Jason Cameron

VSEPR Theory and Molecular Geometry

3 min read

VSEPR Theory and Molecular Geometry

Introduction to VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) theory is a model used to predict the shapes of molecules based on the number of electron domains around a central atom.

Key concepts:

  1. Electron domains repel each other
  2. Molecules arrange themselves to minimize these repulsions
  3. The shape is determined by the number of bonding pairs and lone pairs of electrons

Electron Domains

Bonding Pairs

  • Shared pairs of electrons between atoms
  • Form covalent bonds

Lone Pairs

  • Unshared pairs of electrons on the central atom
  • Do not participate in bonding but influence molecular shape

AXE Notation

AXE notation is used to describe the arrangement of electron domains:

  • A: central atom
  • X: number of bonding domains (atoms bonded to the central atom)
  • E: number of lone pair domains on the central atom

Example: NH₃ has AX₃E notation (3 bonding pairs, 1 lone pair)

Molecular Shapes

The molecular shape depends on the total number of electron domains (X + E) and the number of lone pairs (E).

Linear (AX₂)

  • 2 bonding pairs, 0 lone pairs
  • 180° bond angle
  • Example: CO₂

Bent (AX₂E)

  • 2 bonding pairs, 1 lone pair
  • Approximately 119° bond angle
  • Example: H₂O

Trigonal Planar (AX₃)

  • 3 bonding pairs, 0 lone pairs
  • 120° bond angles
  • Example: BF₃

Trigonal Pyramidal (AX₃E)

  • 3 bonding pairs, 1 lone pair
  • Approximately 107° bond angles
  • Example: NH₃

Tetrahedral (AX₄)

  • 4 bonding pairs, 0 lone pairs
  • 109.5° bond angles
  • Example: CH₄

Trigonal Bipyramidal (AX₅)

  • 5 bonding pairs, 0 lone pairs
  • 90° and 120° bond angles
  • Example: PCl₅

Octahedral (AX₆)

  • 6 bonding pairs, 0 lone pairs
  • 90° bond angles
  • Example: SF₆

Effect of Lone Pairs on Molecular Shape

Lone pairs occupy more space than bonding pairs, leading to slight deviations from ideal geometries:

  1. Lone pairs repel more strongly than bonding pairs
  2. Presence of lone pairs decreases bond angles between bonding pairs
  3. Lone pairs are not visible in the overall molecular shape

Examples of lone pair effects:

  • H₂O: Bent shape due to lone pairs (AX₂E₂)
  • NH₃: Trigonal pyramidal shape due to one lone pair (AX₃E)

Exceptions to VSEPR Theory

While VSEPR theory is generally accurate, there are some exceptions:

  1. Multiple bonds: Treated as a single bonding domain but occupy more space
    Example: CO₂ is linear despite having double bonds

  2. d-orbital involvement: Some compounds with transition metals can have unusual geometries
    Example: [Cu(NH₃)₄]²⁺ is square planar instead of tetrahedral

  3. Very electronegative ligands: Can lead to deviations from expected geometries
    Example: ClF₃ has a T-shaped geometry instead of trigonal bipyramidal

Practice Problems

  1. Predict the molecular geometry of PCl₃.
  2. Draw the Lewis structure and determine the AXE notation for SO₂.
  3. Explain why the H-N-H bond angle in NH₃ is smaller than the H-C-H bond angle in CH₄.

Conclusion

VSEPR theory and molecular geometry are crucial for understanding the three-dimensional structure of molecules. This knowledge is fundamental in predicting and explaining molecular properties, reactivity, and behavior in various chemical and biological systems.

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