AL chem 一問..

NF3 has a pyrimidal shape as N has a lone pair. Thus the structure is not planar. Since N-F bond are polar, the dipole moment cannot cancel out each other, resulting in a non-zero net dipole moment.

BF3 is planar oriented in a trigonal planar shape. The dipole moment of polar bonds cancel each other and hence the net dipole moment is zero

Since nitrogen is a group 5 element, it has 5 outermost electrons. After formation of 3 bonds with the fluoride, there will be one lone pair electron and this would make the molecule in trigonal pyrimidal shape due to the repulsion of the N-F bonds and teh lone pair. It therefore generated a dipole moment. However, in the case of boron, since boron is in group 3, it has only three outermost electron for bonding with the three fluorine atom. Since the resultant BF3 molecule contain no lone pair electron, the molecule is therefore in trigonal planar shape. The molecule is highly symmetric and the no dipole moment in BF3.