Determining Molecular Polarity and Net Dipole Moments
Molecular Geometry: The Deciding Factor for Molecular Polarity
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.
In 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.
In 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°.
- Bent (e.g., H₂O): Bond angle < 109.5° due to lone pairs.
Lone pairs exert stronger repulsion than bonding pairs, reducing bond angles and often resulting in asymmetrical shapes.
Net Dipole Moment: The Measure of Molecular Polarity
Dipole moment
A dipole moment describes the separation of electrical charge in a bond or molecule due to differences in electronegativity.
To evaluate a molecule's overall polarity:
- Assess Bond Dipoles: Determine the direction and magnitude of dipoles for each bond.
- Consider Geometry: Add the bond dipole vectors. If they cancel out, the molecule is non-polar; if they add up, the molecule is polar.
H₂O vs. CO₂
- H₂O: The $O–H$ dipoles point toward oxygen and do not cancel due to the bent geometry. Result: Net dipole moment ≈ 1.85 D (polar).
- CO₂: The $C=O$ dipoles cancel due to the linear geometry. Result: Net dipole moment = 0 D (non-polar).
- It’s a common mistake to assume that all molecules with polar bonds are polar.
- Always consider the molecular geometry to determine if dipoles cancel.
Applications of Molecular Polarity
- Solubility: "Like Dissolves Like"
- Polar molecules (e.g., H₂O) dissolve polar substances (e.g., NaCl, ethanol).
- Non-polar molecules (e.g., hexane) dissolve non-polar substances (e.g., oils, fats).
- Boiling and Melting Points:
- Polar molecules have higher boiling and melting points due to stronger intermolecular forces (e.g., hydrogen bonding in H₂O).
- Biological Relevance:
- Polarity governs molecular interactions in biological systems. For instance, water’s polarity makes it an excellent solvent for biochemical reactions.
- Why is H₂O polar while CO₂ is non-polar?
- How does molecular geometry influence dipole cancellation?
