5.1 VSEPR theory and molecular shapes

VSEPR theory predicts molecular shapes based on electron domain arrangements. It's all about minimizing repulsion between electron pairs around a central atom. This theory helps us understand how molecules look in 3D space.

Knowing molecular shapes is key to grasping chemical properties and reactions. We'll learn how to predict geometries, understand bond angles, and see how lone pairs affect molecular structure. It's like solving a puzzle of atomic Lego pieces!

Molecular Geometry Predictions

VSEPR Theory and Electron Domain Geometry

Top images from around the web for VSEPR Theory and Electron Domain Geometry

• 

  ilovechemistrysomuchthatitscrazy - home View original

  Is this image relevant?

• 

  Molecular Geometry | Boundless Chemistry View original

  Is this image relevant?

• 

  Molecular Structure and Polarity (4.6) – Chemistry 110 View original

  Is this image relevant?

• 

  ilovechemistrysomuchthatitscrazy - home View original

  Is this image relevant?

• 

  Molecular Geometry | Boundless Chemistry View original

  Is this image relevant?

1 of 3

Top images from around the web for VSEPR Theory and Electron Domain Geometry

• 

  ilovechemistrysomuchthatitscrazy - home View original

  Is this image relevant?

• 

  Molecular Geometry | Boundless Chemistry View original

  Is this image relevant?

• 

  Molecular Structure and Polarity (4.6) – Chemistry 110 View original

  Is this image relevant?

• 

  ilovechemistrysomuchthatitscrazy - home View original

  Is this image relevant?

• 

  Molecular Geometry | Boundless Chemistry View original

  Is this image relevant?

1 of 3

• VSEPR theory states electron domains around a central atom arrange to minimize electrostatic repulsion, resulting in a predictable geometry
• Electron domains include bonding pairs (shared electrons in a bond) and lone pairs (non-bonding valence electrons)
• The total number of electron domains (bonding pairs + lone pairs) determines the electron domain geometry
• Electron domain geometries for 2-6 domains:
  • Linear (2 domains)
  • Trigonal planar (3 domains)
  • Tetrahedral (4 domains)
  • Trigonal bipyramidal (5 domains)
  • Octahedral (6 domains)

Predicting Molecular Geometry

• Apply VSEPR theory to predict the geometry of molecules and ions based on the number of electron domains around the central atom
• Examples of molecules and their predicted geometries:
  • $BeF_2$: 2 bonding pairs, 0 lone pairs, linear geometry
  • $BCl_3$: 3 bonding pairs, 0 lone pairs, trigonal planar geometry
  • $CH_4$: 4 bonding pairs, 0 lone pairs, tetrahedral geometry
  • $PCl_5$: 5 bonding pairs, 0 lone pairs, trigonal bipyramidal geometry
  • $SF_6$: 6 bonding pairs, 0 lone pairs, octahedral geometry

Molecular Shape and Bond Angles

Determining Molecular Shape

• Molecular geometry is determined by the arrangement of atoms, not considering lone pairs
• Described by the positions of bonded atoms relative to the central atom
• Molecular geometries for 2-6 bonding pairs:
  • Linear (2 bonding pairs)
  • Trigonal planar (3 bonding pairs)
  • Tetrahedral (4 bonding pairs)
  • Trigonal bipyramidal (5 bonding pairs)
  • Octahedral (6 bonding pairs)

Bond Angles in Molecular Geometries

• Bond angles determined by the arrangement of electron domains and presence of lone pairs
• Ideal bond angles for common geometries:
  • Linear (180°)
  • Trigonal planar (120°)
  • Tetrahedral (109.5°)
  • Trigonal bipyramidal (90° and 120°)
  • Octahedral (90°)
• Examples of molecules and their bond angles:
  • $CO_2$: Linear, 180° bond angles
  • $SO_3$: Trigonal planar, 120° bond angles
  • $SiH_4$: Tetrahedral, 109.5° bond angles

Electron Domain vs Molecular Geometry

Differences Between Electron Domain and Molecular Geometry

• Electron domain geometry considers all electron domains (bonding and lone pairs)
• Molecular geometry only considers the arrangement of bonded atoms
• When there are no lone pairs, electron domain geometry and molecular geometry are the same
• Presence of lone pairs can cause molecular geometry to differ from electron domain geometry

Effect of Lone Pairs on Molecular Geometry

• Lone pairs repel more strongly than bonding pairs
• Presence of lone pairs changes molecular geometry from electron domain geometry
• Example: Tetrahedral electron domain geometry with one lone pair results in trigonal pyramidal molecular geometry

Lone Pairs and Molecular Geometry

Lone Pair Effects on Bond Angles

• Lone pairs occupy more space than bonding pairs and have a greater repulsive effect
• Compress bond angles more than bonding pairs
• As number of lone pairs increases, bond angles deviate more from ideal angles of electron domain geometry
• Increased repulsion from lone pairs causes the deviation

Examples of Lone Pair Effects

• Tetrahedral electron domain geometry with one lone pair:
  • Trigonal pyramidal molecular geometry
  • Bond angles less than 109.5° ($NH_3$)
• Tetrahedral electron domain geometry with two lone pairs:
  • Bent molecular geometry
  • Bond angles less than 109.5° ($H_2O$)
• Trigonal bipyramidal electron domain geometry with one or more lone pairs:
  • Can result in seesaw ($SF_4$), T-shaped ($ClF_3$), or linear ($I_3^-$) molecular geometries
  • Bond angles deviate from 90° and 120°