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Despite F being more electronegative than H, μ(NH₃) > μ(NF₃) because:
ANH₃ has a tetrahedral shape, NF₃ does not
BIn NH₃, the lone-pair dipole reinforces (adds to) the resultant N–H bond moment; in NF₃ the lone-pair dipole opposes the resultant N–F bond moment
CNF₃ is non-polar
DNH₃ has stronger hydrogen bonding
Answer & Solution
Correct answer: B. In NH₃, the lone-pair dipole reinforces (adds to) the resultant N–H bond moment; in NF₃ the lone-pair dipole opposes the resultant N–F bond moment
Both molecules are pyramidal with a lone pair on N. In NH₃ (μ ≈ 1.47 D) the lone-pair orbital dipole is in the **same direction** as the N–H bond-moment resultant. In NF₃ (μ ≈ 0.23 D) the lone-pair dipole **opposes** the N–F bond-moment resultant, partially cancelling it.
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