Net Dipole Moment as the Vector Sum of Bond Dipoles
Chemistry · Chemical Bonding · NEET
A molecule has many polar bonds, and each polar bond is a tiny arrow called a bond dipole. The net (total) dipole moment of the molecule is the vector sum of all these bond arrows, so both their size AND their direction matter. Memory hook: think of each bond as a person pulling a rope. If they all pull in balanced opposite directions, the net pull is zero; if they pull the same way, the pulls add up.
Same two polar bonds, different shape: in linear CO2 the bond dipoles point opposite and cancel (net = 0), while in bent H2O they add up as vectors to give a net dipole of 1.85 D.
Your doubts, answered
Is dipole moment a vector or a scalar quantity?
Dipole moment is a VECTOR. It has both a magnitude (how big) and a direction. By convention the vector arrow points from the positive centre to the negative centre. Because it is a vector, you cannot just add the numbers of the bond dipoles; you must add them like arrows (vector addition), taking the angle between the bonds into account. This is why the shape of the molecule decides the answer.
How do I add two bond dipoles to get the net dipole moment?
Treat each bond dipole as an arrow. If two equal bond dipoles p make an angle theta between them, the resultant is given by the parallelogram law: net dipole = square root of (p^2 + p^2 + 2*p*p*cos theta). Two simple checks: if theta = 180 degrees (bonds point exactly opposite), cos 180 = -1 and the net dipole = 0. If theta = 0 (same direction), they add up fully to 2p.
Why does CO2 have zero dipole moment but water (H2O) does not?
Both have two polar bonds, but the SHAPES differ. CO2 is linear (O=C=O, angle 180 degrees), so the two C=O bond dipoles are equal and point exactly opposite; they cancel and the net dipole is 0. Water is bent (angle 104.5 degrees), so the two O-H bond dipoles do NOT cancel; they add to a net dipole of 1.85 D. Same bonds, different geometry, different result.
If all bonds are polar, why is the net dipole moment sometimes zero?
Because a polar bond only means each individual arrow is non-zero. The MOLECULE can still be non-polar if its shape is symmetric, so the arrows cancel out by vector addition. Examples: CO2 (linear), BF3 (trigonal planar), BeF2 (linear), CH4 and CCl4 (tetrahedral). Each has polar bonds but a symmetric shape, so net dipole = 0. This is a favourite NEET trap.
Does a lone pair also count as a dipole when I add up the arrows?
Yes. A lone pair on the central atom acts like an extra dipole pointing away from the atom, and it adds into the vector sum. This is why NH3 (lone-pair dipole and N-H dipoles point the same way, so they reinforce) has a higher dipole moment than NF3 (lone-pair dipole opposes the N-F dipoles, so they partly cancel), even though F is more electronegative than H.
How is the direction of a bond dipole decided?
The arrow points from the less electronegative atom toward the more electronegative atom, because the electron cloud is pulled toward the more electronegative side (the negative end). For example in H-F the arrow points from H to F. Note: in Lewis structures a crossed arrow is often drawn with the cross on the positive end, pointing to the negative end, which is the same direction as the actual dipole vector.
⚠️ The NEET trap ✗ BF3 has three polar B-F bonds, so it must have a non-zero dipole moment. ✓ BF3 is trigonal planar and fully symmetric, so the three equal B-F bond dipoles (at 120 degrees) cancel by vector addition and the net dipole moment is exactly zero. 🧠 Polar bonds do NOT guarantee a polar molecule. Always check the SHAPE first: symmetric shape means the arrows can cancel to zero.
Real NEET questions
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 net dipole needs a symmetric shape where bond dipoles cancel by vector addition. BF3 (trigonal planar), BeF2 (linear), CO2 (linear) and 1,4-(para)-dichlorobenzene are all symmetric, so their bond dipoles add up to zero. NF3, H2O, NH3, HF and 1,3-(meta)-dichlorobenzene are unsymmetrical, so they have a non-zero dipole moment. Only set (B) is fully symmetric.
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 its four bond dipoles cancel: net mu = 0 (smallest). For the rest, electronegativity difference and geometry give the size of the vector sum: HF (about 1.91 D) > NH3 (about 1.47 D) > H2S (about 0.97 D) > CH4 (0). So the order is HF > NH3 > H2S > CH4, option (B).
NEET 2024
Identify the correct answer.
A · BF3 has non-zero dipole moment
B · Dipole moment of NF3 is greater than that of NH3
C · Three canonical (resonance) forms can be drawn for CO3^2- ion ✓
D · Three resonance structures can be drawn for ozone (O3)
Solution: BF3 is trigonal planar and symmetric, so the bond dipoles cancel and mu = 0 (A is wrong). In NH3 the lone-pair dipole and the N-H bond dipoles point the same way and reinforce (higher mu), while in NF3 the lone-pair dipole opposes the N-F dipoles (lower mu), so NH3 > NF3 (B is wrong). CO3^2- has three equivalent canonical forms (C is correct). Ozone has two main resonance structures, not three (D is wrong).
Solved Chemical Bonding NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A bond dipole is the small dipole moment of one polar bond. It appears when two bonded atoms have different electronegativities, so the shared electrons sit closer to the more electronegative atom. It is drawn as an arrow pointing to the more electronegative (negative) end.
What is the formula to add two equal bond dipoles?
Use the parallelogram law: net dipole = square root of (p^2 + p^2 + 2*p*p*cos theta), where p is each bond dipole and theta is the angle between them. At theta = 180 degrees the net is zero; at theta = 0 the net is 2p.
Which shapes always give zero dipole moment?
Symmetric shapes with identical outer atoms and no reinforcing lone pair: linear (CO2, BeF2), trigonal planar (BF3), tetrahedral (CH4, CCl4), trigonal bipyramidal (PF5) and octahedral (SF6). Their bond dipoles cancel by symmetry.
Why is dipole moment important for NEET?
NEET regularly asks you to compare dipole moments or spot zero-dipole molecules (2019, 2020, 2023, 2024, 2025). To answer fast you must decide the shape, draw the bond arrows, and add them as vectors, not as plain numbers.
What unit is dipole moment measured in?
The Debye (D). 1 D = 3.33564 x 10^-30 coulomb metre. For example, water has a net dipole moment of 1.85 D.
