Molecular Geometry - Video Tutorials & Practice Problems
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True Shape of Molecules that takes into account differences in repulsion between lone pairs and surrounding elements.
Molecular Geometry
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concept
Molecular Geometry Concept 1
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The molecular geometry of a compound can be seen as the true shape of that molecule or compound that takes into account differences in repulsion between lone pairs and surrounding elements. Now because of this, we would treat lone pairs on the central element and surrounding elements as different. So they're not gonna be treated as the same. Now we're gonna take a look at each of the number of electron groups that exist, starting out with 2 electron groups. Now when we have 2 electron groups, we're gonna say here, this is central elements with 2 electron groups that have 0 lone pairs and give only one possible molecular geometry. So if we take a look here, we have 2 electron groups. We said that there are 0 lone pairs, so that means we're gonna have 2 bonding groups possible. Remember, bonding groups are just your surrounding elements, 0 lone pairs. Here are some examples. In all of them, we have a central element. Here we have beryllium, carbon, and carbon again, and they are connected to only 2 surrounding elements. Now it doesn't matter if it's single bonded to them or double bonded or triple bonded. It's still 2 surrounding elements or 2 bonding groups. Visually, we'd see it as our black sphere here, which is our central element, connected to those gray spheres which are surrounding elements. Here, the molecular geometry for all of them would be linear. So when it comes to 2 electron groups around the central element, there's only one possible molecular geometry, and that's a linear molecular geometry.
Two Electron Groups: Linear
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Molecular Geometry Concept 2
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It comes to 3 electron groups, we're going to say this is when central elements with 3 electron groups that can have either 0 or 1 lone pair to give 2 possible molecular geometries. So again, when we have 3 electron groups around our central element, 2 possible shapes can occur. So we have 3 electron groups. In the first situation, we have 3 surrounding elements or 3 bonding groups and 0 lone pairs. Here we have an example of carbon in the center connected to 3 surrounding elements. Its visualization would look like this, and we'd say that its molecular geometry will be trigonal planar. Now another option that could occur is we have 2 surrounding elements or 2 bonding groups and one lone pair on the central element. Here we have our example with 10 in the center with 2 chlorines, and it has 1 lone pair. In its visualization, we have lost the element that was here and it's been replaced by a lone pair. Now remember, lone pairs have an electron cloud which further causes more repulsion, so that's why it looks kind of like that. And its shape kinda fits the name. Here, we'd say that its molecular shape is bent. You might also see the name v shaped, or you might even see the name angular. So just remember all of these names are synonymous for the same thing. They're talking about a molecular structure in this case where we have 2 surrounding elements and one lone pair in terms of this one and other ones like it. Alright. So keep in mind, when you have 3 electron groups around your central element, 2 possible molecular geometries are possible.
Three Electron Groups:
Trigonal Planar, Bent = V-shaped = Angular
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example
Molecular Geometry Example 1
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Determine the molecular geometry for the following anion. Here we have Beryllium connected to 3 hydrogens, and there's a negative charge. Now Beryllium itself is in group 2 a, and we're gonna say because of that it has 2 valence electrons. But here we have 3 hydrogens, so we would need 3 valence electrons. That's gonna be okay because this minus one charge means we've gained an electron from the outside, so we've added an additional electron to beryllium. Remember, hydrogens only have one valence electron since they're in group 1 a, and they only make single bonds. Because this has a charge, we put it in brackets and the charge on the outside. Now if we take a look here, we'd say that beryllium is connected to 3 electron groups, 3 surrounding hydrogens. It has no lone pairs on it. So we have 3 electron groups, 0 lone pairs, and because of that, our molecular geometry would be trigonal planar. So this would be the molecular geometry of our particular anion.
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Molecular Geometry Concept 3
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Central elements with 4 electron groups can have either 0, 1, or 2 lone pairs to give 3 possible molecular geometries. So if we take a look here, we have 4 electron groups, and the way we can split this up is our central element could have 4 surrounding elements and no lone pairs, it could have 3 surrounding elements and 1 lone pair, or it could have 2 surrounding elements and 2 lone pairs. If we take a look here, we have different examples of shapes that fit this criteria. Here we have c h 4 which is methane. Here we have ammonia which is n h 3. In here we have water. They're visualizations. We have carbon in the center with its 4 hydrogens attached to it. Now here for nitrogen, we have our 3 surrounding elements and up top we have our lone pair. Remember, lone pairs have their own electron cloud which causes further repulsion. Water here would have 2 lone pairs on the oxygen which causes further repulsion. Now here, what would the names of the geometries be? Well, here, if we have 4 surrounding elements and no lone pairs, it would be called tetrahedral. For the next one, it kinda looks like a pyramid, a pyramid with 3 legs. So that's why we call it trigonal pyramidal. And then finally, water, we have 2 lone pairs on a central element and 2 surrounding groups. Here it kinda looks familiar to us. We saw a shape similar to this when we talked about 2 electron groups. We'd say that this looks bent or v shaped or angular. Now any one of these three terms could be used to identify this particular shape, so treat them all as the same. So just remember, when we have 4 electron groups, there are 3 possible molecular geometries that are possible.
Four Electron Groups:
Tetrahedral, Trigonal pyramidal, Bent = V-shaped = Angular
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Molecular Geometry Concept 4
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Central elements with 5 electron groups can have 2 0 to 3 lone pairs to get 4 possible molecular geometries. So we can have 0, 1, 2, or 3 lone pairs on our central element. So here we have 5 electron groups, and the way it can break down is we have five surrounding elements or bonding groups and 0 lone pairs on the central element, or it could be 4 to 1, or it could be 3 to 2, or finally it can be 2 to 3. Now here, if we take a look, we have examples. Here we have, phosphorus pentachloride. Here we have Selenium Tetrachloride, here we have, Bromine, Bromine Triiodide, and here we have Xenon Dichloride as examples. Now they're visualizations. Here, if we take a look, visually this would look like 2 pyramids that are stacked on top of each other. And if we think about its 2 3 legged pyramids stacked on top of each other, its molecular geometry name would be trigonal, because each pyramid has 3 legs, and there's 2 pyramids on top of each other by pyramidal. For the next one, if we have 4 surrounding elements and one lone pair on the central element, it would visually look like this. Here, I've added a person here and a person here to help us think of what this would look like in a daily object. So if you look at it, it kind of looks like a seesaw. So that's the name, seesaw. For the next one, if you have 3 surrounding elements and 2 lone pairs on your central element, this here looks like the letter t. So that's why we call it t shaped. And then, finally, if you have 2 surrounding elements and 3 lone pairs on your central element, you look linear, like a straight line. So these would be our different possible molecular geometries if we have 5 electron groups involved in our molecule. Alright? So keep them in mind and just remember visually what they look like. That's a great way to help you remember their name.
Five Electron Groups:
Trigonal Bipyramidal, See-saw, T-shaped, Linear
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example
Molecular Geometry Example 2
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Draw and determine the molecular geometry for the following molecule, s o c l 4. So here, sulfur would go in the center. It's in group 6 a, so it has 6 valence electrons. Halogens, those in elements in group 7 a, only make single bonds when they are a surrounding element. So sulfur is gonna single bond to 4 of the chlorines. The chlorines are in group 7 a, so they have 7 valence electrons. So here they go here. Now remember sulfur can have an expanded octet, which means it can have more than 8 electrons around it, and it'll have to in order to incorporate the oxygen. Oxygen ideally wants to make 2 bonds. Sulfur can expand its octet, so sulfur is gonna help it out and make a double bond to the oxygen. Oxygen is in group 6 a, so it has 6 valence electrons. So here, this would be the shape of our s l c l four structure, and we're gonna say it is connected to 5 bonding groups or surrounding elements, and it has no lone pairs on it. So it would be trigonal by pyramidal. So this would represent its molecular geometry.
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Molecular Geometry Concept 5
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Center elements with 6 electron groups can have 0 to 2 lone pairs to give 3 possible molecular geometries. So if we have 6 electron groups, the combinations that exist are, we can have 6 bonding groups, which are surrounding elements and 0 lone pairs, or we can have 5 surrounding elements and 1 lone pair, or we can have 4 surrounding elements and 2 lone pairs on the central element. Here we have some good examples of this. Here we have sulfur hexafluoride. Here we have chlorine pettabromide, and then here we have xenon tetrachloride. Now the visualizations. For sulfur hexafluoride, we have sulfur in the center with its 6 surrounding elements, 6 surrounding fluorines. Here the molecular geometry would be octahedral. For the next one, we have chlorine pentabromide, and if we were to visually show it, we can see that it looks like a pyramid, and this pyramid has a base that's square shaped. So that's why its name is square pyramidal. So just remember, if you have a central element that has 5 surrounding elements and one lone pair, it's square pyramidal. Then finally, the last one here, we have 4 surrounding elements in 2 lone pairs. Here, it would take on a square shape, and here it's on a flat plane. So that's why this is called square planar or square planar. So these are the 3 molecular geometries that exist if you're dealing with 6 electron groups on your central element.
Six Electron Groups:
Octahedral, Square pyramidal, Square planar
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example
Molecular Geometry Example 3
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Determine the molecular geometry for the following ions. So here we have, Krypton Pentachloride Cation. So Krypton is in group 8 a, so it has 8 valence electrons, but plus 1 means we've lost 1 electron, so now it only has 7. We're going to say here the chlorines, it's gonna single bond to them, and here we're gonna have a lone pair on the bottom. The chlorines are in group 7 a, so they have 7 valence electrons. So when we draw this out, we can see that our Krypton has around it 5 surrounding elements or 5 bonding groups, and it has one lone pair. So remember, when we have this type of situation, we'd say that this is square pyramidal. So this would represent the molecular geometry of Krypton Pentachloride Cation.
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Problem
Problem
Determine the molecular geometry for the following molecule:SeH2Cl2
A
T-shaped
B
Seesaw
C
Square pyramidal
D
Square planar
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Problem
Problem
Determine the molecular geometry for the following molecule:CHClO.
A
Trigonal pyramidal
B
T-shaped
C
Trigonal planar
D
Tetrahedral
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Problem
Problem
Determine the molecular geometry for the following molecule:FSSF.
A
Tetrahedral
B
Bent
C
Trigonal Planar
D
Trigonal Pyramidal
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Problem
Problem
Determine the molecular geometry for the following molecule:IF4–.