Basic Principles of VSEPR Theory

• Electron Repulsion: Electron pairs around a central atom will repel each other, and they arrange themselves in a way that minimizes this repulsion.

• Types of Electron Pairs:

  • Bonding Pairs: Electrons shared between atoms that form bonds.

  • Nonbonding (Lone) Pairs: Electrons not involved in bonding but located on the central atom.

• Effect of Lone Pairs: Lone pairs occupy more space than bonding pairs because they are only attached to one atom, which affects molecular geometry and bond angles.

Electron Geometry vs Molecular Geometry

Remember the Difference:

• Electron Geometry: Arrangement of ALL electron pairs (bonding + lone)

• Molecular Geometry: Arrangement of ATOMS only

Example: Water (H₂O)

• Electron Geometry: Tetrahedral (4 electron pairs)

• Molecular Geometry: Bent (only considering H-O-H arrangement)

Predicting Molecular Shapes Using VSEPR

Step 1: Draw the Lewis Structure

• Use all previous Lewis structure skills

• Include all bonding and lone pairs

• Check formal charges if needed

Step 2: Count Electron Pairs Around Central Atom

• Bonding pairs: Single, double, triple bonds (each counts as 1)

• Lone pairs: Nonbonding electron pairs

• Total electron pairs = bonding pairs + lone pairs

Step 3: Determine Electron Geometry

• Based on total number of electron pairs

• Describes arrangement of ALL electron pairs

Step 4: Consider Lone Pair Positions

• Lone pairs take up more space than bonding pairs

• Prefer positions with maximum space

Step 5: Determine Molecular Geometry

• Describes arrangement of ATOMS only

• Ignore lone pair positions for naming

Special Cases in VSEPR Theory

• Expanded Octets: Atoms in the third period or beyond (like phosphorus in PCl₅ or sulfur in SF₆) can have more than 8 electrons in their valence shells, leading to trigonal bipyramidal or octahedral geometries.

• Multiple Central Atoms: Larger molecules may have more than one central atom, each of which follows VSEPR theory (e.g., ethane, C₂H₆).