Consider the structure of
BF3 as shown below.
BF3 has zero dipole moment Consider the structure of
NH3 as shown below.
In
NH3,N is more electronegative than
H . The direction of moment is same due to N-H bond as that of the lone pair of electrons on nitrogen.
Consider the structure of
NF3 as shown below,
F atom is more electronegative than
N, so all
F atoms pull the electron towards themselves. Therefore, the resultant dipole moment is just opposite to the direction of the lone pair. Therefore, the order of the dipole moment is,
NH3>NF3>BF3