Dipole Moment
Hardchemistry
Among BF₃, NF₃, and ClF₃, the correct order of increasing dipole moment is:
Select the correct option:
Solution
Incorrect! Answer:
BF₃ < NF₃ < ClF₃
- 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.
- 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.
- 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.
- Order: BF3(0)<NF3(0.23)<ClF3(0.6).
- Key Insight: Molecular symmetry determines whether individual bond dipoles cancel. Higher asymmetry generally leads to larger net dipole moment.
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About This Question
- Subject
- chemistry
- Chapter
- chemical bonding and molecular structure
- Topic
- dipole moment
- Difficulty
- Hard
- Year
- 2025
Solution
Correct Answer:
BF₃ < NF₃ < ClF₃
- 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.
- 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.
- 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.
- Order: BF3(0)<NF3(0.23)<ClF3(0.6).
- Key Insight: Molecular symmetry determines whether individual bond dipoles cancel. Higher asymmetry generally leads to larger net dipole moment.
This hard difficulty chemistry question is from the chapter chemical bonding and molecular structure, covering the topic of dipole moment. It appeared in the 2025 exam.
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