if we have two units connected to the central one, do not like each other. They repel each other and they want to be as far apart from each other as possible. So, in order to make the most optimized geometry, what we want to do is to push these two units away from each other at 180°. So, if I put them like this, just straight away from each other, it is 180° and this is going to be the optimal geometry for this structure that has two units connected to the central one.

What about if we have three units connected to the central unit? So, let’s put the central unit 1, 2, and 3. How can we arrange them to be as far away from each other as possible? You can pause the video for a second and try to get the optimal geometry and then check the answer here. So, for the three units what we need to do is put the central unit here and the other three at 120°. So, one-we can put on the top and then two on the sides, like this, and all the lengths are also going to be the same. All the angles here are 120°. The same thing, of course, can also be represented by pointing the two groups up and then one of them pointing down. It doesn’t really matter how we show them, these can be shown in any orientation and it’s the same geometry so don’t be confused about that. What is more important is that the angles are 120° because this unit on the central atom do not like each other they want to be as far away from each other as possible.

So, what if I have 4 units connected to the central unit? Let me put here 1, 2, 3 and 4. How would you arrange these to get an optical geometry, so that the groups are as far away from each other as possible? You can pause the video for a second and try to get the optimal geometry. Okay, there’s a high chance that if you’re trying to put four groups as far away from each other as possible then you might have come up with a geometry where they are at 90°. Let’s put 90 here and this refers to all the angles here. This is normal. Everyone is going to predict 90° unless you know the answer, unless you know that it is not 90°.

What happens is that when you have four groups connected, you can actually switch to a 3D arrangement and you can say that the two groups are going to be pointing on these sides and then I can have two other groups, one of them pointing towards me represented with a wedge line and the other one pointing away from me, represented with a dash line. This can also be drawn from the side. So, if I put it like that and put the groups, one pointing up and one pointing down on this side, and so now this is going to be pointing towards us and this is going to point away from us. So, that’s a 3D object and can be drawn in different ways but what is important is to understand that this is the optimal geometry and the angels turn out to be 109.5°. It can vary for different molecules but, averagely, it is the optimal angle considering that all these groups, units are the same. So, 90° is not correct and we are not going to look at this.

This is called linear. This is called trigonal planar and this is called tetrahedral.

When we switch to molecules, we need to remember that the central unit is always going to be an atom. So, this has to be an atom while the side groups connected to it can be either an atom, and can be a really long chain, many atoms, part of the molecule but at least one atom, or it can be electrons depending what we have on this side, atoms electrons, we are going to take a slightly different approach to name these structures.  For example, if I have methane, the formula of methane is CH4, the central atom here is the carbon as we know from the Lewis structure. If I put the carbon, I’m going to have 4 hydrogens connected to it- 1, 2, 3, 4. Because I have 4 units around a central atom, it is a tetrahedral geometry. On the other hand, if you draw, for example, the Lewis structure of ammonia, NH3, we remember that is a lone pair on the nitrogen, based on the Lewis structures. What happens here, is that the nitrogen will be in the middle and I have one hydrogen, lets’ say put it up and then I have another hydrogen, and then another hydrogen here pointing towards us, and this lone pair, which is also considered a unit, is going to be pointing away from us. Let me just put two dots here and this will be the electrons.

These four are arranged as a tetrahedron and this is a tetrahedral geometry but that refers to the electronic geometry. This is electronic geometry because I’m also considering the electrons here, the lone pairs of electrons. If we need to name the molecular geometrywe ignore this lone pair.

Ammonia can also be drawn this way. If I put the nitrogen here, I’m going to pull the lone pairs on the top and I’m going to put the hydrogen and then hydrogen here and then one is pointing away from us. Tetrahedral is symmetrical geometry so it doesn’t matter on which side I put any of the groups so, don’t be confused about that.  So, when I name this as a molecular geometry I’m going to ignore the electrons and you may wonder why but one way to explain this, is the lone pairs cannot be seen under any modern microscope while atoms can because of the nuclei. Nuclei are denser and heavier which makes them visible under certain microscopes. So, if I draw the ammonia with this approach, I will have the three hydrogens and it will be called pyramidal, not planar, it is not planar because not everything is in the plane of this paper or the drawing board.

To summarize the names of the molecular and electron geometry you can use this little table. In this representation, we are seeing AX, AX, and then we have AXE. X is the terminal atom and the E is the lone pair of electrons.

For example, this is the this is what we looked for the ammonia. If I have three atoms around the central nitrogen and one lone pair, we are going to ignore the lone pair and we will have a trigonal pyramidal geometry. Same, if we ignore the two lone pairs on water, we will have a bent molecular geometry.

For any molecule, in order to draw the geometry, follow this steps of VSEPR theory:

  • First what you need to do is draw the Lewis Structure with the atoms and all the electrons. Unless we have the correct Lewis Structure, with the will not be able to determine the geometry.
  • Second, you are going to count how many atoms and lone pairs of electrons are connected to the central unit. So, this is not how many electrons but how many pairs, how many pairs of electrons we have on the central unit. Then we’re going to arrange them to minimize the repulsion which means that put them as far away from each other as possible and then we’ll go based on the chart to actually name the geometry.

For example, let’s do it for water. So, H2O and if you check based on the Lewis structure, we going to see that the oxygen has two lone pairs of electrons. Now, we are going to put the oxygen in the middle, and I’m counting the steric number. So, Steric Number (SN) refers to how many atoms and lone pairs I have around the central unit. I have two hydrogens, so, I have two atoms and I have two lone pairs of electrons so the steric number is four. I have four things, four units around the central atom which means that I need to put them in a tetrahedral geometry. So, let’s put them together here. Hydrogens first, so this will be the 2 hydrogens one on top and one on the side and I’m going to put the lone pairs of electrons. So, four units arranged around a central unit at 109°, about 109°. Finally, if I want to name the molecular geometry based on our procedure, we going to ignore the lone pairs because we don’t count them in naming the molecular geometry. If you do that, you are going to have oxygen with two hydrogens like that and now this corresponds to the bent geometry. Bent geometry can result from different electronic geometries. For example, hypothetically, you can have a central atom connected to a lone pair and you can have two other atoms let’s say B & B and as an electron geometry, the angle here is 120° but if I discard the lone pair of electrons, I’m going to have a bent geometry and the angel here is 120° because it is the lone pair that pushes these two groups down and keeping them at 120°. So, this is bent and this is bent but here the angle is about 109° so there’s no unique angle for the bent geometry. It depends on the electronic geometry of the molecule.

Let’s do another example, something that has a double or triple bond. Carbon dioxide, CO2. Based on the Lewis Structure, the carbon will be in the middle, oxygens on the side and each oxygen has a double bond. Put two lone pairs of electrons. Now, we are going to look at the central atom, carbon, and determine the steric number. The steric number is equal to two atoms plus no lone pairs on carbon, carbon has no lone pairs, so this is two and it’s not important whether this is a double bond or a single bond. Even if it was a triple band, it’s not possible but even if it was, we would still look at the number of atoms only.

With steric number two, the only way to bring it to the optimal geometry, is to put them at 180° and this will be a linear geometry.

And finally, one other way to talk about VSEPR theory is to look at the balloons model. The central unit here is going to be the center where the two balloons or two balloons are connected so right here this is the central unit, right there to the central unit and we have the central unit here. So, if we have two balloons connected, the angle is going to be 180. If we have three balloons connected the angle is going to be 120 and here it’s going to be about 109°. This is actually a pretty good representation because in fact when you’re saying the two atoms are connected, lets’ say a carbon with a hydrogen, it’s coming from an overlap of the orbitals. Carbon has an orbital that’s a sp3 or sp2, whatever it can be and then hydrogen also has an orbital- s orbital. So, the orbitals are represented as balloons and if I want to arrange the orbitals in an optimal way, I am going to get to this geometry. Check the examples below, do some more practice to get confident in this topic.

Practice

1.

Draw the Lewis Structures and determine the electronic and molecular geometries for the following molecules: (a) BF3, (b) CH2O, (c) HCN, (d) BeCl2, (e) CH2Cl2, (f) SOCl2, (g) SO2

Check your answers

(a) trigonal planar, trigonal planar

(b) trigonal planar, trigonal planar

(c) linear, linear

(d) linear, linear

(e) tetrahedral, tetrahedral,

(f) tetrahedral, trigonal pyramidal

(g) trigonal planar, bent

Solution

2.

Determine the electronic and molecular geometries for the following ions: (a) CO32-, (b) SO32-, (c) ClO4, (d) NH4+

Check your answers

(a) trigonal planar, trigonal planar

(b) tetrahedral, trigonal pyramidal

(c) tetrahedral, tetrahedral

(d) tetrahedral, tetrahedral

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