Dipole Moment: What It Is and How to Compare Molecules
Chemistry · Chemical Bonding · NEET
Dipole moment (symbol μ) measures how polar a molecule is. It depends on two things: how big each bond dipole is (electronegativity difference) AND the shape of the molecule (do the bond dipoles cancel or add up). If a molecule is symmetric, the bond dipoles cancel and μ = 0, so the molecule is non-polar. Memory hook: "Shape decides the fate" — even strong polar bonds give zero dipole if the shape is symmetric (like CO2, BF3, BeF2).
Left: symmetric molecules (CO2 linear, BF3 planar) have bond dipoles that cancel, so μ = 0 and they are non-polar. Right: bent H2O cannot cancel its dipoles, giving a net μ of 1.85 D. Shape, not just electronegativity, decides polarity.
Your doubts, answered
Why is the dipole moment of BF3 zero even though B-F bonds are polar?
Each B-F bond is strongly polar because fluorine pulls electrons much harder than boron. But BF3 is trigonal planar: the three B-F bonds point 120 degrees apart in a flat symmetric shape. When you add the three bond dipoles as vectors, the resultant of any two exactly cancels the third. So the net dipole μ = 0 and BF3 is non-polar. This is why 'shape decides the fate' — polar bonds can still give a non-polar molecule.
Why is NH3 dipole moment greater than NF3, even though F is more electronegative than H?
Both NH3 and NF3 are pyramidal and both have a lone pair on nitrogen. The lone pair itself makes a dipole (the orbital dipole). In NH3, N is more electronegative than H, so the N-H bond dipoles point toward N — the same direction as the lone-pair dipole. They ADD up, giving a large μ (about 1.47 D). In NF3, F is more electronegative than N, so the N-F bond dipoles point away from N — opposite to the lone-pair dipole. They partly CANCEL, giving a small μ (about 0.24 D). So μ(NH3) > μ(NF3). NEET loves this reversal.
How do I decide if a molecule has zero dipole moment?
Ask one question: is the molecule symmetric so that all bond dipoles cancel? Zero-dipole (non-polar) shapes with identical outer atoms: linear (CO2, BeF2, BeCl2), trigonal planar (BF3), tetrahedral (CH4, CCl4), trigonal bipyramidal (PCl5, SbCl5), octahedral (SF6), and para (1,4)-disubstituted benzene. If the molecule is bent, pyramidal, or has a lone pair breaking the symmetry (H2O, NH3, NF3, SO2, NO2), μ is not zero and it is polar.
What two factors control the size of the dipole moment?
Factor 1 is the electronegativity difference of the bonded atoms — bigger difference means a bigger bond dipole (so HF has a large μ). Factor 2 is the molecular geometry — the net μ is the vector sum of all bond dipoles, so the shape decides whether they add or cancel. You must check BOTH. Comparing dipole moments using only electronegativity is the most common NEET mistake.
Why is 1,4-dichlorobenzene non-polar but 1,3-dichlorobenzene polar?
In para (1,4)-dichlorobenzene the two C-Cl bond dipoles point in exactly opposite directions across the ring, so they cancel and μ = 0. In meta (1,3)-dichlorobenzene the two C-Cl dipoles are at an angle (120 degrees apart), so they do not cancel and there is a net dipole. Same atoms, different positions, different answer — position and symmetry matter.
⚠️ The NEET trap ✗ F is more electronegative than H, so NF3 must have a larger dipole moment than NH3. ✓ NH3 has the larger dipole moment. In NH3 the N-H bond dipoles and the lone-pair dipole point the same way and ADD; in NF3 the N-F bond dipoles oppose the lone-pair dipole and CANCEL. So μ(NH3) about 1.47 D > μ(NF3) about 0.24 D. 🧠 Electronegativity alone is a trap — the lone pair direction and shape decide the winner. NEET 2024 and 2019 Odisha both tested exactly this.
Real NEET questions
NEET 2019 (Odisha)
Which of the following is the correct order of dipole moment?
A · NH3 < BF3 < NF3 < H2O
B · BF3 < NF3 < NH3 < H2O ✓
C · BF3 < NH3 < NF3 < H2O
D · H2O < NF3 < NH3 < BF3
Solution: BF3 is trigonal planar and symmetric, so its bond dipoles cancel: μ = 0 (smallest). In NF3 the N-F bond dipoles oppose the lone-pair dipole, giving a small μ. In NH3 the N-H bond dipoles reinforce the lone-pair dipole, giving a larger μ. H2O is bent (104.5 degrees) with the largest μ (1.85 D). So BF3 < NF3 < NH3 < H2O.
NEET 2020
Which of the following set of molecules will have zero dipole moment?
A · Nitrogen trifluoride, beryllium difluoride, water, 1,3-dichlorobenzene
B · Boron trifluoride, beryllium difluoride, carbon dioxide, 1,4-dichlorobenzene ✓
C · Ammonia, beryllium difluoride, water, 1,4-dichlorobenzene
D · Boron trifluoride, hydrogen fluoride, carbon dioxide, 1,3-dichlorobenzene
Solution: Zero dipole needs a symmetric shape where bond dipoles cancel. BF3 (trigonal planar), BeF2 (linear), CO2 (linear) and 1,4-(para)-dichlorobenzene are all symmetric, so μ = 0 for each. The polar members in the other options — NF3, H2O, NH3, HF and 1,3-(meta)-dichlorobenzene — all have a non-zero dipole. Only option B has all four with μ = 0.
NEET 2023 (Phase 2)
The correct order of dipole moments for molecules NH3, H2S, CH4 and HF, is
A · NH3 > HF > CH4 > H2S
B · HF > NH3 > H2S > CH4 ✓
C · CH4 > H2S > NH3 > HF
D · H2S > NH3 > HF > CH4
Solution: CH4 is tetrahedral and symmetric, so μ = 0 (last). HF has the largest electronegativity difference and is a simple diatomic, so it has the highest μ (about 1.91 D). NH3 (about 1.47 D) is more polar than H2S (about 0.97 D) because N is more electronegative than S and the N-H lone-pair dipole reinforces it. Order: HF > NH3 > H2S > CH4.
Solved Chemical Bonding NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the product of the charge separation and the distance between the charges (μ = Q × d), measured in Debye (D). It tells you how polar a molecule is.
What is the unit of dipole moment?
The Debye (D). 1 D = 3.33564 × 10^-30 coulomb metre. The unit is named after chemist Peter Debye.
Does a molecule with polar bonds always have a dipole moment?
No. If the molecule is symmetric, the polar bond dipoles cancel and the net μ = 0. CO2, BF3, CCl4 and SF6 all have polar bonds but zero dipole moment.
Why is H2O bent and CO2 linear even though both are triatomic?
Water's oxygen has two lone pairs that push the O-H bonds down to a bent 104.5 degree shape, so its dipoles do not cancel (μ = 1.85 D). CO2 has no lone pairs on carbon, so it stays linear and its two C=O dipoles cancel (μ = 0). This shows shape controls the dipole moment.
How does dipole moment help predict shape for NEET?
If a molecule is known to have μ = 0, its shape must be symmetric (linear, trigonal planar, tetrahedral, etc.). This is a fast checking tool alongside VSEPR theory, which predicts the actual geometry from electron pairs.
