explain the dipole moment of NB3 is lower than NH3.

WELL THAT IS BECAUSE DIPOLE MOMENT IS A VECTOR QUANTITY AND HENCE RESULTANT DIPOLE MOMENT DEPENDS ON THE DIRECTIONOF INDIVIDUAL DIPOLE MOMENTS.

IN NH₃, NITROGEN IS much MORE ELECTRONEGATIVE(that they have greater electronegativity difference) THAN HYDROGEN SO THE EFFECTIVE DIPOLE MOMENT IS SAME AS THE DIRECTION IN WHICH THE 3 N-H DIPOLES.

IN NB3, NITROGEN IS AGAIN MORE ELECTRONEGATIVE BUT THE ELECTRONEGATIVITY DIFFERENCE IS LESS, hence the dipole moment would also be less because

 

HOPE THIS HELPS!

THANK YOU

@ mintu 
below arethe reasons 
1- both molecules are pyramidal in shape with aloan pairof electron in nitrogen atom .. 
2- the orbital dipole moment due to loan pair is in the same direction , as the resultant dipolemoment of the bonds . 
3- thirdly , the most electronegativity power of nitrogen will comes into play . 
HOPE IT CLEARS YOUR DOUBT 
ALL THE BEST ..