Dipole moment is a vector quantity. It is defined as the product of the magnitude of the charge and the distance between the centres of the positive and negative charges.
Dipole moment of
NF3 and
NH3 The dipole moment of
NH3 is larger than
NF3 because in
NH3, the dipole moment vector of the bond and the lone pairs are in the same direction, while in
NF3, molecule, the dipole moment vector of the lone pair and the bond pairs are opposite in direction.
Dipole moment of
H2O and
NH3 They both have dipole moment because they do not have regular geometries. Dipole moment of
H2O is more than
NH3 because
O is more electronegative than
N.
Dipole moment of
BF3 It is zero because it has symmetrical (triangular planar) structure.
Therefore, the correct order of dipole moments of molecules is
H2O‌ (III) ‌>NH3(‌ I ‌)>NF3(‌ IV) ‌)>BF3‌ (II) ‌