# Dipole Moments

A dipole consists of a positive and an equal negative charge separated by a distance within a molecule. The degree of polarity of a bond is given by the dipole moment (

*Î¼*), which is the product of charge (*e*) and the distance (*d*) between them:*Î¼*=*d*Ã—*e*, where*e*is the electronic charge and*d*is the distance between the atomic centers.1

*d*= 10^{â€“18}esu-cm = 3.336 Ã— 10^{â€“30}C-m.# Applications of dipole moment measurements

- Diatomic molecules: Diatomic molecules such as H
_{2}, Cl_{2}, and N_{2}have no dipole moment; so these molecules are symmetrical. *Triatomic molecules*: Some of these molecules possess zero dipole moment; so they have a symmetrical linear structure, e.g., CO_{2}, CS_{2}, and HgCl_{2}. Others such as water and sulphur dioxide have definite dipole moments. They are said to have angular or bent structure or V-shaped structure.*Tetratomic molecules*: Some molecules such as BCl_{3}have zero dipole moment. They are said to possess a flat and symmetrical (triangular) structure. Other examples are BF_{3}, BBr_{3}, CO_{3 }^{2â€“}, and NO^{â€“ }_{3}. PCl_{3}, AsCl_{3}, NH_{3}, PH_{3}, AsH_{3}, and H_{3}O^{+}have appreciable dipole moments. They possess trigonal pyramidal structures._{4}have zero dipole moment. So they possess symmetrical tetrahedral structures with C atom at the center of the tetrahedron.*cis*- and*trans*-isomers of organic compounds. The*trans*-isomer, which is symmetrical, has zero dipole moment while the*cis*-isomer has a definite dipole moment. For example,*cis*-dibromoethylene (*Î¼*= 1.4 D) and*trans*-dibromoethylene (*Î¼*= 0).*Dipole moment in aromatic ring system*: We know that when substituted benzene is treated with a reagent, different products (namely*ortho*,*meta*, and*para*) are formed. In order to find their dipole moment, we need to know about the nature of the groups linked to the benzene ring.**Case (i):**When X and Y both are electron pushing or electron withdrawing. Let the bond dipole of C â€“â€“ X bond be represented by*Î¼*_{1}and that of C â€“â€“ Y bond by*Î¼*_{2}. Now let us assume that the electron-pushing groups have positive bond moment and the electron-withdrawing groups have negative bond moment.**Case (ii):**When X is electron pushing and Y is electron withdrawing or vice versa. Let the bond moment of Câ€“X dipole be*Î¼*_{1}and that of Câ€“Y dipole be*Î¼*_{2}.

# Dipole moment and percentage ionic character

The percentage of ionic character in a compound having some covalent character can be calculated by the following equation:

% ionic character =

Theoretical dipole moment is confined to when we assume that the bond is 100% ionic, and it is broken into ions. Observed dipole moment is with respect to fractional charges on the atoms of the bond.