Question
Carbon dioxide (CO₂) has two polar C=O bonds. Yet its dipole moment is zero. Explain why CO₂ is nonpolar, and contrast this with water (H₂O), which has polar O-H bonds and a net dipole moment.
This exact conceptual question appeared in JEE Main 2023 and is a NEET favourite — high weightage in chemical bonding.
Solution — Step by Step
The electronegativity of oxygen (3.5) is much higher than carbon (2.5). This difference means each C=O bond has a dipole — electrons are pulled toward oxygen.
Each C=O bond dipole moment ≈ 2.3 D, pointing from C → O.
CO₂ is a linear molecule — bond angle 180°. The two C=O bonds point in exactly opposite directions along the same axis.
Both dipole vectors have equal magnitude but opposite direction.
Dipole moment is a vector quantity. We don’t add magnitudes — we add vectors.
The two equal and opposite vectors cancel completely. Net dipole = zero.
Water has a bent geometry — bond angle ≈ 104.5°. The two O-H dipoles do NOT point in opposite directions.
When we resolve the two O-H vectors, their resultant is a net dipole pointing from the hydrogen side toward oxygen. This is why water has μ = 1.85 D.
The rule: Polar bonds do not guarantee a polar molecule. Whether dipoles cancel depends entirely on molecular geometry.
Symmetric arrangements (linear, trigonal planar, tetrahedral with identical substituents) → dipoles cancel → nonpolar. Asymmetric arrangements → net dipole → polar.
Why This Works
Dipole moment is defined as , where is the charge separation and is the bond length. Since it’s a vector, direction matters as much as magnitude.
In CO₂, the molecule’s linear shape is forced by the absence of lone pairs on carbon (VSEPR: 2 bond pairs, 0 lone pairs → linear). This geometry is the sole reason the dipoles cancel. If CO₂ were bent — even slightly — it would have a net dipole.
Water, by contrast, has 2 lone pairs on oxygen that push the bond angle down from the ideal 109.5° to 104.5°. This bend is precisely what prevents cancellation. The lone pairs that make water “bent” are also what make it polar — and that polarity is why water is such an extraordinary solvent.
Quick geometry → polarity check for PYQs:
- Linear (2 identical groups): nonpolar — CO₂, CS₂, BeF₂
- Trigonal planar (3 identical groups): nonpolar — BF₃, BCl₃
- Tetrahedral (4 identical groups): nonpolar — CCl₄, SiF₄
- Bent, pyramidal, or asymmetric substituents: almost always polar
This pattern solves 80% of dipole moment MCQs in 10 seconds.
Alternative Method
We can use the bond moment cancellation rule algebraically.
For a molecule with bond angle between the two A-B bonds, the net dipole is:
For CO₂: , so .
For H₂O: , so .
This formula is especially useful when questions give you the bond angle and ask you to compare dipole moments across a series — a pattern common in JEE Advanced.
Common Mistake
“CO₂ has polar bonds, so it must be polar.”
This is the most common error on this question — students conflate bond polarity with molecular polarity. They’re different things.
Bond polarity depends on electronegativity difference alone. Molecular polarity depends on both electronegativity difference and geometry. Always check the shape before concluding about the molecule.
A symmetric arrangement of identical polar bonds always gives zero net dipole, no matter how polar each individual bond is.