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Molecular Properties
Key Concepts & Variables
| Term | Meaning |
|---|---|
| Bonding pairs | Pairs of electrons shared between two atoms to form a covalent bond |
| Lone pairs | Pairs of electrons on the central atom not involved in bonding (they push harder) |
| Bond angle | Angle between two bonds; lone pairs squeeze bond angles smaller |
| Polarity | Uneven electron sharing creates a dipole (positive and negative ends) |
| Tetrahedral | 4 electron pairs → 109.5° bond angle (e.g. CH4) |
| Trigonal planar | 3 electron pairs → 120° bond angle (e.g. BF3) |
| Bent / V-shape | 2 bonding + 2 lone pairs → ~104.5° (e.g. H2O) |
Lone pairs repel more strongly than bonding pairs, so they squash bond angles. Water is bent (104.5°) instead of linear because of its two lone pairs on oxygen.
About VSEPR Theory and Molecular Shape
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the 3D shape of molecules based on the idea that electron pairs around a central atom arrange themselves to minimise repulsion. Bonding pairs and lone pairs are both considered when determining the electron geometry, but the molecular shape describes only the positions of the atoms.
For example, water (H2O) has 4 electron pairs around oxygen (2 bonding, 2 lone), giving a tetrahedral electron geometry but a bent molecular shape with a bond angle of 104.5°. Understanding molecular shape is essential for predicting properties such as polarity, boiling points and solubility.