A Quick Overview On Dipole Moment

The separation of charges in any system leads to a dipole moment. Both ionic and covalently bonded compounds develop dipole moments. The main cause for the development of the dipole moment is the electronegativity difference between chemically bonded atoms or elements.

Where Do Dipole Moments Occur?

Dipole Moments mostly occur between two ions in an ionic bond or between two molecules when they share a covalent bond. The main reason for the rise of the dipole moment is due to the difference in the electronegativity of the atoms of compounds formed. In the size of the dipole moment the distance of the bond also plays a crucial role in determining the magnitude of the dipole moment. Dipole moment is actually the measurement of the polarity of the molecules. 

Therefore in other words, the dipole moment is created when the atoms of a molecule share the electrons unequally. This usually occurs when one atom is more electronegative than the other atom which results in more pulling of the electron cloud by the higher electronegative atom. It also happens with the atom bearing the lone pair of electrons and the difference in the vector points of the electronegativity in a similar way. One of the most common examples is the water molecule that consists of one highly electronegative oxygen atom and two electropositive hydrogen atoms. Thus the difference in the electronegativity combined with the presence of the lone pair of electrons on the oxygen atom gives it a partial negative charge and the hydrogen atom the positive charges.    

Polar character is the separation of positive and negative charges, in a compound. This measurement of polar character of a chemical bond in a molecule, between two atoms, is given by bond dipole moment. Bond dipole moment is considered as a vector quantity, as it has both magnitude and direction. For example,

Figure 1. Dipole moment in HCl

δ+ and δ- indicate positive and negative charges, which are separated by distance d. These charges are equal in magnitude but opposite in sign.

Important Points

Bond dipole moment differs from the total dipole moment in polyatomic molecules. e.

Bond dipole moment is the dipole moment between the single bond of a diatomic molecule, while the total dipole moment in a polyatomic molecule is the vector sum of all the bond dipoles.

Thus, total molecular dipole moment depends on the factors like- differences in the sizes of the two atoms, hybridization of the orbitals, direction of lone pair electrons.

Dipole moments can also be zero, when opposite two bond dipoles cancel each other. 

In chemistry, the representation of the dipole moment is given a little differently with the arrow symbol. Dipole moment is represented by an arrow, with cross (+) on one side. The arrow side denotes the negative sign, while the + side denotes the positive sign. 

Figure 2. Representation of dipole moment

The arrow signifies the shifting of electron density in the molecule. 

Dipole Moment Formula

The dipole moment is established when the two electrical charges that are of equal magnitude but of opposite signs are separated by a distance. The dipole moment ( μ ) is also used to determine the size of the dipole.dipole moment which is equal to the distance between the charge X the charge is measured in the Debye unit where 1 Debye = 3.34×10-30Cm.   Dipole moment definition can be given as the product of magnitude of electric charge of the molecule and the internuclear distance between the atoms in a molecule. It is given by the equation:

Dipole moment (µ) = Charge (Q) × Distance of separation (d)

(µ) = (Q) × (d)

where, μ is the bond dipole moment, Q is the magnitude of the partial charges 𝛿+ and 𝛿–, and d is the distance between 𝛿+ and 𝛿–.

It is measured in Debye units, represented by D.

D = 3.33564 ×10-30 Cm; C = Coulomb, m = meter.

The dipole moment of a molecule can be calculated by another primary Equation that is mentioned below:

μ =∑q_ir_i

Where,

μ  is the dipole moment vector

q_i is the magnitude of the ith charge, and

R_i is the vector representing the position of ith charge.

The dipole moment acts in the direction of the vector quantity. For example water (H₂O) as a lone pair of electrons on the oxygen atom and its structure according to the VSEPR theory is bent and thus the vectors representing the dipole moment of each of the bonds do not cancel each other out. Thus, water molecules are considered polar in nature.

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The vector always points from positive to negative in the individual bond dipoles and also on both the molecular dipole moment. Therefore the larger the difference between the electronegativity of the two atoms, the more will be the electronegative activity of the bond. So as to establish a polar bond, the difference in electronegativity needs to be large. When each of the bond electronegativities are added together, the dipole moment points in the direction of the vector quantity of each of the bonds.

The bond dipole is interrupted when the charges separated over a distance r are between the partial charges Q+  and Q− (or the more commonly used terms δ+ - δ−). The orientation of the dipole is along the axis of the bond. When a simple system is considered where a single electron and a proton is separated by a fixed distance, if the distance between the electron and the proton is smaller and they are close together, the dipole moment of the degree of polarity decreases. However if the distance between the proton and electron increases and they get far apart from each other, the dipole moment increases.

Dipole Moment Chemistry

1. In the diatomic molecule of HCl, the dipole moment of the HCl molecule is the same as dipole moment of HCl bond, which is 1.03D.

Figure 3. Dipole moment of HCl 

2. In beryllium fluoride molecule, the dipole moment is zero. BeF2 has a linear shape. There exist two individual bond dipole moments, which cancel each other resulting in the net dipole moment zero. This is because in BeF2 molecule, the bond dipole moments are equal in magnitude and opposite in direction.

Figure 4. Dipole moment of BeF2

3. In the triatomic CO2 (carbon dioxide) molecule, the dipole moment is zero. Due to the linear structure of the molecule, the dipole moment of C=O bond (2.3D) on one side of the molecule gets canceled by that on the other side of the molecule, resulting in a net zero dipole moment.

Figure 5. Dipole moment of CO2

4. In a triatomic H₂O water molecules, the dipole moment is 1.84D. Due to the bent structure of the water molecule, the dipole moment is not zero. This is due to the resultant dipole moments of 2 O-H bonds, inclined at 104.5 degrees, with 2 lone pairs on oxygen atoms.

Figure 6. Dipole moment of H2O

5. In tetra-atomic boron trihydride (BH3), the dipole moment is zero, but that of ammonia (NH₃) is 1.49D. This is because BH₃ has a symmetrical structure and the 3 B-H bonds are placed at an angle of 120 degrees to each other. As the 3 bonds are in a single plane, dipole moments cancel each other, with net dipole moment equal to zero. On the other hand, NH₃ has a pyramidal structure, with 3 N-H bonds and a lone pair on nitrogen atom. This gives the resultant dipole moment as 1.49D.

Figure 7. Dipole moment of NH3 and BH3

Also, when we consider NH₃ and NF₃ molecules, both have 3 N-H bonds and a lone pair on nitrogen atoms but the resultant dipole moment of NF₃ is less than that of NH3.  This is because the dipole formed between the lone pair and nitrogen atom differs in both NH₃ and NF₃ molecules. Fluorine, being more electronegative than nitrogen, will attract all the shared electrons towards it from nitrogen in opposite direction to net dipole moment. Thus, the resultant dipole moment of NF₃ decreases. While nitrogen being more electronegative than hydrogen, it will attract all the shared electrons towards it from hydrogen in the same direction to net dipole moment due to N-H bonds. Thus, the resultant dipole moment of NH3 increases.

Figure 8. Dipole moment of NF3 and NH3

6. In CH₄ (methane) and CCl₄ (carbon tetrachloride) molecules, the dipole moments are zero. These two, CH₄ and CCl₄ molecules, have a symmetrical tetrahedral shape. Therefore, dipole moments of C-H bonds in CH4 cancel out each other and result in zero dipole moment, same in CC_l4 molecule dipole moment of C-Cl bonds cancel out each other and result in zero dipole moment.

 Figure 9. Dipole moment of CH4 and CCl4 

While in CH₃Cl (methyl chloride) molecule, even though it has a tetrahedral structure, its dipole moment is not zero. This is because the structure of methyl chloride is not symmetrical and the dipole moments of bonds C-Cl and C-H are not equal. Thus, the resultant dipole moment comes to 1.86 D.

Figure 10. Dipole moment of CH3Cl

Uses of Dipole moment

In finding the polar nature of the bond: As the magnitude of dipole moment increases, more will be the polar nature of the bond. Molecules with zero dipole moment are non-polar, while molecules with dipole moment are said to be polar.

In finding the structure (shape) of the molecules: Molecules with specific dipole moment values will be bent or angular in shape and not have a symmetrical structures. While molecules with zero dipole moment will have an asymmetrical shape.

In finding symmetry of the molecules: Molecules having two or more polar bonds would not be symmetrical and possess some dipole moment. Examples: H2O = 1.84D; CH₃Cl (methyl chloride) = 1.86 D. If similar atoms in the molecule are attached to the central atom with resultant dipole moment zero, then such molecules will have symmetrical structures. Examples: CO₂, CH₄

In distinguishing between cis- and trans-isomers: Generally, isomer with higher dipole moment would be trans-isomer and isomer with lower dipole moment would be cis-isomer.

In distinguishing between ortho, meta and para-isomers: para-isomer will have dipole moment zero, while ortho-isomers have dipole moment greater than that of meta-isomer.