Question

The N-F bond is more polar than N-H bond, but NF3 has smaller dipole moment than NH3. Explain the curious result.

NH₃: µ= 1.5 D, NF₃: µ= 0.2D

Short answer

N—F bond is more polar than N—H but, NF3 is less polar than NH3 because in NF3 individual polarity face towards F therefore net will cancel (since it is very less). But in NH3 N is more electronegative than H therefore this all add up and net is more.

Step-by-step solution

Step 1

Dipole moment in NH3 Dipole moment in NF3

In NH3, N is more electronegative than H. Therefore, N pulls the electrons from H towards itself and the direction of dipole moment due to the N-H bonds will be in the same direction as that of the lone pair of electrons on Nitrogen.

In NF3, F is more electronegative than N. Therefore, F atoms (all 3) pull the electrons towards themselves and the resultant moment is opposite to the directions of that of the lone pair of electrons on N atom. Therefore, the net dipole decreases(Dipole moment is a vector quantity).

NH3 dipole moment is much higher than that of NF3.