Dipole Moment

Question

Among BF₃, NF₃, and ClF₃, the correct order of increasing dipole moment is:

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1. **Analyse BF₃**: BF₃ is trigonal planar (sp² hybridised, no lone pairs). The three B–F bond dipoles are symmetrically arranged at 120° and cancel out completely. Dipole moment = **0 D**.
2. **Analyse NF₃**: NF₃ is trigonal pyramidal (sp³, one lone pair). Although each N–F bond is polar, the lone pair on nitrogen and the three bond dipoles point in similar directions (both away from N toward the base), so they partially oppose each other. Net dipole moment ≈ **0.23 D** — small but non-zero.
3. **Analyse ClF₃**: ClF₃ is T-shaped (sp³d, two equatorial lone pairs). The asymmetric geometry means bond dipoles do not cancel. The lone pairs reinforce the net dipole. Dipole moment ≈ **0.6 D**.
4. **Order**: $BF_3 (0) < NF_3 (0.23) < ClF_3 (0.6)$.
5. **Key Insight**: Molecular symmetry determines whether individual bond dipoles cancel. Higher asymmetry generally leads to larger net dipole moment.

Dipole Moment

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