Summary
The shapes of molecules are determined by the Valence Shell Electron Pair Repulsion (VSEPR) Theory, which predicts molecular geometry based on electron pair repulsion around a central atom.
- VSEPR Theory — a model used to predict the geometry of a molecule based on electron pair repulsion. Example: Methane (CH4) has a tetrahedral shape.
- Bond Angle — the angle between two covalent bonds from the same atom. Example: In water (H2O), the bond angle is 104.5°.
- Bond Length — the distance between the nuclei of two covalently bonded atoms. Example: Longer bonds are weaker than shorter ones.
- Tetrahedral Shape — a molecular shape where a central atom is surrounded by four atoms at 109.5° angles. Example: Methane (CH4).
- Trigonal Planar Shape — a molecular shape where a central atom is surrounded by three atoms at 120° angles. Example: Boron trifluoride (BF3).
- Linear Shape — a molecular shape where a central atom is surrounded by two atoms at 180° angles. Example: Carbon dioxide (CO2).
Exam Tips
Key Definitions to Remember
- VSEPR Theory
- Bond Angle
- Bond Length
Common Confusions
- Confusing bond angles with bond lengths
- Misidentifying molecular shapes based on the number of electron pairs
Typical Exam Questions
- What is the shape of methane (CH4)? Tetrahedral with bond angles of 109.5°
- How does VSEPR theory predict molecular shapes? By considering the repulsion between electron pairs around the central atom
- What is the bond angle in water (H2O)? 104.5°
What Examiners Usually Test
- Ability to predict molecular shapes using VSEPR theory
- Understanding of bond angles and their relation to molecular shapes
- Differences between bond length and bond angle