A dipole moment is a measurement of the separation of two opposite electrical charges. Dipole moments are a vector quantity. The magnitude is equal to the charge multiplied by the distance between the charges and the direction is from negative charge to positive charge:
μ = q · r
where μ is the dipole moment, q is the magnitude of the separated charge, and r is the distance between the charges.
Dipole moments are measured in the SI units of coulomb·meters (C m), but because the charges tend to be very small in magnitude, the historical unit for a dipole moment is the Debye. One Debye is approximately 3.33 x 10-30 C·m. A typical dipole moment for a molecule is about 1 D.
Significance of the Dipole Moment
In chemistry, dipole moments are applied to the distribution of electrons between two bonded atoms. The existence of a dipole moment is the difference between polar and nonpolar bonds. Molecules with a net dipole moment are polar molecules. If the net dipole moment is zero or very, very small, the bond and molecule are considered to be nonpolar. Atoms that have similar electronegativity values tend to form chemical bonds with a very small dipole moment.
Example Dipole Moment Values
The dipole moment is dependent on temperature, so tables that list the values should state the temperature. At 25°C, the dipole moment of cyclohexane is 0. It is 1.5 for chloroform and 4.1 for dimethyl sulfoxide.
Calculating the Dipole Moment of Water
Using a water molecule (H2O), it's possible to calculate the magnitude and direction of the dipole moment. By comparing the electronegativity values of hydrogen and oxygen, there is a difference of 1.2e for each hydrogen-oxygen chemical bond. Oxygen has a higher electronegativity than hydrogen, so it exerts a stronger attraction on the electrons shared by the atoms. Also, oxygen has two lone electron pairs. So, you know the dipole moment must point toward the oxygen atoms. The dipole moment is calculated by multiplying the distance between the hydrogen and oxygen atoms by the difference in their charge. Then, the angle between the atoms is used to find the net dipole moment. The angle formed by a water molecule is known to be 104.5° and the bond moment of the O-H bond is -1.5D.
μ = 2(1.5)cos(104.5°/2) = 1.84 D