Answer:
c. PF₅
Explanation:
The dipole moment of a molecule is the sum of the moments of the bonds, which are vectors, so to know its value it's important to know the orientation of the molecule or its geometry.
The moment represents the difference of electronegativity of the atoms when it is close, the moment is considered 0, and the bond is called nonpolar. When there's a huge difference in electronegativity, the moment is different from 0, and the bond is polar.
In the molecule of NH₃, the central atom (N) has five valence electrons, and only 3 electrons are being shared (1 with each H), so there's a lone pair at the central. According to the VSPER theory, this lone pair and the bonding pair will repulse, so the geometry will be pyramidal. Nitrogen and Hydrogen have a huge difference in electronegativity, and because of the geometry, the moments are not canceled, and the dipole moment is different from 0.
NO₂ has a similar characteristic. The central atom, N, will have a lone pair of electrons, so the geometry here will be angular. Because of the difference of electronegativity of the elements, the bonds are polar, and because of the geometry the moment is not canceled, then, the dipole moment is different from 0.
In PF₅, the central atom P has 5 valence electrons and shares all of them ( 1 with each F), so there are no lone pairs. Because of that, it has a bipyramidal geometry, and although there's a difference of electronegativity, the moments are canceled, and the dipole moment is 0.
The molecule of HCN has linear geometry, but the moments are not canceled, because has different values. The bond between C and H is almost nonpolar, and the bond between C and N has a huge moment, so the dipole moment is different from 0.
