Of `BF_(3) " and " NF_(3)`, dipolemoment is observed for `NF_(3)` and not for `BF_(3)`. Why?

Explanation

In polyatomic molecules the dipolemoment of the molecule is the resultant of vector sum of the dipolemoment of various bonds.
`BF_(3)` molecule has symmetric planar triangular structure. The threee equal bond dipoles point to the corners of a triangle at an angle of `120^(@)1` and cancel the effect on each other.

The three bond moments give a net sum of zero as the resultant of any two is equal and opposite to the third.

In `NF_(3)` the three N - F bonds have dipolemoments. Also the lone pair present on nitrogen atom show dipolemoment in the opposite direction to the resultant N - F bond dipolemoments.
Though the orbital dipolemoment due to lone pair and the resultant dipole moment of three N - F bonds are in opposite direction there remains some resultant dipolemoment since they do not cancel completely.