Question

Bf3 is nonpolar but nf3 is polar why

Answer 1

It depends how you look at it. The electronegativies of boron, nitrogen and fluorine are roughly 2, 7 and 9 so an individual B-F bond is more polarized than an individual N-F bond. However, the shapes of NF3 and BF3 are different. NF3 is pyramidal and hence has a net dipole moment whereas BF3 has a symmetrical trigonal planar geometry and the dipole moments cancel out to zero.

Answer 2

BF3is nonpolar as it has trigonal planar structure(sp2hybridization) which is symetrical in all respects bonded electrons are situtaed at vertices of equilateral triangle so resultant dipole is 0 but NF3is pyramidal fluorine being more electronegative its resulatant dipole is downwards hence not 0