What is Molecular Polarity?
Molecular polarity describes whether a molecule has a net electric dipole moment. It depends on both the electronegativity differences in individual bonds and the overall 3D symmetry of the molecule. Polar molecules dissolve in polar solvents; nonpolar in nonpolar solvents.
A molecule is polar if it has a net dipole moment (asymmetric charge distribution). Polarity requires both polar bonds AND an asymmetric 3D geometry. Symmetry cancels dipoles; asymmetry adds them into a net dipole.
- •Bent geometry (asymmetric)
- •O is more electronegative than H
- •Net dipole moment ≠ 0
- •Dissolves in water, charged/ionic
- •Linear geometry (symmetric)
- •C–O bonds are polar individually
- •Dipoles cancel each other
- •Net dipole moment = 0
Step-by-step worked examples
Is HCl (hydrogen chloride) polar or nonpolar?
H and Cl have different electronegativities (H = 2.1, Cl = 3.0). The H–Cl bond is polar. Linear geometry with only 2 atoms → no way to cancel the dipole. Net dipole moment ≠ 0. HCl is POLAR.
Is CCl₄ (carbon tetrachloride) polar or nonpolar?
C–Cl bonds are polar (Cl is more electronegative). Tetrahedral geometry: 4 identical Cl atoms symmetrically arranged. Dipoles on all 4 C–Cl bonds cancel each other (vector sum = 0). Net dipole moment = 0. CCl₄ is NONPOLAR.
Is NH₃ (ammonia) polar or nonpolar?
N is more electronegative than H; N–H bonds are polar. Trigonal pyramidal geometry (3 H atoms + 1 lone pair on N). Asymmetric shape: dipoles on N–H bonds add up. The lone pair also points in a direction, adding to the dipole. Net dipole moment ≠ 0. NH₃ is POLAR.
Flashcards
Quick quiz
Q1.What determines if a molecule is polar?
Q2.Why is H₂O polar but CO₂ is nonpolar?
Q3.In NH₃, which direction does the dipole point?
Q4.A nonpolar molecule has…
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Common mistakes
Thinking polar bonds always = polar molecule. — Correct: Polar bonds are necessary but not sufficient. Symmetric geometry cancels dipoles (nonpolar).
Ignoring the role of lone pairs in creating dipoles. — Correct: Lone pairs affect molecular geometry and can contribute to net dipole (e.g., NH₃).
Assuming nonpolar means no electron attractive force. — Correct: Nonpolar means no NET dipole. Individual bonds are still polar; they just cancel.
Not checking molecular geometry before deciding polarity. — Correct: You must know both 3D shape and bond polarities to predict polarity.
FAQ
What is the relationship between polarity and solubility?
"Like dissolves like." Polar molecules dissolve in polar solvents (water); nonpolar in nonpolar (hexane, oil). Polarity determines intermolecular forces.
Can a symmetric molecule with polar bonds be nonpolar?
Yes. Perfect symmetry cancels all dipoles (net dipole = 0). Example: BF₃ has polar B–F bonds but is nonpolar (trigonal planar).
How do you measure polarity?
By dipole moment (measured in Debyes, D). Larger dipole moment = more polar. Nonpolar = 0 D.
Why does polarity matter?
Polarity determines solubility, boiling point, intermolecular forces (hydrogen bonding), and how molecules interact with solvents, membranes, and other molecules.