Question

which of the following has zero dipole moment? also please explain to me how to determine the answer

a. \(NH_3\)
b. \(NO_2\)
c. \(PF_5\)
d. \(HCN\)

Answer 1

\[PF _{5}\] has a zero dipole moment as it doesn't have a lone pair of electrons. the geometry is trigonal bipyramidal. The axial P-F bonds "cancel" each other. The equatorial P-F bonds are oriented as trigonal planar, so those three (like in BF3) "cancel" as well.

Dipole moment of a compound is different from the dipole moment of a bond. You need to look at ALL bonds for the overall dipole moment of a molecule.

Answer 2

sorry im a bit slow...generally how do you know if something has zero dipole moment?

Answer 3

Ok, one of the best things to do is to draw each molecules lewis diagram and then figure out the shape of the molecule using the number of lone pair electron clouds and bond pair electron clouds around the central atom (you might want to draw the 3D structure of the molecule too)
So in the case of phosphorous pentafluoride when you draw the lewis structure there will be no lone pair electron clouds and 5 bond pair electron clouds which means it will have a trigonal bipyramid shape
With this structure you can reason that the forces applied to the central atom by the flourine atoms that go into and come out of the plane of the page (the dotted and bold lines) will 'cancel' each other out and the other 3 fluorine atoms will be around the phosphorous atom in a trigonal planar shape so the forces they apply will be 'cancelled' as well. This means that the net force on the central atom is zero, that the molecule as a whole will be non-polar and the molecule will have a zero dipole moment