📚 Introduction to VSEPR Theory
VSEPR (Valence Shell Electron Pair Repulsion) theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. The central idea of VSEPR theory is that electron pairs surrounding an atom repel each other, and in order to minimize this repulsion, the electron pairs will arrange themselves as far apart as possible.
📐 Key Concepts of VSEPR Theory
- ⚛️ Electron Groups: Identify the number of electron groups (bonding pairs and lone pairs) around the central atom. A single, double, or triple bond counts as one electron group.
- ↔️ Repulsion: Electron groups repel each other. Lone pair-lone pair repulsion is stronger than lone pair-bonding pair repulsion, which is stronger than bonding pair-bonding pair repulsion.
- ✨ Molecular Geometry: The arrangement of atoms in space, determined by the number of bonding pairs.
- 🔮 Electron Geometry: The arrangement of electron groups around the central atom (bonding and non-bonding).
📝 Determining Molecular Shape
To predict the shape of a molecule using VSEPR theory, follow these steps:
- ✍️ Draw the Lewis structure of the molecule.
- 🔢 Count the number of electron groups around the central atom.
- 📐 Determine the electron geometry based on the number of electron groups (e.g., 2 groups = linear, 3 groups = trigonal planar, 4 groups = tetrahedral).
- 👁️🗨️ Determine the molecular geometry based on the number of bonding and lone pairs.
📊 Common Molecular Geometries
| Electron Groups |
Lone Pairs |
Electron Geometry |
Molecular Geometry |
Bond Angle(s) |
Example |
| 2 |
0 |
Linear |
Linear |
180° |
$CO_2$ |
| 3 |
0 |
Trigonal Planar |
Trigonal Planar |
120° |
$BF_3$ |
| 3 |
1 |
Trigonal Planar |
Bent |
<120° |
$SO_2$ |
| 4 |
0 |
Tetrahedral |
Tetrahedral |
109.5° |
$CH_4$ |
| 4 |
1 |
Tetrahedral |
Trigonal Pyramidal |
<109.5° |
$NH_3$ |
| 4 |
2 |
Tetrahedral |
Bent |
<109.5° |
$H_2O$ |
🌡️ Factors Affecting Bond Angles
- 👤 Lone Pairs: Lone pairs exert greater repulsive force than bonding pairs, reducing bond angles.
- ⚡ Electronegativity: Differences in electronegativity between the central atom and surrounding atoms can affect bond angles.
- ⚖️ Size of Atoms: Bulky atoms or groups attached to the central atom can increase bond angles due to steric repulsion.
🧪 Practice Quiz
Determine the molecular geometry and approximate bond angles for the following molecules:
- $BeCl_2$
- $PCl_3$
- $SF_6$
- $XeF_4$
- $H_2S$
Answers:
- Linear, 180°
- Trigonal Pyramidal, <109.5°
- Octahedral, 90°
- Square Planar, 90°
- Bent, <109.5°
