Electron Geometry (Simplified): Study with Video Lessons, Practice Problems & Examples

Electron geometry is determined by the number of electron groups around a central atom, including lone pairs and surrounding elements. With 2 electron groups, the geometry is linear; with 3, it is trigonal planar; and with 4, it is tetrahedral. Remember that "tri" signifies three and "tetra" signifies four. Understanding these geometries is crucial for predicting molecular shapes and reactivity, which are foundational concepts in organic chemistry and biochemistry.

Electron Geometry (Simplified) Concept 1

So here we're going to say that the electron geometry represents the simplest system for geometrical shapes that focuses on the number of electron groups around the central element. We're going to treat lone pairs and surrounding elements as the same. Alright. So, we can have 2, 3, or 4 electron groups on the central element. So remember, when you have 2 electron groups on the central element, that's just 2 surrounding elements. When we have 3, then there are 2 possibilities. We could have here 3 surrounding elements or 2 surrounding elements and 1 lone pair. With 4 electron groups, there are multiple possibilities here, which is showing 2 possibilities, where it's 4 surrounding groups or 2 surrounding groups and 2 lone pairs. Remember, the one that I'm not showing that we talked about in earlier videos is you could have 3 surrounding elements and one lone pair as well, and that would still constitute a 4 electron group structure. Now the number of electron groups determines the electron geometry. When you have 2 surrounding groups, your electron geometry is linear. So a good way to remember linear is that 2 points in a straight line. So, 2 points, 2 surrounding elements in a straight line. Linear means a line. When you have 3 electron groups, then your electron geometry is trigonal or trigonal planar or planar. So again, depending on where you are in the country, you might pronounce it differently. Trigonal planar, trigonal planar, trigonal planar, trigonal planar. It's all the same thing. Here, a good way to remember this is tri equals 3. It starts out with tri, tri means 3, 3 electron groups. Finally, if your electron group is 4, then your electron geometry is tetrahedral. A good way to remember tetrahedral is tetra means 4. Right? So tetra here means 4, 4 electron groups. So when it comes to 2, 3, and 4 electron groups, remember we have linear, trigonal planar, and tetrahedral.

Electron Geometry (Simplified) Example 1

Determine the electron geometry for the hydrogen sulfide molecule or H₂S. Alright. So the electron geometry is based on your electron group number. If we take a look here, sulfur is connected to 2 lone pairs and 2 surrounding elements. Remember, your electron group total equals lone pairs on the central element plus the bonding groups on the central element, where bonding groups mean your surrounding elements. So we have 2 lone pairs and 2 surrounding elements; hence, our electron group is 4. "Tetra" means 4, so the electron geometry here will be tetrahedral. So, tetrahedral will be the shape for this hydrogen sulfide molecule.

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

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

Electron Geometry (Simplified) Example 2

Now recall there are many possible Lewis dot structures that exist, but there are rules to draw the best structure. Here we have to determine the electron geometry for the following molecule of CH₂O. Alright. So here the number of valence electrons; we have carbon, which is in group 4A, hydrogen, which is in group 1A, and there's 2 of them, and we have oxygen, which is in group 6A. So that's 12 total valence electrons.

We put carbon in the center because, remember, hydrogen never goes in the center. We form single bonds to the hydrogen and to the oxygen initially. We place enough electrons around the central elements so they follow the octet rule. But remember, hydrogen doesn't follow the octet rule; it follows the duet rule, so it only needs a single bond and it's fine. At this moment, we've used all 12 of our electrons, so there's none left. Remember, bonding preferences: oxygen wants to make 2 bonds and carbon ideally wants to have 4. To accommodate both of them, we'd remove 1 of the lone pairs on oxygen and use it to make a double bond. So this would be the structure of our compound of CH₂O, also known as formaldehyde.

So, we'd say it has how many groups attached to it, the central element? 1, 2, 3 electron groups. Remember, when you have 3 electron groups, 3 is tri, so this is trigonal planar or planar. Thus, this will be the electron geometry of the following compound.

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

• A. 2
• B. 3
• C. 4
• D. 1

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

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

Here’s what students ask on this topic:

What is electron geometry and how is it determined?

Electron geometry refers to the spatial arrangement of electron groups around a central atom, including both bonding pairs and lone pairs of electrons. It is determined by the number of electron groups surrounding the central atom. For example, if there are 2 electron groups, the geometry is linear; with 3 electron groups, it is trigonal planar; and with 4 electron groups, it is tetrahedral. Understanding electron geometry is crucial for predicting molecular shapes and reactivity, which are foundational concepts in organic chemistry and biochemistry.

How does the number of electron groups affect the electron geometry?

The number of electron groups around a central atom directly determines its electron geometry. With 2 electron groups, the geometry is linear, meaning the groups are arranged in a straight line. With 3 electron groups, the geometry is trigonal planar, forming a triangular shape in a single plane. With 4 electron groups, the geometry is tetrahedral, forming a three-dimensional shape with four faces. These geometries help predict the molecular shape and reactivity of compounds.

What is the difference between electron geometry and molecular geometry?

Electron geometry considers all electron groups around the central atom, including lone pairs and bonding pairs, to determine the spatial arrangement. Molecular geometry, on the other hand, only considers the arrangement of atoms (bonding pairs) and ignores lone pairs. For example, a molecule with 4 electron groups (including lone pairs) has a tetrahedral electron geometry, but if one of those groups is a lone pair, the molecular geometry would be trigonal pyramidal.

Why is understanding electron geometry important in chemistry?

Understanding electron geometry is crucial in chemistry because it helps predict the shape and reactivity of molecules. The spatial arrangement of electron groups around a central atom influences how molecules interact with each other, their physical properties, and their chemical behavior. This knowledge is foundational in fields like organic chemistry and biochemistry, where the shape of molecules affects their function and interactions.

How can you remember the different types of electron geometries?

A good way to remember the different types of electron geometries is by associating them with the number of electron groups. For 2 electron groups, think of a straight line (linear). For 3 electron groups, remember 'tri' means three, forming a trigonal planar shape. For 4 electron groups, 'tetra' means four, forming a tetrahedral shape. These mnemonics can help you quickly recall the geometries based on the number of electron groups.