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Why BF3, CO2, BeF2 and CCl4 Have Zero Dipole Moment

Chemistry · Chemical Bonding · NEET

BF3, CO2, BeF2 and CCl4 have zero net dipole moment because their shapes are fully symmetric. Each bond IS polar, but the bond dipoles point in opposite (or evenly spread) directions and cancel out, so the total adds up to zero. Memory hook: "symmetric shape, dipoles cancel, mu = 0" — even polar bonds give a nonpolar molecule if the geometry is balanced.
Symmetric shape → bond dipoles cancel → net μ = 0CO2 (linear)OCOequal, opposite → cancelBF3 (trigonal planar)BFFF120° apart → cancelCCl4 (tetrahedral)CClClClCl109.5° even → cancelAll three: polar bonds, but net dipole moment = 0
Bond dipoles (blue arrows) exist in CO2, BF3 and CCl4 because each bond is polar, but the symmetric linear, trigonal planar and tetrahedral shapes make the arrows cancel, so the net dipole moment is zero.

Your doubts, answered

If B-F bonds in BF3 are polar, why is the dipole moment zero?

Each B-F bond IS polar because fluorine pulls electrons more than boron. So each bond has its own small arrow (bond dipole) pointing toward F. But BF3 is trigonal planar: the three B-F bonds spread out flat at exactly 120 degrees. The three equal arrows point away from the centre in a balanced star shape, so their vector sum is zero. Polar bonds + symmetric shape = zero net dipole. This is the exact trap NEET tests.

Is CO2 polar or nonpolar, and why?

CO2 is nonpolar overall, even though each C=O bond is polar. CO2 is a LINEAR molecule (O=C=O, 180 degrees). The two C=O bond dipoles are equal in size but point in exactly opposite directions. Two equal opposite arrows cancel, so net dipole moment = 0. Compare with water (H2O), which is BENT, so its two O-H dipoles do NOT cancel and water is polar.

Why is CCl4 dipole moment zero when C-Cl bonds are polar?

CCl4 is tetrahedral (109.5 degrees). All four C-Cl bonds are identical and point to the four corners of a tetrahedron, spread evenly in 3D. Because the four equal bond dipoles are symmetrically arranged around the carbon, they cancel completely, giving mu = 0. If you replaced one Cl with H (making CHCl3), the symmetry breaks and the dipole moment is no longer zero.

Why does BeF2 have no dipole moment?

BeF2 is a linear molecule (F-Be-F, 180 degrees). Beryllium has no lone pair on the central atom (incomplete octet, only 2 bond pairs). The two Be-F bond dipoles are equal and opposite, exactly like CO2, so they cancel and net mu = 0.

What is the difference between a bond dipole and the net dipole moment?

A bond dipole is the polarity of ONE bond, caused by the electronegativity difference between the two atoms. The net (molecular) dipole moment is the VECTOR SUM of all the bond dipoles plus any lone-pair dipole. A molecule can have polar bonds but still zero net dipole if the shape makes those bond dipoles cancel. Never judge molecular polarity from bonds alone — you must add them as arrows using the geometry.

How do I quickly tell if a molecule has zero dipole moment in the exam?

Check the shape. If the central atom has NO lone pair and all outer atoms are the SAME, the molecule is symmetric and mu = 0. This covers linear (BeF2, CO2), trigonal planar (BF3, SO3), tetrahedral (CCl4, CH4), and also para-dichlorobenzene. If there is a lone pair on the central atom (NH3, H2O, NF3) or the outer atoms differ (CHCl3), the shape is unsymmetrical and mu is NOT zero.

⚠️ The NEET trap
✗ BF3 has polar B-F bonds, so BF3 must be a polar molecule with a non-zero dipole moment.
✓ BF3 is trigonal planar and symmetric, so the three equal B-F bond dipoles cancel and the net dipole moment is ZERO — even though each bond is polar.
🧠 Polar bonds do NOT guarantee a polar molecule. Shape decides. Symmetric shape means the arrows cancel.

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: BF3 (trigonal planar), BeF2 (linear) and CO2 (linear) are all symmetric, and 1,4-(para)-dichlorobenzene is also symmetric, so in each case the equal bond dipoles cancel and net dipole moment = 0. The wrong options contain NF3, H2O, NH3, HF or 1,3-(meta)-dichlorobenzene, which are unsymmetrical and have non-zero dipole moments. Correct answer: 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: Option A is WRONG: BF3 is trigonal planar and symmetric, so its dipole moment is zero, not non-zero. Option B is wrong: in NH3 the bond dipoles and lone-pair dipole add up (higher mu) while in NF3 they oppose (lower mu), so mu(NH3) > mu(NF3). Option D is wrong: ozone has two main resonance structures. Option C is correct: CO3^2- has three equivalent canonical forms. Correct answer: C.

Solved Chemical Bonding NEET PYQs

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Frequently asked

Does zero dipole moment mean the bonds are nonpolar?

No. Zero dipole moment means the whole molecule is nonpolar. The individual bonds (B-F, C=O, Be-F, C-Cl) are still polar. The bond dipoles simply cancel because of the symmetric shape.

Which common molecules have zero dipole moment for NEET?

Learn these: BeF2, BeCl2, CO2, CS2 (linear); BF3, BCl3, SO3 (trigonal planar); CH4, CCl4, SiF4 (tetrahedral); PCl5 (trigonal bipyramidal); SF6 (octahedral); and para-dichlorobenzene, benzene, naphthalene. All are symmetric with no unbalanced lone pair.

Why is H2O polar but CO2 is not, when both are triatomic?

CO2 is linear (180 degrees) so its two C=O dipoles point opposite and cancel. H2O is bent (104.5 degrees) because oxygen has two lone pairs, so its two O-H dipoles do not cancel and add to give a net dipole of about 1.85 D.

Why does CCl4 have zero dipole moment but CHCl3 does not?

CCl4 has four identical C-Cl bonds arranged symmetrically in a tetrahedron, so the dipoles cancel. In CHCl3 one bond is C-H (different from C-Cl), so the symmetry is broken and the dipoles no longer fully cancel, giving a net dipole moment (about 1.04 D).

What is the unit of dipole moment?

Dipole moment is measured in the debye (D), where 1 D = 3.33 x 10^-30 coulomb metre. A value of 0 D means the molecule is nonpolar.

Related concepts:
Why NH3 has higher dipole moment than NF3Net dipole moment as vector sum of bond dipolesPolar vs nonpolar covalent bondOrder of dipole moment of common moleculesVSEPR shapes when central atom has no lone pair
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