Determining Molecular Polarity and Net Dipole Moments
Molecular Geometry: The Deciding Factor for Molecular Polarity
Definition
Molecular geometry
Molecular geometry is the three-dimensional arrangement of atoms in a molecule, determined by the repulsion between electron pairs around the central atom.
How Geometry Influences Polarity
- Even if a molecule contains polar bonds, its overall polarity depends on its molecular geometry, the 3D arrangement of atoms.
- Geometry determines whether the individual bond dipoles cancel out (non-polar molecule) or add up to form a net dipole moment (polar molecule).
Symmetrical Geometry:
In symmetrical molecules (e.g., CO₂, BF₃), the bond dipoles cancel each other out, leading to a non-polar molecule.
ExampleIn CO₂, the two polar C=O bonds are arranged linearly at 180°, so their dipoles point in opposite directions and cancel.
Asymmetrical Geometry:
In asymmetrical molecules (e.g., H₂O, NH₃), the bond dipoles do not cancel, resulting in a polar molecule.
ExampleIn H₂O, the bent shape means the O–H bond dipoles add up, creating a net dipole moment.
Using VSEPR Theory to Predict Geometry
- The Valence Shell Electron Pair Repulsion (VSEPR) model predicts molecular shapes by considering the repulsion between electron domains (bonding and lone pairs).
- Key geometries include:
- Linear (e.g., CO₂): Bond angle = 180°.
- Trigonal planar (e.g., BF₃): Bond angle = 120°.
- Tetrahedral (e.g., CH₄): Bond angle = 109.5°.
