VSEPR Theory and Molecular Geometry - Chemistry

1. Fundamental Concepts

Definition: VSEPR Theory (Valence Shell Electron Pair Repulsion Theory) is used to predict the three-dimensional shape of molecules based on the idea that electron domains repel each other and arrange themselves as far apart as possible.
Electron Pairs: Valence electron pairs, whether bonding or non-bonding, will arrange themselves around the central atom to minimize repulsion.
Electron Geometry: The spatial arrangement of all valence electron pairs (bonding + lone pairs) around the central atom.
Molecular Geometry: The arrangement of atoms in space, determined by the number of bonding and non-bonding electron pairs around the central atom.

2. Key Concepts

Bonding Pairs (bp): Electron pairs shared between the central atom and surrounding atoms (e.g., single, double, or triple bonds are all counted as one bonding pair in VSEPR calculations).

Lone Pairs (lp): Non-bonding electron pairs on the central atom, which exert stronger repulsion than bonding pairs.

Repulsion Strength Order: Lone pair-lone pair (lp-lp) > Lone pair-bonding pair (lp-bp) > Bonding pair-bonding pair (bp-bp). (More lone pairs → smaller bond angles)

VSEPR Table:

3. Examples

Example 1 (Easy)

Problem: Which of the following molecules has a bent shape due to the presence of lone pairs on the central atom?
A. CO₂
B. H₂O
C. CH₄
D. BCl₃

Step-by-Step Solution:

Determine the molecular shape based on the electron pair arrangement.
A. CO₂ has no lone pairs and is linear.
B. H₂O has two lone pairs and is bent.
C. CH₄ has no lone pairs and is tetrahedral.
D. BCl₃ has no lone pairs and is trigonal planar.
Therefore, the correct answer is B.

Example 2 (Medium)

Problem: Determine the molecular geometry of a molecule with 5 bonding pairs and 1 lone pair around the central atom.
A. Trigonal bipyramidal
B. Octahedral
C. Square pyramidal
D. T-shaped

Step-by-Step Solution:

Determine total electron domains: 5 bonding pairs and 1 lone pair → Total electron domains = 6
Determine the molecular geometry: Square pyramidal (due to the one lone pair).
Therefore, the correct answer is C.

Example 3 (Hard)

Problem: Consider the three molecules CO₂, SO₂, and H₂O. Identify the molecular geometry of each. Rank them in order of increasing bond angle.

Step-by-Step Solution:

Structure analysis of CO₂: The central C atom forms 2 double bonds with O atoms. → Electron domains = 2 bonding pairs → Molecular geometry = linear → Bond angle: 180°
Structure analysis of SO₂: The central S atom forms 2 double bonds with O atoms, and has 1 lone pair. → Electron domains = 2 bonding pairs + 1 lone pair → Molecular geometry = bent → Bond angle: ≈119°
Structure analysis of H₂O: The central O atom forms 2 single bonds with H atoms, and has 2 lone pairs. → Electron domains = 2 bonding pairs + 2 lone pairs → Molecular geometry = bent → Bond angle: ≈104.5°
Conclusion: Molecular geometry: CO₂: Linear. SO₂: Bent. H₂O: Bent. Ranking of bond angles (increasing): H₂O < SO₂ < CO₂.

4. Problem-Solving Techniques

Draw Lewis Structures: Start by drawing the Lewis structure to identify the central atom and the number of bonding and non-bonding electron pairs.
Count Electron Domains: Count the total number of bonding and non-bonding electron pairs around the central atom. (Remember: Multiple bonds still count as one domain.)
Determine Electron Pair Geometry: Use the VSEPR table to determine the electron pair geometry based on the number of electron pairs.
Determine Molecular Geometry: Adjust the electron pair geometry to account for lone pairs and determine the molecular geometry.