Solved: CHEMISTRY Question for NEET

NEET CHEMISTRYMedium

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

$NH_3 < BF_3 < NF_3 < H_2O$

$BF_3 < NF_3 < NH_3 < H_2O$

$BF_3 < NH_3 < NF_3 < H_2O$

$H_2O < NF_3 < NH_3 < BF_3$

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.

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, $\mu(NH_3) > \mu(NF_3)$ .

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).
Conclusion: The increasing order is $BF_3 (0) < NF_3 < NH_3 < H_2O$ .

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