When the atoms in a molecule share their electrons unequally, they create what is called a dipole moment . This phenomenon occurs when one atom is more electronegative than another, causing that atom to attract more strongly from the shared pair of electrons, or when an atom has a lone pair of electrons and the difference in electronegativity points in the same direction.
One of the most common examples is the water molecule, which is made up of one oxygen and two hydrogen atom. Differences in electronegativity and lone electrons give oxygen a partial negative charge and each hydrogen a partial positive charge.
bond dipole moment
The bond dipole moment, or chemical dipole moment , is the dipole moment between the single bond in a diatomic molecule, while the total dipole moment in a polyatomic molecule is the vector sum of all bond dipoles. Thus, the bond dipole moment differs from the total dipole moment in polyatomic molecules. Thus, the total molecular dipole moment depends on factors such as differences in atomic size, hybridization of the orbitals, and the direction of the lone pair electrons. The dipole moment can also be smaller when two opposite dipole bonds cancel.
In chemistry, the representation of the dipole moment is given a little differently by the arrow symbol (->). That being said, the dipole moment is represented by an arrow with a cross (+) on one side. The arrow side denotes the negative sign, while the cross (+) side denotes the positive sign. Here, the arrow indicates the shift in electron density in the molecule.
Dipole moment representation
dipole moment formula
The definition of the dipole moment can be given as the product of the magnitude of the electronic charge of the molecule and the internuclear distance between the atoms of a molecule and is given by the following equation:
Dipole moment (μ) = Charge (Q) x Separation distance (d). That is, that (μ) = (Q) x (d)
Where (μ) is the bond dipole moment, Q is the magnitude of the partial charges δ + and δ – , and the distance between δ + and δ – .
On the other hand, the dipole moment is measured in debye units , represented by D. Where 1 D= 3.33564 x 10 -30 C x m. Here C = Coulomb and m = meter.
Example of how to calculate a dipole moment
For this example, we will use the water molecule, which can be used to determine the direction and magnitude of the dipole moment. Based on the electronegativities of oxygen and hydrogen, the difference is 1.2e for each of the hydrogen-oxygen bonds. So, since oxygen is the most electronegative atom, it has a greater attraction for shared electrons; it also has two lone pairs of electrons. Therefore, we can conclude that the dipole moment is between the two hydrogen atoms and the oxygen atom.
Using the above equation, the dipole moment is calculated to be 1.84 D by multiplying the distance between the oxygen and hydrogen atoms by the difference in charge between them, and then finding the components of each that point in the direction of the net dipole moment. (the angle of the molecule is 104.5˚).
The binding moment of the OH bond is 1.5 D, so the net dipole moment is:
(μ)= 2(1.5) cos (104.5˚/2) = 1.84D
Uses of dipole moment
- To find the polar nature of the bond. As the magnitude of the dipole moment increases, so does the polar nature of the bond. Molecules with zero dipole moment are nonpolar, while molecules with zero dipole moment are considered polar.
- To find the structure (shape) of molecules. Molecules with specific values of dipole moment will have a curved or angular shape and will not have a symmetrical structure, while molecules with zero dipole moment will have a symmetrical shape.
- To determine the percentage of ionic character of a bond. This percentage is the amount of electrons shared between two atoms, where a limited sharing of electrons corresponds to a high percentage of ionic character. To determine the percentage of ionic character of a bond, the electronegativities of the atoms are used to predict the distribution of electrons between them.
- To find the symmetry of molecules. Molecules with two or more polar bonds are not symmetric and have a certain dipole moment. Example: H 2 O = 1.84D and CH 3 CI (methyl chloride) = 1.86D. If similar atoms in the molecule are attached to the central atom with a resultant dipole moment of zero, then such molecules will have symmetrical structures. Example: CO 2 (carbon dioxide) and CH 4 (methane).
- To distinguish between cis and trans isomers. In general, the isomer with the higher dipole moment is the trans isomer and the isomer with the lower dipole moment is the cis isomer.
- To distinguish between ortho, meta and para isomers. The para isomer will have a zero dipole moment, while the ortho-isomer will have a higher dipole moment than the meta-isomer.
Carbon Dioxide C02 
Methane CH4
References
http://www.biorom.uma.es/contenido/JCorzo/temascompletos/InteraccionesNC/dipolares/dipolar1.htm
http://hyperphysics.phy-astr.gsu.edu/hbasees/electric/dipole.html
Physics and Chemistry 2nd year of Baccalaureate. Editorial Santillana (Spain) – INVESTIGA Series, 2021. Various authors
