Easy VSEPR Geometry Practice Questions

Concept Explanation

VSEPR geometry, or Valence Shell Electron Pair Repulsion theory, is a chemical model used to predict the three-dimensional shapes of individual molecules based on the idea that electron pairs around a central atom will naturally arrange themselves as far apart as possible to minimize repulsion. By understanding how bonding pairs and lone pairs interact, students can determine the spatial arrangement of atoms, which is critical for predicting physical and chemical properties. To master this, you often need to start with Lewis Structure practice questions to correctly identify the number of electron domains.

The core principle of VSEPR is that all electron groups, whether they are single, double, or triple bonds, or non-bonding lone pairs, are negatively charged. Because like charges repel, these groups push away from each other. The specific geometry depends on the Steric Number, which is the sum of the number of atoms bonded to the central atom and the number of lone pairs on that central atom. For instance, according to LibreTexts Chemistry, a molecule with a steric number of 4 will adopt a tetrahedral electron-group geometry to maximize the distance between the four groups.

Key VSEPR Shapes for Beginners

Steric Number Lone Pairs Molecular Geometry Ideal Bond Angle 2 0 Linear 180° 3 0 Trigonal Planar 120° 4 0 Tetrahedral 109.5° 4 1 Trigonal Pyramidal < 109.5° 4 2 Bent < 109.5°

It is important to distinguish between electron geometry (the arrangement of all electron domains) and molecular geometry (the arrangement of only the atoms). While lone pairs affect the shape, they are "invisible" in the final molecular name. Learning these shapes is often the first step before moving on to VSEPR Geometry practice questions at a more advanced level or exploring Polarity Determination practice questions.

Solved Examples

The following examples demonstrate how to apply VSEPR theory step-by-step to determine molecular shapes.

Example 1: Beryllium Chloride (BeCl₂)

1. Identify the central atom: Beryllium (Be).

2. Count valence electrons: Be has 2, each Cl has 7. Total = 16.

3. Draw the Lewis structure: Be is in the center with two single bonds to Cl atoms. Be has no lone pairs (it is an octet exception).

4. Determine the steric number: 2 bonding atoms + 0 lone pairs = 2.

5. Apply VSEPR: Two groups are furthest apart at 180°. The geometry is Linear.

Example 2: Boron Trifluoride (BF₃)

1. Identify the central atom: Boron (B).

2. Count valence electrons: B has 3, each F has 7. Total = 24.

3. Draw the Lewis structure: B is in the center with three single bonds to F atoms. B has no lone pairs.

4. Determine the steric number: 3 bonding atoms + 0 lone pairs = 3.

5. Apply VSEPR: Three groups are furthest apart at 120°. The geometry is Trigonal Planar.

Example 3: Water (H₂O)

1. Identify the central atom: Oxygen (O).

2. Count valence electrons: O has 6, each H has 1. Total = 8.

3. Draw the Lewis structure: O is in the center with two single bonds to H atoms and two lone pairs.

4. Determine the steric number: 2 bonding atoms + 2 lone pairs = 4.

5. Apply VSEPR: The electron geometry is tetrahedral, but since there are two lone pairs, the molecular geometry is Bent.

Practice Questions

Test your knowledge with these easy VSEPR geometry practice questions. Try to determine the molecular shape for each molecule listed below.

1. What is the molecular geometry of Carbon Dioxide (CO₂)?

2. Determine the molecular shape of Methane (CH₄).

3. What is the geometry of the Ammonia (NH₃) molecule?

4. Identify the geometry of Sulfur Dioxide (SO₂), considering it has one lone pair on the sulfur atom.

5. Predict the shape of Carbon Tetrachloride (CCl₄).

6. What is the molecular geometry of the Nitrite ion (NO₂⁻)?

7. Determine the shape of Phosphorus Trichloride (PCl₃).

8. What is the geometry of the Silane molecule (SiH₄)?

9. Identify the shape of Boron Trichloride (BCl₃).

10. Predict the geometry of the Hydronium ion (H₃O⁺).

Answers & Explanations

Review the detailed explanations below to verify your logic for each practice question.

1. Linear: CO₂ has two double bonds and zero lone pairs on the carbon atom. This results in a steric number of 2, creating a straight-line arrangement with a 180° bond angle.

2. Tetrahedral: CH₄ has four single bonds and zero lone pairs on the carbon. With a steric number of 4, the hydrogen atoms spread out into a tetrahedron with 109.5° angles.

3. Trigonal Pyramidal: NH₃ has three bonding pairs and one lone pair. While the electron geometry is tetrahedral (steric number 4), the lone pair pushes the bonds down, creating a pyramid shape.

4. Bent: SO₂ has two bonding regions (double bonds) and one lone pair. For more information on how lone pairs affect geometry, see Khan Academy's VSEPR resources. The steric number of 3 leads to a bent shape.

5. Tetrahedral: CCl₄ consists of a central carbon bonded to four chlorine atoms with no lone pairs. This is a classic steric number 4 arrangement.

6. Bent: NO₂⁻ has two oxygen atoms bonded to nitrogen and one lone pair on the nitrogen. This results in a steric number of 3, leading to a bent molecular geometry.

7. Trigonal Pyramidal: PCl₃ is similar to ammonia; phosphorus has five valence electrons, uses three for bonding, and retains one lone pair.

8. Tetrahedral: Silicon is in the same group as carbon, so SiH₄ behaves exactly like CH₄ with four bonding pairs and no lone pairs.

9. Trigonal Planar: BCl₃ has three bonding pairs and no lone pairs on the boron atom, creating a flat triangle with 120° angles.

10. Trigonal Pyramidal: H₃O⁺ has three bonding pairs and one lone pair on the oxygen atom, making it structurally similar to ammonia.

Quick Quiz

Frequently Asked Questions

What is the difference between electron geometry and molecular geometry?

Electron geometry describes the arrangement of all electron domains (bonds and lone pairs) around a central atom, while molecular geometry describes only the spatial arrangement of the actual atoms. If there are no lone pairs on the central atom, the electron and molecular geometries are identical.

Why do lone pairs change the bond angles in a molecule?

Lone pairs occupy more space than bonding pairs because they are attracted to only one nucleus, whereas bonding pairs are shared between two. This increased volume allows lone pairs to exert a stronger repulsive force, pushing the bonding pairs closer together and reducing the bond angles.

How do you calculate the steric number?

The steric number is calculated by adding the number of atoms bonded to the central atom to the number of lone pairs residing on that central atom. This single number determines the base electron geometry of the molecule.

Can a molecule with double bonds still be linear?

Yes, molecules with double bonds can be linear if the central atom has a steric number of 2. For example, carbon dioxide (CO₂) has two double bonds and no lone pairs, resulting in a linear shape.

Does VSEPR theory predict the exact bond angles?

VSEPR theory provides ideal bond angles based on perfect symmetry, but real-world angles often deviate slightly due to differences in atom size and the presence of lone pairs. For example, while the ideal tetrahedral angle is 109.5°, the angle in water is approximately 104.5° because of lone pair repulsion.

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