Electron Geometry: Videos & Practice Problems

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

Electron geometry is a fundamental concept in chemistry that focuses on the arrangement of electron groups around a central atom. These electron groups can include both bonding pairs (surrounding elements) and lone pairs of electrons, which are treated equivalently for the purpose of determining geometry. Understanding the different types of electron geometries is essential for predicting molecular shapes and behaviors.

When a molecule has 2 electron groups, such as in carbon dioxide (CO₂), the electron geometry is classified as linear. A helpful mnemonic is to remember that a straight line connects two points, reflecting the two electron groups.

For molecules with 3 electron groups, the geometry is termed trigonal planar. The prefix "tri-" indicates three, making it easier to recall. This configuration can arise from three surrounding atoms or a combination of two atoms and one lone pair.

When there are 4 electron groups, the geometry is referred to as tetrahedral. The prefix "tetra-" signifies four, which aligns with the total number of electron groups present.

In cases where there are 5 electron groups, the geometry is known as trigonal bipyramidal. This name can be remembered by breaking it down: "tri-" for three and "bi-" for two, which together sum to five.

Finally, when a molecule has 6 electron groups, the geometry is classified as octahedral. Although "octa-" suggests eight, it can be remembered through the analogy of an octopus, which has eight tentacles. By subtracting the two eyes, we arrive at six, correlating with the six electron groups.

In summary, recognizing the number of electron groups and their arrangement is crucial for understanding molecular geometry. Each geometry has a specific name that can be remembered through various mnemonic devices, aiding in the mastery of this essential chemistry concept.

Electron Geometry Example

The electron geometry of the hydrogen sulfide molecule (H₂S) is determined by the arrangement of electron groups around the central sulfur atom. In H₂S, sulfur is the central atom and is connected to two hydrogen atoms, while also having two lone pairs of electrons. This results in a total of four electron groups surrounding the sulfur atom.When analyzing the electron groups, it is important to note that both the bonded atoms (the two hydrogen atoms) and the lone pairs contribute to the overall geometry. With four electron groups, the geometry is classified as tetrahedral, which is derived from the arrangement of four groups in three-dimensional space.Thus, the electron geometry of hydrogen sulfide is tetrahedral, reflecting the spatial distribution of the electron groups around the sulfur atom.

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

Answer: (A)

Drawing Structures and Electron Geometry Example

In understanding the molecular structure of formaldehyde (CH₂O), it is essential to follow specific rules for drawing Lewis dot structures. The first step involves identifying the central atom, which is typically the least electronegative element. In this case, while hydrogen is the least electronegative, it cannot occupy the central position; thus, carbon is chosen as the central atom. Carbon, belonging to group 4A, has four valence electrons.

Hydrogens, which are in group 1A, can only form single bonds. Since carbon must form four bonds and already has two bonds with the two hydrogen atoms, it requires two additional bonds. The only remaining element is oxygen, which is in group 6A and has six valence electrons. To satisfy the bonding requirements, carbon forms a double bond with oxygen.

When analyzing the electron geometry, we observe that carbon is connected to three electron groups: two from the hydrogen atoms and one from the double bond with oxygen. The presence of three electron groups indicates a trigonal planar geometry. Therefore, the electronic geometry of the formaldehyde molecule is classified as trigonal planar, reflecting its molecular shape.

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

Answer: (B)

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

To determine the electron geometry of the molecule tellurium tetrabromide (TeBr₄), we start by analyzing the valence electrons of the central atom, tellurium (Te). Being in group 6A, tellurium has 6 valence electrons. Each bromine (Br), which is in group 7A, contributes 1 electron for bonding, resulting in 4 single bonds between tellurium and the four bromine atoms. This utilizes 4 of the 6 valence electrons from tellurium.

After forming these bonds, tellurium is left with 2 unpaired electrons, which exist as a lone pair. Therefore, the total number of electron groups around the tellurium atom is 5: 4 bonding pairs (from the Te-Br bonds) and 1 lone pair.

To visualize the electron geometry, we recognize that with 5 electron groups, the arrangement adopts a trigonal bipyramidal shape. This geometry is characterized by three equatorial positions and two axial positions, allowing for optimal spatial distribution of the electron groups to minimize repulsion according to VSEPR (Valence Shell Electron Pair Repulsion) theory.

In summary, the electron geometry for TeBr₄ is trigonal bipyramidal, reflecting the presence of 5 electron groups around the central tellurium atom.

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

Answer: (C)

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.