The bond angle of a molecule is determined by its molecular geometry, which is in turn determined by the number of bonding and non-bonding (lone pairs) electron groups around the central atom. Here's a breakdown of how to find bond angles:
1. Determine the Lewis Structure
First, you need to draw the Lewis structure of the molecule. This shows how the atoms are connected and the locations of all bonding and non-bonding electron pairs. Accurate Lewis structures are crucial for predicting molecular geometry and thus bond angles.
2. Determine the Electron Geometry
The electron geometry refers to the arrangement of all electron groups (bonding and lone pairs) around the central atom. The number of electron groups determines the basic geometry. Here's a quick overview:
| Number of Electron Groups | Electron Geometry |
|---|---|
| 2 | Linear |
| 3 | Trigonal Planar |
| 4 | Tetrahedral |
| 5 | Trigonal Bipyramidal |
| 6 | Octahedral |
3. Determine the Molecular Geometry
The molecular geometry takes into account the positions of only the atoms in the molecule. Lone pairs influence the bond angles because they are more repulsive than bonding pairs. The presence of lone pairs can alter the ideal bond angles predicted by the electron geometry.
4. Predict the Bond Angles
Based on the molecular geometry, you can predict the approximate bond angles. Here's how different geometries affect bond angles:
- Linear: 180° (e.g., CO₂)
- Trigonal Planar: 120° (e.g., BF₃)
- Tetrahedral: 109.5° (e.g., CH₄)
- Bent (from Trigonal Planar): < 120° (e.g., SO₂, approximately 119°) - Lone pair repulsions reduce the angle.
- Trigonal Pyramidal (from Tetrahedral): < 109.5° (e.g., NH₃, approximately 107°) - Lone pair repulsions reduce the angle.
- Bent (from Tetrahedral): << 109.5° (e.g., H₂O, approximately 104.5°) - Two lone pairs cause a greater reduction in bond angle.
- Trigonal Bipyramidal and Octahedral: These geometries have multiple bond angles that depend on the arrangement of atoms and lone pairs. These arrangements and angles are more complex.
Important Considerations:
-
Lone Pair Repulsion: Lone pairs repel bonding pairs more strongly than bonding pairs repel each other. This reduces the bond angles in molecules with lone pairs.
-
Electronegativity Differences: Significant differences in electronegativity between the central atom and surrounding atoms can also affect bond angles slightly.
Example:
Let's consider water (H₂O).
- Lewis Structure: Oxygen is the central atom, bonded to two hydrogen atoms, with two lone pairs of electrons.
- Electron Geometry: Four electron groups around oxygen indicate a tetrahedral electron geometry.
- Molecular Geometry: The arrangement of only the atoms (two hydrogen atoms and the central oxygen) gives a bent shape.
- Bond Angle: Due to the two lone pairs, the H-O-H bond angle is compressed from the ideal tetrahedral angle of 109.5° to approximately 104.5°.
By following these steps, you can determine the bond angles in various molecules.
