CHEM 110-Debye
1/30/2003
Polar Covalent Bonds: Bond Dipole Moments

Dipole Moment = (magnitude of charge separated) x (distance of separation)

The amount of charge that is separated within a polar covalent bond due to the unequal sharing of the bonding electrons depends in an extremely complicated manner on the electronegativity difference between the two bonding partners. Qualitatively, the greater this difference in electronegativities, the larger the magnitude of the charge separated within the covalent bond. The bonding partner with the higher electronegativity displays the extra negative charge (negative end because its nucleus is now surrounded by slightly more lectrons than it contains protons); the bonding partner with the lower electronegativity suffers the lack of negative charge (positive end because the charge of its nucleus is no longer completely shielded). The distance of charge separation depends primarily on the length of the bond and this bond length is affected strongly by the atomic radii of the bonding partners.

In the calculation of the dipole moment, the charge separation acts as a scaling factor for the electronegativity differences of the bonding partners. Even small electronegativity differences between the partners in a long bond will result in significant bond dipole moments. The bond dipole moments given below are averages calculated using molecules of known structure and for which the molecular dipole moment has been measured. As you will quickly see, the results are a bit confusing when analyzed considering only electronegativity differences between the bonding partners.

Bond
A-B
Electronegativities (EN)
DEN
Bond
Dipole Moment
A
B
Cl-F
3.0
4.0
1.0
0.9 D
Cl-Cl
3.0
3.0
0.0
0.0
Cl-Br
3.0
2.8
0.2
0.6
H-F
2.1
4.0
1.9
1.92
H-Cl
2.1
3.0
0.9
1.08
H-Br
2.1
2.8
0.7
0.78
H-I
2.1
2.5
0.4
0.38
H-O
2.1
3.5
1.4
1.5
H-N
2.1
3.0
0.9
1.3
H-P
2.1
2.1
0.0
0.4
H-C
2.1
2.5
0.4
0.0
F-C
4.0
2.5
1.5
1.4
Cl-C
3.0
2.5
0.5
1.5
Br-C
2.8
2.5
0.3
1.4
I-C
2.5
2.5
0.0
1.2
Br-C
2.8
2.5
0.3
1.4