VSEPR theory, which stands for Valence Shell Electron Pair Repulsion theory, explains molecular shape based on a simple idea:
Electron pairs around a central atom repel each other, and they arrange themselves as far apart as possible to minimize this repulsion.
Here’s how it works step by step:
- Count the valence electrons around the central atom.
- Determine the number of electron pairs (bonding and lone pairs).
- Predict the electron pair geometry by arranging these pairs to minimize repulsion.
- Determine the molecular shape based on how many of these pairs are bonding (to other atoms) and how many are lone pairs (non-bonding).
Key points:
- Lone pairs repel more strongly than bonding pairs, so they can slightly distort bond angles.
- The shape is named according to the positions of the atoms only, not the lone pairs.
Examples:
- CH₄ (methane): 4 bonding pairs, 0 lone pairs → tetrahedral shape.
- NH₃ (ammonia): 3 bonding pairs, 1 lone pair → trigonal pyramidal shape.
- H₂O (water): 2 bonding pairs, 2 lone pairs → bent shape.
So, VSEPR theory essentially predicts molecular shapes by considering repulsion between electron pairs in the valence shell of the central atom.