Read Molecular Symmetry Before Predicting Polarity
A molecule is nonpolar when its individual bond dipoles cancel by symmetry, even if the individual bonds are themselves quite polar — polarity is a vector-sum question, not a 'does it have an electronegative atom' question.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Symmetric shapes cancel bond dipoles; asymmetric ones don't
XeF₄ (square planar) and PF₃Cl₂ vs PCl₃F₂ comparisons hinge entirely on which specific positions the more electronegative atoms occupy — a trigonal bipyramidal molecule with all-axial or all-equatorial identical substituents can be nonpolar, but mixing substituent positions breaks that symmetry and gives a net dipole.
The NH₃ vs NF₃ dipole moment reversal
NH₃ has a larger dipole moment than NF₃ despite F being more electronegative than H, because the lone pair's own contribution to the molecular dipole points the SAME direction as the N-H bond dipoles in NH₃, but OPPOSITE to the N-F bond dipoles in NF₃ — the lone pair's contribution can reinforce or oppose the bond dipoles depending on the substituent.
Ranking dipole moment magnitude across a substituted series
For a series like CH₃F/CH₃Cl/CH₃Br (or similarly substituted molecules), dipole moment doesn't simply track electronegativity of the halogen — bond length and the resulting charge separation both matter, so CH₃Cl can have the largest dipole moment in some such series even though F is more electronegative than Cl, because the C-Cl bond dipole benefits from a longer bond length despite lower electronegativity difference.
OlympIQ exam lens
- ✓Polarity depends on the vector sum of bond dipoles, not on whether polar bonds are present.
- ✓A molecule with a rotational/reflective symmetry axis that maps each bond dipole onto an equal-and-opposite partner is nonpolar.
- ✓The lone pair's own dipole contribution can add to or subtract from bond dipoles — this is why NH₃ > NF₃ in dipole moment despite F being more electronegative than H.
- ✓Dipole moment magnitude in a substituted series depends on both electronegativity difference AND bond length — don't rank by electronegativity alone.
Don't assume the more electronegative substituent always gives the larger dipole moment — check whether the lone pair's contribution reinforces or opposes the bond dipoles (as in the NH₃ vs NF₃ reversal), and remember bond length affects the dipole too.
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