Chapter 10.1 - VSEPR Theory

CHEM 1000H INTRODUCTORY CHEMISTRY I CHAPTER 10.1 - VSEPR THEORY

  • Instructor: Dr. Shannon Accettone

  • Semester: Fall 2024

MOLECULAR SHAPES

  • Molecular shape significantly influences chemical properties and reactivity.

  • Lewis structures help understand bonding but do not predict molecular shapes.

DETERMINING MOLECULAR SHAPE

  • Electron pairs, including lone pairs and bond pairs, repel each other due to Coulombic forces.

  • Predicted molecular shape is based on maximizing the distance between electron pairs to minimize repulsion.

ELECTRON DOMAINS

  • Electron domains refer to lone pairs or bonds around a central atom.

  • Each lone pair counts as one electron domain.

  • Multiple bonds (double/triple) count as a single electron domain.

VSEPR THEORY

  • VSEPR (Valence Shell Electron Pair Repulsion) theory aids in predicting molecular shapes based on electron domain arrangement.

  • The optimal arrangement minimizes repulsion between electron domains.

REPRESENTING 3D STRUCTURES

  • VSEPR theory helps visualize the 3D arrangement of electron domains.

ELECTRON DOMAIN GEOMETRY

  • Electron domain geometry is determined by the count of electron domains in the molecule's Lewis structure.

  • The geometry aims to keep electron domains as far apart as possible.

MOLECULAR GEOMETRY

  • Molecular geometry differs from electron domain geometry, focusing on the positions of only the atoms, excluding lone pairs.

PREDICTING MOLECULAR SHAPES USING VSEPR

  1. Draw the Lewis structure and count the electron domains around the central atom.

    • Each bond and nonbonding pair counts as one domain.

  2. Determine the electron domain geometry by arranging domains to minimize repulsions.

  3. Identify molecular geometry based on the arrangement of bonded atoms.

VSEPR NOTATION

  • Notation: AXmEn

    • A = central atom

    • X = bonded atoms

    • m = number of bonding electron domains (integer)

    • E = number of lone pairs on the central atom

    • Total electron domains = m + n.

2 ELECTRON DOMAINS (AX2)

  • Arrangement: Linear geometry

  • Angle: 180°

3 ELECTRON DOMAINS (AX3)

  • Geometry: Trigonal planar

  • Bond angle: 120°

  • Variations: AX2E (bent, <120°) based on lone pairs.

4 ELECTRON DOMAINS (AX4)

  • Geometry: Tetrahedral

  • Bond angle: 109.5°

  • Variations: AX3E (trigonal pyramidal) and AX2E2 (bent, <109.5°).

5 ELECTRON DOMAINS (AX5)

  • Geometry: Trigonal bipyramidal

  • Bond angles: 90° (axial), 120° (equatorial)

  • Variants include: AX4E (seesaw), AX3E2 (T-shaped), AX2E3 (linear).

6 ELECTRON DOMAINS (AX6)

  • Geometry: Octahedral

  • Bond angle: 90°

  • Variations include AX5E (square pyramidal) and AX4E2 (square planar).

IDEAL VS. NON-IDEAL BOND ANGLES

  • VSEPR provides reasonable geometry predictions, but bond angles can deviate due to electron density changes.

EFFECT OF MULTIPLE BONDS ON BOND ANGLES

  • Double/triple bonds exert greater repulsion than single bonds, influencing bond angles in a molecule.

EFFECT OF LONE PAIRS ON BOND ANGLES

  • Lone pairs are larger than bonding pairs, create greater repulsion leading to reduced bond angles.

BOND POLARITY AND DIPOLE MOMENTS

  • Polarity arises from differences in electronegativity, resulting in uneven electron sharing.

  • The overall polarity of a molecule depends on its molecular shape and vector addition of bond dipoles.

MOLECULAR SHAPE AND POLARITY

  • A polar molecule may have nonpolar bonds depending on symmetrical arrangements that can cancel dipoles.

  • Electron domain geometries indicate polarity but changes in atom types/molecular shapes can affect it.

LEARNING CHECKS

  • Engage with questions related to predictions of molecular geometry and understanding of bond polarity.