Molecular Geometry & VSEPR Theory Notes

Molecular Shape: Importance

• Molecular shape affects reactivity and melting/boiling points.

• Molecular shape can be predicted.

Determining 3D Shape

• Electrons (bonding and non-bonding) repel each other and are positioned as far apart as possible.

• The number of peripheral atoms influences 3D shape.

VSEPR Theory

• Predicts 3D shape based on electron-pair repulsion.

• Each electron pair (bonding or non-bonding/single, double & triple electron domain) = 1 electron domain.

• Electron Geometry: Number of electron domains.

Predicting 3D Shape

1. Identify all electron pairs (bonding & non-bonding).

2. Count electron domains around the central atom and arrange them to minimize repulsion.

3. Classify the molecular geometry (arrangement of bonding electron domains).

Key Definitions

• Electron Geometry: Distribution of electron domains.

• Molecular Geometry: Arrangement of atoms.

Impact of Non-bonding Electrons

• Non-bonding electrons repel bonding electrons, influencing molecular shape.

• Electron domain geometry and molecular domain geometry are not always the same.

Molecular Shape: 2 Electron Domains

• Geometry is always linear; minimizes repulsion.

Molecular Shape: 3 Electron Domains

• Electron domain geometry: Trigonal planar (120° bond angle).

  • All bonding: Molecular domain is trigonal planar.

  • One non-bonding: Molecular domain is bent.

Molecular Shape: 4 Electron Domains

• Electron domain geometry: Tetrahedral (109.5° bond angle).

  • All bonding: Molecular domain is tetrahedral.

  • One non-bonding: Molecular domain is trigonal pyramidal.

  • Two non-bonding: Molecular domain is bent.

Molecular Shape: 5 Electron Domains

• Electron Domain geometry: Trigonal bipyramidal.

  • Molecular geometries: Trigonal bipyramidal, seesaw, T-shaped, linear.

Molecular Shape: 6 Electron Domains

• All positions are equivalent in the octahedral domain.

  • Molecular geometries: Octahedral, square pyramidal, square planar.

Influence of Lone Pairs

• Lone pairs affect molecular shape, influencing physical & chemical properties.

Molecular Polarity

• Non-polar molecules:

  • Identical polar bonds pulling in opposite directions cancel out (e.g., $CO<em>2$, $BF</em>3$, $CH_4$).

• Polar molecules:

  • Polar bonds do not cancel out due to arrangement (e.g., $NH<em>3$, $H</em>2O$, $CH_3Cl$).

• Lone pairs repel other electrons more strongly than bonding pairs

Summary

• VSEPR predicts atom arrangement.

• Bonding and lone pairs influence molecular geometry.

• Bond polarity and shape predict molecular polarity.