VSEPR Notes

VSEPR Model Overview

• Date of Focus: September 26, 2025

• Edition: Focus 2E

• Topics Covered:

  • VSEPR Model and Molecular Shapes

  • Basic VSEPR

  • Lone Pairs

  • Polar Molecules

  • Previous Topics: Properties of Bonds

VSEPR Model for Molecular Shape

• Definition: VSEPR stands for Valence Shell Electron Pair Repulsion Model, deriving directly from Lewis structures.

• Rule 1: Electrons repel each other.

  • Examples:

  • Beryllium Chloride (BeCl2) exhibits an incomplete octet.

  • Boron Trifluoride (BF3) also shows an incomplete octet.

  • Geometries:

  • Linear configuration (BeCl2, BF3)

  • Trigonal planar configuration.

  • Carbon Dioxide (CO2) is also linear due to its double bonds treated as a single unit.

VSEPR Rules and Shape Designations

• Electron Pair Configuration:

  • Identify the configuration with lone pairs (electron arrangement).

  • Name the shape excluding lone pairs (molecular shape).

• AXnEm Notation:

  • In this notation, 'A' stands for the central atom, 'X' indicates surrounding atoms, and 'E' represents lone pairs.

  • Example: Methane (CH4) has 4 substituents and a bond angle of 109.5°. The geometry is tetrahedral.

• Rule 3: Consider lone pairs when determining shape.

  • Configurations:

  • For AX4E0:

    • Tetrahedral (109.5°)

  • For AX3E1:

    • Trigonal pyramidal (e.g., NH3 with a bond angle of 107°)

  • For AX2E2:

    • Bent shape (e.g., H2O with a bond angle of 105°)

• Rule 4: Different types of repulsions:

  • Lone pair-lone pair > Lone pair-atom > Atom-atom.

• All configurations typically involve four things bound to the central atom.

More Than One Central Atom

• Guideline: Treat multiple central atoms independently.

• Example:

  • In acetaldehyde (CH3CHO), the lone pairs are not directly shown but are assumed in the Lewis structure alongside geometrical forms like tetrahedral and trigonal planar.

Geometries for Five Bonding Pairs

• Five Attachments to Central Atom:

  • Notation: AX5

  • Geometries and Angles:

  • Example: Phosphorus Pentachloride (PCl5) has a trigonal bipyramidal geometry with bond angles of 90° and 120°.

  • For AX4E:

  • Example: Sulfur Tetrafluoride (SF4), which has a see-saw geometry.

  • For AX3E2:

  • Example: Bromine Trifluoride (BrF3), which has a T-shaped geometry.

  • All of these configurations exhibit expanded valence.

Geometries for Six Bonding Pairs

• Six Attachments to Central Atom:

  • Notation: AX6

  • Geometries and Examples:

  • Sulfur Hexafluoride (SF6) has an octahedral shape, where all positions are equivalent.

  • Bromine Pentafluoride (BrF5) has a square pyramidal geometry.

  • Xenon Tetrafluoride (XeF4) exhibits a square planar configuration.

• In these cases too, all configurations feature expanded valence.

Identifying Polarity in Molecules

• Polar Bond Definition: A polar bond arises when electrons are not evenly distributed due to differing electronegativities among atoms.

• Polar Molecule Definition: An overall molecule is classified as polar if it has a non-zero dipole moment (where polar bonds do not cancel each other’s effects).

• Electronegativity Values:

  • Carbon (C) = 2.55

  • Oxygen (O) = 3.44

  • Hydrogen (H) = 2.20

• Example:

  • In water (H2O):

  • Dipoles do not cancel; therefore, the molecule is polar.

  • In carbon dioxide (CO2):

  • Dipoles cancel each other out due to linear shape, hence it is non-polar.

Review of CH4 Polarity

• Molecular Structure:

  • Tetrahedral geometry in CH4, capable of being described as fitting into a cube configuration.

• Electronegativity Analysis:

  • C (2.55) vs H (2.20) results in dipoles that cancel out, leading to non-polar molecule classification.

Practice Problems

• Example 1: Nitrite Ion (NO2-)

  • Write the Lewis structure:

  • Calculate VSEPR formula and molecular shape.

  • Identified Geometry: Bent shape with bond angle of ~120°.

  • Polarity Assessment: Yes, because the dipoles do not cancel out.

• Example 2: Xenon Tetroxide (XeO4)

  • Write Lewis structure:

  • Count Electrons: 8 (Xe), 24 (4O), resulting in 32 total.

  • Expand valence, check for formal charges.

  • VSEPR: AX4, shape: tetrahedral, bond angle: 109.5°.

  • Polarity Assessment: No, same reasoning as CH4, the dipoles cancel out.

• Example 3: Thionyl chloride (SOCl2)

  • Write Lewis structure:

  • Count Electrons: 6 (S), 6 (O), 14 (Cl), yields 26 total.

  • Assess molecular geometry using VSEPR notation: AX3E (trigonal pyramidal).

  • Bond Angle: Approximately 109.5° but may be slightly less.

  • Polarity Assessment: Yes, the dipoles do not cancel, thus classified as polar.