Question 1

What does the crossed arrow in a bond dipole mean?

Accepted Answer

In the chemistry convention, the crossed end marks the δ+ side and the arrowhead points toward the δ− side. The arrowhead is placed toward the more electronegative atom in that bond.

Question 2

Why can a molecule with polar bonds have zero dipole moment?

Accepted Answer

Molecular dipole moment is a vector sum, not an arithmetic sum of bond-dipole magnitudes. In linear CO₂, the two equal bond dipoles point oppositely; in BF₃ and tetrahedral CH₄ or CCl₄, symmetry makes all bond dipoles cancel exactly.

Question 3

Why are H₂O and NH₃ polar molecules?

Accepted Answer

H₂O is bent, so its two O–H bond dipoles do not oppose each other and add toward oxygen. NH₃ is trigonal pyramidal, so its three N–H bond dipoles also add toward nitrogen, producing a nonzero resultant.

Question 4

How should the dipole moments of NH₃, H₂O and CH₃Cl be compared?

Accepted Answer

The exact order is NH₃ at 1.47 D, H₂O at 1.85 D and CH₃Cl at 1.87 D. The H₂O–CH₃Cl difference is only 0.02 D, and CH₃Cl must be evaluated from the full vector sum of its C–Cl and C–H bond dipoles.

Question 5

Does zero molecular dipole mean every bond is nonpolar?

Accepted Answer

No. CO₂, BF₃, CH₄ and CCl₄ contain polar individual bonds in the dipole model, but their geometry causes the vector resultants to cancel. A zero molecular resultant has no direction even though the individual crossed bond arrows remain visible.

Dipole Moment Lab — Bond Polarity and Molecular Shape Decide Polarity

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