Electron Geometry - Online Tutor, Practice Problems & Exam Prep

Understanding electron geometry is crucial for grasping the spatial arrangement of electron groups around a central atom in a molecule. Electron geometry considers both lone pairs and bonded atoms equally. For two electron groups, the geometry is linear, as exemplified by carbon dioxide. With three electron groups, the shape is trigonal planar, which can be remembered since 'tri' signifies three. A molecule with four electron groups adopts a tetrahedral geometry, 'tetra' indicating four. For five electron groups, the shape is trigonal bipyramidal, with 'tri' and 'bi' adding up to five. Lastly, six electron groups result in an octahedral geometry; think of an octopus with eight limbs minus two, leaving six. These geometries help predict molecular shapes and properties, which are fundamental concepts in chemistry.

Electron Geometry Concept 1

Hey everyone. So when we say electron geometry, realize that it represents the simplest system for geometrical shapes that focuses on the number of electron groups around the central element. And when it comes to the electron geometry, we're going to treat lone pairs and surrounding elements as the same.

So if we take a look here at this chart, we are breaking things down in terms of number of electron groups, the orbital shapes where we just give certain examples that fit this particular row, the electron geometry name and then a memory tool to help us remember that electron geometry name. So here we're going to start out with the first one and let's say that our molecule has two electron groups. Here we have carbon dioxide as our example. Carbon is a central element, it has two surrounding elements, so that's why it has two electron groups. The geometry here, the electron geometry, would be called linear. And a good way to remember linear is that recall that there are two points in a straight line. OK, so line for linear and two points because there's two electron groups. So again we have carbon in the center and we have two electron groups, these two oxygens.

For the next one we have 3 electron groups. And remember, these electron groups could either be 3 surrounding elements, or they could be two surrounding elements and 1 lone pair. Remember, we're treating lone pairs and surrounding elements as the same. In this case, the electron geometry name would be trigonal planar. Now the beauty of chemistry is that we can pronounce things differently, but all that really matters is that you get the spelling correct. OK, so you might hear trigonal planar, trigonal planar, trigonal planner, trigonal planner. None of that matters. Just make sure you're spelling it correctly. And how to remember this? Well, we're going to say we have 3 electron groups and it starts with Tri, Tri meaning 3.

Next, we have 4 electron groups, and remember that could just be different situations where they're all surrounding elements or they're surrounding elements with a mixture of lone pairs. As long as it adds up to 4 electron groups, it fits in this row. Here the electron geometry name would be tetrahedral. Tetra means 4, 4 electron groups.

Now the next two. Let's look at the next two. We have 5 electron groups for the next one. So the central element has five electron groups around it, which could be all surrounding elements, or a combination of surrounding elements and lone pairs, right? So here we have 3 surrounding elements, 2 lone pairs. Still, that adds up to five. Here we have two surrounding elements and three lone pairs. Now the name for this one. This one's a bit longer. This is trigonal or trigonal bipyramidal. Now, Jules, how am I supposed to remember this name? Well, we know that it has five electron groups, and we're going to say here tri means 3. Bi here means two. 3 + 2 will give me 5.

All right. Finally, we have 6 electron groups. So again, that could be a combination of all surrounding elements that add up to six, or some combination of surrounding elements and lone pairs. This one, its name is octahedral. This one is going to require a little bit more imagination in terms of remembering it. Octa, you might say, hey Jules, Octa means 8, but here we're talking about 6 electron groups. What we can play on this idea of octa meaning 8. So Octa, octopus. So an octopus has eight tentacles, right? Or 8 legs and it has two eyes, so 8 tentacles minus two, 8 - 2 gives me 6, 6 electron groups. So remember if you have 6 electron groups, your electron geometry is octahedral.

So just remember some of these memory tools if it's hard for you to remember a particular electron geometry name and you'll be able to get through this. Remember, in the case of electron geometry, lone pairs on the central element and surrounding elements are treated as the same.

Electron Geometry Example 1

Determine the electron geometry for the hydrogen sulfide molecule H₂S. So if we take a look at hydrogen sulfide or yeah, sulfide molecule, we're going to see here that it is a center element sulfur connected to how many electron groups it's connected to, 1, 2, 3, 4 electron groups.

Remember, electron groups represent lone pairs on the central element as well as surrounding elements directly connected to the central element. So we have 4 electron groups, and remember 4 is connected to Tetra. Because it's connected to Tetra.

This electron geometry will be tetrahedral, so hydrogen sulfide has a tetrahedral electron geometry.

Determine the electron geometry for the carbon disulfide molecule, CS₂. 

A. Linear

B. Bent

C. Trigonal planar

D. Tetrahedral

• A. Linear
• B. Bent
• C. Trigonal planar
• D. Tetrahedral

Electron Geometry Example 2

Recall there are many possible Lewis dot structures that exist, but there are rules to draw the best structure. So for this example it says determine the electron geometry for the following molecule CH₂O. This is also called formaldehyde. So if we take a look here we place the least electronegative element in the center. Now that would be hydrogen. But remember we have another rule when it comes to hydrogen. It can never go in the center. So as a result, it's carbon that has to go in the center.

Carbon is in Group 4A, so it has four valence electrons. And remember, hydrogens are in Group 1A and they can only form single bonds. Carbon itself has to make four bonds. It's already making two bonds. It needs two more. The only element that's left is oxygen, so carbon would have to form a double bond to that oxygen. Carbon oxygens in Group 6A, so it has six valence electrons, and here we're showing all of them. So this would represent the structure or molecular shape of formaldehyde.

Now if we take a look, we'd say that the carbon is connected to how many electron groups it's connected to. 123 electron groups? Remember 3. We say try equals 3 because we're saying try. That would mean that the electronic geometry is trigonal planar, so this would be the electronic geometry or molecular shape of the formaldehyde molecule.

Determine the number of electron groups for the following cation:AsBr₂⁺. 

A. 2

B. 3

C. 4

D. 1

• A. 2
• B. 3
• C. 4
• D. 1

Draw and determine the electron geometry for the following molecule:TeBr₄

Draw and determine the electron geometry for the following molecule. Here we have tellurium connected to 4 bromines. Tellurium is in group 6A, so it has 6 valence electrons. Remember, halogens, when not in the center, only make single bonds. We're going to use 4 out of our 6 valence electrons for tellurium to connect to the 4 bromines. Bromines are in group 7A, so they have 7 valence electrons. So, there go each of the bromines. And then we still have 2 electrons we're not using for tellurium; they would exist as a lone pair on tellurium. Now remember, we draw the structure this way on purpose. If you haven't taken a look, go back and review my video on molecular shapes. Here, if we take a look, we're going to have how many electron groups on tellurium? We have 1, 2, 3, 4, 5 electron groups on tellurium. Remember, we have 5 electron groups. 'Tri' means 3, 'bi' means 2, and when you add them together, it gives us 5. This is going to be trigonal bipyramidal. Two words. So, this would be the electron geometry for TeBr4.

Determine the electron geometry of the nitrogen atom within methylamine, CH₃NH₂.  

A. Linear

B. Trigonal Pyramidal

C. Tetrahedral

D. Trigonal Planar

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

Here’s what students ask on this topic:

What is electron geometry?

Electron geometry, also known as electronic geometry, refers to the spatial arrangement of electron pairs around a central atom in a molecule. This includes both the bonding electron pairs, which participate in forming bonds with other atoms, and the lone pairs, which are not shared with other atoms and remain localized on the central atom.

The electron geometry is determined by the repulsion between these electron pairs, as described by the Valence Shell Electron Pair Repulsion (VSEPR) theory. According to VSEPR theory, electron pairs will arrange themselves as far apart as possible to minimize repulsion, resulting in specific geometric shapes. Common electron geometries include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. The geometry is crucial for understanding the molecule's shape, bond angles, and potential reactivity.

How do you determine electron geometry?

Electron geometry, also known as electron-pair geometry, is determined by the arrangement of electron pairs around a central atom. To determine this, you follow the VSEPR (Valence Shell Electron Pair Repulsion) theory, which is based on the idea that electron pairs repel each other and will therefore arrange themselves as far apart as possible to minimize repulsion.

Here's a step-by-step guide:

1. Write the Lewis Structure: Draw the molecule's Lewis structure to determine the number of bonding pairs (shared between atoms) and lone pairs (non-bonding pairs) on the central atom.
2. Count Electron Pairs: Count the total number of electron pairs around the central atom, including both bonding pairs and lone pairs.
3. Apply VSEPR Shapes: Use the total number of electron pairs to determine the molecular shape based on VSEPR theory. For example, two electron pairs result in a linear shape, three in a trigonal planar, four in a tetrahedral, five in a trigonal bipyramidal, and six in an octahedral shape.

Remember that the electron geometry considers all electron pairs, while the molecular geometry describes the arrangement of only the atoms (bonding pairs). Lone pairs affect the geometry since they take up space and can push bonding pairs closer together.

What is the difference between electron geometry and molecular geometry?

Electron geometry and molecular geometry are both ways of describing the shape of a molecule, but they focus on different aspects.

Electron geometry, also known as electronic geometry, considers all electron pairs (bonding and non-bonding) surrounding a central atom and describes how these electron pairs are arranged in space. The electron pairs repel each other and tend to occupy positions that minimize repulsion, leading to specific geometric arrangements. Common electron geometries include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.

Molecular geometry, on the other hand, describes the spatial arrangement of only the atoms in a molecule, ignoring the non-bonding electron pairs. It is the shape that is formed by the positions of the nuclei of the atoms, which is influenced by the electron geometry but can be different due to the presence of lone pairs. For example, while the electron geometry of a molecule with four electron pairs is tetrahedral, if one of those pairs is a lone pair, the molecular geometry would be described as trigonal pyramidal.