Dipole Moment & Molecular Structure

CompleteFundamentals of Chemistry

Dipole moment measures charge separation in a bond and tells us whether a molecule is polar, non-polar, or even bent vs linear.

Full notes

Definition: Dipole moment (μ) is the product of the magnitude of charge (q) and the distance (d) between the charges: μ = q × d. It is measured in Debye units (D), where 1 D = 3.336 × 10⁻³⁰ C·m.

1. How a dipole arises

A covalent bond between two different atoms is polar because the more electronegative atom pulls the shared electron pair closer. This produces partial charges: δ+ on the electropositive atom and δ− on the electronegative atom. The vector sum of all bond moments in a molecule gives the net dipole moment.

2. Predicting molecular shape

A molecule with polar bonds may be non-polar overall if the bond dipoles cancel by symmetry. Examples: CO₂ (linear, O=C=O) has μ = 0 because the two C=O bond moments point in opposite directions. BF₃ (trigonal planar) and CCl₄ (tetrahedral) also have μ = 0. In contrast, H₂O (bent, 104.5°) has μ = 1.85 D and NH₃ (trigonal pyramidal) has μ = 1.47 D because their bond moments do not cancel.

Molecule	Shape	μ (D)
CO₂	Linear	0
BF₃	Trigonal planar	0
CCl₄	Tetrahedral	0
H₂O	Bent	1.85
NH₃	Trigonal pyramidal	1.47
HCl	Diatomic	1.08

3. Percentage ionic character

From the observed dipole moment we can estimate how ionic a bond is. For HCl, if the bond were fully ionic, μ would be e × d ≈ 6.1 D. The observed value is 1.08 D, so % ionic character ≈ (μ observed / μ ionic) × 100 ≈ 17–20%. This shows HCl is a polar covalent bond, not a perfect ion pair.

Note: Exam point: dipole moment is a vector — write the μ = q × d formula and a shape-based argument (symmetry cancellation) for any molecule asked.

Exam notes

Definition → μ = q × d (Debye). Vector quantity; net μ = vector sum of bond moments.

Non-polar despite polar bonds when symmetry cancels moments: CO₂, BF₃, CCl₄, CH₄, SF₆.
Polar when shape is asymmetric: H₂O, NH₃, HCl, CHCl₃.
% ionic character = (μ_obs / μ_ionic) × 100, μ_ionic = e × bond length.

Answer structure: definition → μ = q×d → effect of symmetry (examples CO₂ vs H₂O) → one numerical illustration → conclusion.

Important points

Dipole moment is a vector; direction is from δ+ to δ−.
Greater the difference in electronegativity, greater the bond moment, but shape decides net μ.
μ is used to distinguish o/m/p isomers and cis/trans isomers.
Benzene (μ = 0) vs phenol (μ ≈ 1.7 D).

Common mistakes

Saying CO₂ is polar because each C=O bond is polar — the moments cancel.
Forgetting units (Debye) in the final answer.

Next van der Waals Forces & Ion-Dipole Interactions