What is VSEPR Theory?

The VSEPR (Valence Shell Electron Pair Repulsion) Theory is a model used to predict the shape of molecules based on the repulsion between electron pairs in the valence shell of atoms. According to this theory, electron pairs (both bonding and lone pairs) will arrange themselves as far apart as possible to minimize repulsion, resulting in specific molecular shapes.

Shapes of Molecules: CH₄, NH₃, and H₂O

1. CH₄ (Methane)

• Electron Pairs: CH₄ has 4 bonding pairs and no lone pairs around the central carbon atom.
• Shape: According to VSEPR theory, these 4 bonding pairs will arrange themselves in a tetrahedral shape to minimize repulsion, with bond angles of 109.5°.
• Illustration: The shape of CH₄ is like a pyramid with the carbon atom at the center and the hydrogen atoms at the corners.

2. NH₃ (Ammonia)

• Electron Pairs: NH₃ has 3 bonding pairs and 1 lone pair of electrons on the nitrogen atom.
• Shape: The 3 bonding pairs arrange themselves in a trigonal pyramidal shape, while the lone pair pushes the bonds down, resulting in bond angles of about 107°.
• Illustration: NH₃ resembles a pyramid with nitrogen at the top and hydrogen atoms at the base, with the lone pair exerting repulsion from the top.

3. H₂O (Water)

• Electron Pairs: H₂O has 2 bonding pairs and 2 lone pairs on the oxygen atom.
• Shape: The 2 bonding pairs and 2 lone pairs arrange in a bent or angular shape, with bond angles of approximately 104.5° due to the lone pair repulsion.
• Illustration: H₂O has a “V” shape, with the oxygen atom at the vertex and the hydrogen atoms at the ends.

Summary

• CH₄: Tetrahedral shape.
• NH₃: Trigonal pyramidal shape.
• H₂O: Bent shape. These shapes are determined by the number of bonding and lone electron pairs in each molecule according to the VSEPR theory.