Ch. 9 Molecular Geometries (Mod)

Molecular Geometry and Bonding Theories

• Overview of Chapter 9 by James F. Kirby, Quinnipiac University, Hamden, CT.

Molecular Shapes

• Lewis Structures: Show bonding and lone pairs but do not depict the shape of molecules.

• Shape Determination: Lewis Structures assist in determining molecular shapes, particularly for small molecules with central atoms.

• Common shapes for two- and three-atom systems are discussed.

What Determines the Shape of a Molecule?

• Electron Repulsion: Electron pairs (bonding or nonbonding) repel each other.

• Prediction of Shapes: Shapes predicted by arranging electron pairs as far from each other as possible.

• VSEPR Model: Stands for Valence-Shell Electron-Pair Repulsion, a crucial model in predicting molecular shapes.

Electron Domains

• Definition: Directions to which electrons point; includes both bonding and lone pairs.

• Example: A central atom (A) with four electron domains.

VSEPR Model Explained

• Best Arrangement: The arrangement that minimizes the repulsions among electron domains.

• Analogy: Balloon analogy—maximizing distance between electron pairs to minimize repulsion.

Electron-Domain Geometries

• Counting Domains: To determine electron-domain geometries, count total lone pairs and bonds on the central atom.

• A table is provided showing geometries for 2-6 electron domains around a central atom.

Determining Molecular Geometries

• Electron-Domain to Molecular Geometry: After establishing electron-domain geometry, analyze bonded atoms to find the molecular geometry.

• Reference Tables (9.2 and 9.3) include potential molecular geometries based on electron domains.

Linear Electron Domain

• Molecular Geometry: Only linear geometry is possible; remains linear when there are only two atoms regardless of electron domains.

Trigonal Planar Electron Domain

• Possible Geometries:

  • Trigonal Planar: All domains are bonding.

  • Bent: One nonbonding pair present.

Tetrahedral Electron Domain

• Possible Geometries:

  • Tetrahedral: All bonding pairs.

  • Trigonal Pyramidal: One nonbonding pair present.

  • Bent: Two nonbonding pairs are present.

Nonbonding Pairs and Bond Angle

• Size of Nonbonding Pairs: Larger than bonding pairs, resulting in greater repulsive effects.

• Effect on Bond Angles: Compression of angles observed due to increased repulsions.

Multiple Bonds and Bond Angles

• Bond Type Effects:

  • Double/Triple Bonds: Have larger electron domains.

  • Repulsive Force: These bonds exert more force than single bonds, affecting bond angles.

Expanding Beyond the Octet Rule

• Breaking the Octet Rule: Some elements can make more than four bonds or have multiple electron domains.

• Result: Increased geometries such as trigonal bipyramidal (5 domains) and octahedral (6 domains).

Trigonal Bipyramidal Electron Domain

• Distinct Positions:

  • Axial Position: Vertical placements.

  • Equatorial Position: Horizontal placements.

• Lone Pairs: Prefer equatorial positions due to lesser repulsion.

Trigonal Bipyramidal Molecular Geometries

• Geometrical Variations: Four distinct molecular geometries available:

  • Trigonal Bipyramidal

  • Seesaw

  • T-shaped

  • Linear

Octahedral Electron Domain

• Uniform Positions: All positions in octahedral geometry are equivalent.

• Potential Geometries: Three molecular geometries:

  • Octahedral

  • Square Pyramidal

  • Square Planar