VSEPR

September 26, 2025 Focus 2E: The VSEPR Model Today

Introduction

Topics Covered

• Basic VSEPR Theory

• Lone Pairs

• Polar Molecules

• Review of Properties of Bonds

VSEPR Model for Molecular Shape

Definition

VSEPR Model: The Valence Shell Electron Pair Repulsion Model, derived from Lewis structures. This model predicts molecular shapes based on the repulsion between electron pairs in the valence shell of central atoms.

Key Rules of VSEPR

1. Electrons repel each other: This fundamental rule explains how molecular shapes are determined.

   • Example: BeCl₂ and BF₃ are noted for showcasing incomplete octets with shapes like linear and trigonal planar respectively.

2. Multiple bond electrons: Electrons involved in multiple bonds are treated as a single unit for the purpose of determining geometry.

   • Example: CO₂ exhibits a linear shape due to this treatment.

More VSEPR Rules

Configurations and Shapes

• Determine the electron arrangement including lone pairs (denoted as AXnEm).

• Name the molecular shape excluding lone pairs.

Example: CH₄

• VSEPR Notation: AX₄ (4 substituents)

• Bond Angle: 109.5°

• Molecular Shape: Tetrahedral

Rule for Lone Pairs

3. Effect of Lone Pairs: Lone pairs exert a stronger repulsive force compared to bond pairs.

   • Examples:

     • NH₃ is AX₃E₁ and has a trigonal pyramid shape with bond angle of 107°.

     • H₂O is AX₂E₂ with a bent shape and bond angle of 105°.

4. Repulsion Order: The order of repulsions is as follows:

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

Summary

• Configurations with Four Tied Atoms: All discussed structures have four groups around the central atom.

More Than One “Central Atom?”

Treating Molecules Independently

• For molecules with more than one central atom, treat them independently using their Lewis structures.

  • Example: CH₃COH (acetaldehyde) has distinct VSEPR geometries.

More VSEPR Geometries

Five Bonds Around a Central Atom

• Configuration: AX₅

• Bond angles: 90° and 120°.

• Example: PCl₅ features a trigonal bipyramidal shape.

  • Central Atom Positions: Axial positions (top and bottom) vs. equatorial (three). Ensures clear geometric distinction.

  • Examples of Replacement: SF₄ (see-saw shape) and BrF₃ (T-shaped). Each configuration achieves an expanded valence.

Further VSEPR Geometries

Six Bonds Around a Central Atom

• Configuration: AX₆, indicating all positions of the shapes are equivalent.

• Example: SF₆ exhibits an octahedral configuration.

• Other Examples:

  • BrF₅ forms a square pyramid shape.

  • XeF₄ results in a square planar configuration.

  • All configurations discussed exhibit expanded valence.

Determining Polarity of Molecules

Concepts of Polarity

• Polar Bond: A bond is considered polar if electrons are not evenly distributed between atoms that have different electronegativities.

• Polar Molecule: An overall molecule exhibits a nonzero dipole moment when its polar bonds do not cancel each other out.

Electronegativity Values

• C (Carbon): 2.55

• O (Oxygen): 3.44

• H (Hydrogen): 2.20

Example Analysis

• Using modern conventions:

  • Polar molecules are indicated with δ- (negative dipole) and δ+ (positive dipole) notations, highlighting partial charges. Molecules like CH₄ seem nonpolar, while H₂O exhibits polarity due to lack of cancellation in dipole moments.

Clarifications on CH₄

• Behavior of CH₄ is analyzed:

• Electronegativity results lead to cancellation of dipole moments hence interpreted as non-polar despite having polar bonds. Spatial arrangement forms a tetrahedral structure.

Practice Problems

Problem Sets

For Molecule: NO₂⁻ (Nitrite)

1. Lewis Structure: Start with central N having less electronegativity, surrounded by O atoms.

2. Electrons Calculation: 5e⁻ (from N) + 2(6e⁻ from O) = 12e⁻.

3. Total with the extra charge: 18e⁻.

4. Formal Charges:

   • N does not have a complete octet, leading to resonance.

   • VSEPR Configuration: AX₂E leading to a bent shape with bond angles around 120°.

5. Polarity Assessment: Yes, it is polar due to non-cancellation of dipoles.

Problem for XeO₄

1. Lewis Structure: Central Xe and four oxygens yield 32 electrons.

2. VSEPR Configuration: AX₄ results in a tetrahedral molecular shape.

3. Bond Angles: Matches previous tetrahedral examples with 109.5° angles.

4. Polarity Assessment: Not polar, similar cancellation reasoning as CH₄.

Problem for SOCl₂

1. Lewis Structure: S central with 6e⁻ from S, 6 from O, and 14 from Cl brings total to 26 electrons.

2. VSEPR Configuration: AX₃E indicates a trigonal pyramid molecular shape.

3. Bond Angle Assessment: Similar to NH₃ at about 109.5° but slightly less.

4. Polarity Assessment: Yes, as dipoles do not cancel out.

Yes, you are correct for the most part. In the VSEPR notation $AXnMm$:$E$ represents a lone pair of electrons on the central atom. Each $E$ unit signifies one lone pair.
$m$ denotes the number of those

Determining whether a molecule is always polar or always nonpolar depends on its molecular shape and the distribution of electron density, which is heavily influenced by the presence of lone pairs and the symmetry of the molecule, assuming polar bonds are present.

Shapes That Are Always Nonpolar (assuming all peripheral atoms are identical):

• Linear (AX₂): If two identical atoms are bonded to a central atom, the bond dipoles cancel out (e.g., $CO₂$).

• Trigonal Planar (AX₃): If three identical atoms are bonded to a central atom in a flat, triangular arrangement, the bond dipoles cancel (e.g., $BF₃$).

• Tetrahedral (AX₄): If four identical atoms are bonded to a central atom, the symmetrical 3D arrangement causes dipoles to cancel (e.g., $CH₄$, $CCl₄$).

• Trigonal Bipyramidal (AX₅): If five identical atoms are symmetrically arranged around a central atom, bond dipoles cancel (e.g., $PCl₅$).

• Octahedral (AX₆): If six identical atoms are symmetrically arranged around a central atom, bond dipoles cancel (e.g., $SF₆$).

• Square Planar (AX₄E₂): Even though there are lone pairs, if the four atoms are identical and arranged in a perfect square plane, the lone pairs are opposite each other, and the bond dipoles also cancel out (e.g., $XeF₄$).

Shapes That Are Always Polar (even with identical peripheral atoms, due to inherent asymmetry or lone pairs):

• Bent (AX₂E₁ or AX₂E₂): The presence of one or two lone pairs creates an asymmetrical distribution of electrons, preventing the bond dipoles from canceling (e.g., $SO₂$, $H₂O$).

• Trigonal Pyramidal (AX₃E₁): A single lone pair pushes the bonded atoms downwards, creating a pyramid shape and an uneven distribution of charge (e.g., $NH₃$, $SOCl₂$).

• See-Saw (AX₄E₁): The single lone pair and four bonded atoms result in an asymmetrical geometry, meaning dipoles will not cancel (e.g., $SF₄$).

• T-shaped (AX₃E₂): Two lone pairs and three bonded atoms create an asymmetrical 'T' shape that prevents dipole cancellation (e.g., $BrF₃$).

• Square Pyramidal (AX₅E₁): The single lone pair on one side of the central atom guarantees an uneven charge distribution, making the molecule polar (e.g., $BrF₅$).

It's crucial to remember that if the peripheral atoms are not identical, even the inherently nonpolar shapes (like tetrahedral) can become polar because the bond dipole strengths will differ, leading to a net dipole moment.