Arrange the following species in increasing order of their dipole moments: $NH_3, NF_3, BF_3, H_2O$

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Accepted Answer

1. **Analyze $BF_3$:** Boron trifluoride has a trigonal planar geometry ($sp^2$ hybridisation). Due to its symmetry, the individual bond dipoles cancel each other out completely. 
   - $\mu = 0$.
2. **Analyze $NF_3$ vs $NH_3$:** Both have a trigonal pyramidal structure with one lone pair on Nitrogen.
   - In $NH_3$, the bond moments (N-H) point towards Nitrogen (N is more electronegative). The orbital dipole of the lone pair also points upwards. They reinforce each other (Vector addition).
   - In $NF_3$, the bond moments (N-F) point towards Fluorine (F is more electronegative). The orbital dipole points upwards. They oppose each other (Vector subtraction).
   - Therefore, $\mu(NH_3) > \mu(NF_3)$.
3. **Analyze $H_2O$:** Water has a bent structure with two lone pairs on Oxygen. The large electronegativity difference between O and H, combined with the contribution of two lone pairs, results in the highest dipole moment among the group (approx 1.85 D).
4. **Conclusion:** The increasing order is $BF_3 (0) < NF_3 < NH_3 < H_2O$.

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