JEE Main 2024 · 4 April · morning · Single correct · +4 / −1
Which one of the following molecules has maximum dipole moment?
- (a) CH₄
- (b) NF₃
- (c) NH₃
- (d) PF₅
Answer and solution
Answer (c) NH₃
Solution
Chemistry ma’am · 3 steps
Look at where the lone pair points: in NH₃ it adds to the bond dipoles, in NF₃ it fights them.
- Symmetric ones first. CH₄ is tetrahedral and PF₅ is trigonal bipyramidal. In both, the bond dipoles are arranged symmetrically and cancel, so the net dipole moment is zero.
- Same shape, different pull. NH₃ and NF₃ are both pyramidal with one lone pair on N. The lone pair's moment points away from N, towards the lone pair.
Molecule Bond dipoles point Lone pair moment Net dipole CH₄ outwards, symmetric none 0 NH₃ towards N (N is more electronegative than H) same direction as bond dipoles 1.47 D, they add NF₃ away from N, towards F opposite to bond dipoles about 0.23 D, they cancel PF₅ outwards, symmetric none 0 - So. Only in NH₃ do the lone pair and the bond dipoles reinforce each other, so NH₃ (c) has the maximum dipole moment.
The idea to keep. Symmetric shapes give zero dipole. For pyramidal molecules, check whether the lone pair moment points the same way as the bond dipoles or against them.
Answer from the NTA final answer key. Solution by PrepWiser.