Question

dipolemoment of BF3 is zero but NH3 is not zero . explain

Answer 1

in NH3 nitrogen has lone pair of electron due to presence of lone pairs there is a net dipole moment in NH3 and is polar

Answer 2

First of all you should know that dipole moment is a vector quantity ,and all the laws of vector will be followed
In BF3
Boron has no lone pair therefore the bond pair - bond pair repulsion is negligible ,and due to this the vector sum of the dipole moment it becomes zero
In NH3
Nitrogen has a lone pair and due to the presence of this lone pair a repulsion is caused which makes the geometry of the compound distorted. Hence the vector sum of the dipole moment not becomes zero .