Molecular Geometry: Videos & Practice Problems

Understanding the molecular geometry of compounds is crucial as it reveals the true shape of molecules, considering the repulsion between lone pairs and bonded atoms. Molecular geometry is determined by the number of electron groups around a central atom, which can include bonded atoms (bonding groups) and lone pairs. With two electron groups and no lone pairs, the geometry is linear. Three electron groups can form either trigonal planar or bent geometries, depending on whether there are zero or one lone pairs, respectively. Four electron groups can result in tetrahedral, trigonal pyramidal, or bent shapes, contingent on the presence of zero, one, or two lone pairs. Five electron groups lead to trigonal bipyramidal, seesaw, T-shaped, or linear geometries, with zero to three lone pairs influencing the outcome. Lastly, six electron groups can create octahedral, square pyramidal, or square planar geometries, again determined by the number of lone pairs, ranging from zero to two. Recognizing these geometries helps predict molecular behavior and properties.

Molecular Geometry with Two Electron Groups

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 going to be treated as the same.

Now we're going to take a look at each of the number of electron groups that exist, starting out with two electron groups. Now when we have two electron groups, we're going to say here this is central elements with two electron groups that have 0 lone pairs and give only one possible molecular geometry.

So if we take a look here we have two electron groups. We say that there are zero loan pairs, so that means we're going to have two bonding groups possible. Remember bonding groups are just your surrounding elements, 0 lone pairs. Here are some examples and all of them we have a central element here we have beryllium, carbon and carbon again and they are connected to only two 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 two bonding groups visually would see it as our black spear here, which is our central element connected to those grey spheres which are surrounding elements here. The molecular geometry for all of them would be linear.

So when it comes to two electron groups around the central element, there's only one possible molecular geometry, and that's a linear molecular geometry.

Molecular Geometry with Three Electron Groups

When it comes to three electron groups, we're going to say this is when central elements with three electron groups that can have either zero or one lone pair to get two 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 three bonding groups and 0 lone pairs. Here we have an example of carbon in the center connected to three surrounding elements. Its visualization would look like this and we'd say that it's molecular geometry would be trigonal planar.

Now another option that could occur is we have two surrounding elements or two bonding groups and 1 lone pair on the central element. Here we have our example with tin in the center with two chlorines and it has one 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 kind of 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 two surrounding elements and 1 lone pair in terms of this one and other ones like it right. So keep in mind when you have 3 electron groups around your central element, 2 possible molecular geometries are possible.

Trigonal Planar Example

Determine the molecular geometry for the following anion. Here we have beryllium connected to three hydrogens and there's a negative charge. Now beryllium itself is in Group 2A, and we're going to say because of that it has two valence electrons. But here we have 3 hydrogens, so we would need 3 valence electrons. That's going to be OK because this -1 charge means we gained an electron from the outside, so we've added additional electron to beryllium.

Remember, hydrogens only have one valence electron, since they're in Group 1A, and they only make single bonds. Because this has a charge, we'd put in brackets and the charge on the outside. Now if we take a look here, we'd say that beryllium is connected to three 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. This would be the molecular geometry of our particular anion.

Molecular Geometry with Four Electron Groups

Central elements with four electron groups can have either zero, one or two lone pairs to give three 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 center element could have 4 surrounding elements and no lone pairs. It could have three surrounding elements and 1 lone pair, or it could have two 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 methane, which is CH₄. Here we have ammonia, which is NH₃. And here we have water, H₂O. Their visualizations show carbon in the center with its four hydrogens attached to it. Now here for nitrogen, we have our three surrounding elements and up top we'd have our lone pair. Remember, lone pairs have their own electron cloud which causes further repulsion. Water here would have two lone pairs on the oxygen, which causes further repulsion.

Now here, what are the names of the geometries? Well, if we have 4 surrounding elements and no lone pairs, it would be called tetrahedral. For the next one, it kind of looks like a pyramid with three legs, so that's why we call it trigonal pyramidal. And then finally, for water, we have two lone pairs on the central element and two surrounding groups. Here it kind of looks familiar to us. We saw shapes similar to this when we talked about two 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. Just remember, when we have 4 electron groups, there are three possible molecular geometries that are possible.

Molecular Geometry with Five Electron Groups

Central elements with five electron groups can have zero to 3 lone pairs to give four possible molecular geometries. So we can have 0, 1, 2, or three lone pairs on our central element. So here we have 5 electron groups and the way it can break down is we have 5 surrounding elements or bonding groups and 0 lone pairs on the central element. Or it could be 4 to 1 or it could be three to two or finally it can be two to three.

Now here if we take a look, we have examples. Here we have phosphorus pentachloride, here we have selenium tetrafluoride, here we have bromine, bromine, tri iodide and here we have xenon dichloride as examples. Now their 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 it's 2-3 legged pyramids stacked on top of each other, its molecular geometry name would be trigonal because each pyramid has three legs and there's two pyramids on top of each other by pyramidal.

For the next one, if we have 4 surrounding elements and 1 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 two 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 two surrounding elements and three lone pairs on your central element, you look linear, look like a straight line. So these would be our different possible molecular geometries if we have 5 electron groups involved in our molecule.

All right, so keep them in mind and just remember visually what they look like. That's a great way to help you remember their name.

Trigonal Bipyramidal Example

Draw and determine the molecular geometry for the following molecule. So CL₄. So here sulfur would go in the center. It's in Group 6A so it has six valence electrons. Halogens, those in elements in Group 7A only make single bonds when they are our surrounding element. So sulfur is going to single bond to four of the chlorines.

The chlorines are in Group 7/8, 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 will have to in order to incorporate the oxygen. Oxygen ideally wants to make two bonds. Sulfur can expand its octet, so sulfur is going to help it out and make a double bond to the oxygen.

Oxygens in Group 6A, so it has six valence electrons. So here this would be the shape of our SLCL₄ structure and we're going to say it is connected to five bonding groups or surrounding elements and it has no lone pairs on it so it would be trigonal by pyramidal so. So this would represent its molecular geometry.

Molecular Geometry with Six Electron Groups

Center elements with six electron groups can have zero to 2 lone pairs to give three possible molecular geometries. So if we have 6 electron groups, the combinations that exist are we can have six bonding groups which are surrounding elements and 0 lone pairs. Or we could have 5 surrounding elements and 1 lone pair. Or we could have 4 surrounding elements and two lone pairs on the central element.

Here we have some good examples of this. Here we have sulfur hexafluoride, here we have chlorine, PETA, bromide and then here we have Xenon tetrachloride. Now their visualizations for sulfur tech on hexa. On fluoride we have sulfur in the center with its six 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 five surrounding elements in one lone pair, it's square pyramidal.

Then finally the last one. Here we have 4 surrounding elements and two loanpayers. 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 three molecular geometries that exist if you're dealing with six electron groups on your central element.

Square Pyramidal Example

Determine the molecular geometry for the following ion. So here we have Krypton, pentachloride cation. So Krypton is in Group 8A, so it has 8 valence electrons. But plus one means we've lost one electron so now it only has seven.

We're going to say here the chlorines, it's going to single bond to them and here we're going to have a lone pair on the bottom. The chlorines are in Group 7A, 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 five 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.

Determine the molecular geometry for the following molecule:SeH₂Cl₂

• A. T-shaped
• B. Seesaw
• C. Square pyramidal
• D. Square planar

Answer: (B)

Determine the molecular geometry for the following molecule:CHClO.

• A. Trigonal pyramidal
• B. T-shaped
• C. Trigonal planar
• D. Tetrahedral

Answer: (C)

Determine the molecular geometry for the following molecule:FSSF.

• A. Tetrahedral
• B. Bent
• C. Trigonal Planar
• D. Trigonal Pyramidal

Answer: (B)

Determine the molecular geometry for the following molecule:IF₄^–.

• A. Square planar
• B. Square pyramidal
• C. Trigonal bipyramidal
• D. Seesaw

Answer: (A)

Here’s what students ask on this topic:

How do you determine molecular geometry?

To determine the molecular geometry of a molecule, you'll need to follow the VSEPR (Valence Shell Electron Pair Repulsion) theory, which states that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. Here are the steps to determine molecular geometry:

1. Draw the Lewis Structure: Determine the total number of valence electrons for the molecule, arrange them to satisfy the octet rule for each atom, and pair any remaining electrons.
2. Count Electron Pairs: Identify the number of bonding pairs (shared electrons) and lone pairs (non-bonding electrons) around the central atom.
3. Determine Electron Pair Geometry: Use the number of electron pairs to determine the electron pair geometry. For example, two pairs give a linear geometry, three a trigonal planar, four a tetrahedral, and so on.
4. Identify Molecular Geometry: Consider the positions of the atoms only. Lone pairs affect the geometry, making it different from the electron pair geometry. For instance, if there's a tetrahedral electron pair geometry but one lone pair, the molecular geometry is trigonal pyramidal.
5. Use VSEPR Notation: Optionally, you can describe the geometry using AXE notation, where 'A' is the central atom, 'X' is the

What is molecular geometry?

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It's a way to describe the shape that a molecule takes based on the positions of its nuclei and the electron pairs that bond the atoms together. The geometry is determined by the number of electron pairs, both bonding and lone pairs, around the central atom, following the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory states that electron pairs will arrange themselves as far apart as possible to minimize repulsion between them.

The geometry of a molecule can significantly influence its physical and chemical properties, including reactivity, polarity, phase of matter, color, magnetism, and biological activity. Common molecular geometries include linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. Understanding molecular geometry is crucial in fields such as chemistry, biochemistry, and pharmacology, where the shape of a molecule can affect how it interacts with other substances.

How does electron pair repulsion determine the molecular geometry?

Electron pair repulsion is a key factor in determining molecular geometry due to the VSEPR (Valence Shell Electron Pair Repulsion) theory. According to this theory, electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion between their negative charges. This is because electron pairs, both bonding (shared between atoms) and non-bonding (lone pairs), repel each other.

The specific arrangement of these electron pairs in three-dimensional space dictates the shape of the molecule. For example, if there are four electron pairs around the central atom and they are all bonding pairs, they will repel each other equally and adopt a tetrahedral geometry. If there is one lone pair and three bonding pairs, the geometry becomes trigonal pyramidal, as the lone pair exerts a slightly greater repulsive force and distorts the shape.

Thus, by considering the number of lone pairs and bonding pairs around a central atom, one can predict the molecular geometry using the VSEPR model.