Shapes of Molecules - VSEPR Theory Summary

Linear Molecules

• Molecules made of one atom bonded to another are linear.

VSEPR Theory

• Valence-Shell Electron-Pair Repulsion (VSEPR) Theory predicts molecular shapes based on electron pair repulsion.
• Electron pairs occupy localized orbitals and arrange themselves to maximize distance between each other.

Arrangement of Electron Pairs

• Linear: 2 electron pairs (180°).
• Trigonal planar: 3 electron pairs (120°).
• Tetrahedral: 4 electron pairs (109.5°).

Methane Example

• Methane (CH₄) has a tetrahedral arrangement with 4 shared electron pairs.
• Bond angles are confirmed as 109.5°.

Shapes with Lone Pairs

• Lone pairs occupy larger orbitals and exert stronger repulsions.
• Ammonia (NH₃) has a trigonal pyramidal shape with bond angles of 107.3° due to one lone pair's repulsion.
• Water (H₂O) has an angular shape with bond angles of 104.5° due to two lone pairs pushing shared pairs together.

Molecules with Multiple Bonds

• In double bonds, two shared pairs count as one region; similarly for triple bonds.
• Formaldehyde (H₂C=O) has a trigonal planar shape with angles slightly less than 120° due to double bond repulsion.
• Carbon Dioxide (CO₂) is linear due to two double bonds, which support a linear arrangement.

Summary of Bonding Arrangements

• Molecular shapes are determined by electron pair arrangements:
• 2 electron pairs: Linear
• 3 electron pairs: Trigonal planar
• 4 electron pairs: Tetrahedral

Practice Problem

• Predict molecular shapes using VSEPR Theory for: a) C₂, b) NF₃, c) H₂S.