Dipole Moment - Overview, Definition, Formula, FAQs

A dipole moment is a measure of the separation of positive and negative charges in a molecule, indicating the polarity of the molecule. It arises when there is an uneven distribution of electron density, resulting in partial positive and negative charges at different ends of the molecule. The dipole moment is a vector quantity, having both magnitude and direction. The magnitude of the dipole moment is given by the product of the charge difference and the distance between the charges. It is expressed in Debye units (D).

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This Story also Contains

1. Dipole Moment
2. Some Solved Examples
3. Summary

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Dipole Moment

When the covalent bond forms between two different atoms then because of the difference in electronegativity the electrons get shifted towards the more electronegative atom and thus form the polar covalent bond and this polar molecule is known as a dipole molecule. The more electronegative atom occupies a partial negative charge (δ-) and the other atom possesses a partial positive charge (δ+). This separation of charge gives rise to a bond dipole moment. The magnitude of a bond dipole moment is represented by the Greek letter mu (µ) and is given by the formula as shown below, where Q is the magnitude of the partial charges (determined by the electronegativity difference) and r is the distance between the charges: μ=Qr

(a) There is a small difference in electronegativity between C and H, represented as a short vector. (b) The electronegativity difference between B and F is much larger, so the vector representing the bond moment is much longer.

For diatomic molecules, there is only one bond, so its bond dipole moment determines the molecular polarity. Homonuclear diatomic molecules such as Br₂ and N₂ have no difference in electronegativity, so their dipole moment is zero. For heteronuclear molecules such as CO, there is a small dipole moment. For HF, there is a larger dipole moment because there is a larger difference in electronegativity.

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When a molecule contains more than one bond, the geometry must be taken into account. If the bonds in a molecule are arranged such that their dipole moments cancel, then the molecule is non-polar. For example, in the case of CO₂ as shown in the figure given below. Each of the bonds is polar, but the molecule as a whole is non-polar. The dipole moments cancel each other because they are pointed in opposite directions.

In the case of the water molecule, the Lewis structure again shows that there are two bonds to a central atom, and the electronegativity difference again shows that each of these bonds has a nonzero bond moment. In this case, however, the molecular structure is bent because of the lone pairs on O, and the two bond moments do not cancel. Therefore, water does have a net dipole moment and is a polar molecule (dipole).

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Some Solved Examples

Example 1: The most polar compound among the following is :

1)

2)

3)

4)

Solution

Resultant dipole moment -

Let XY and XZ be two polar bonds inclined at an angle θ their dipole moments are μ1 and μ2

Resultant μR=μ12+μ22+2μ1μ2cos⁡θ

In the bond dipole vector of the C-F bond is not subtractive.

Example 2: The molecule which has zero dipole moment is :

1)CH₂Cl₂

2) BF₃

3)NF₃

4)ClO₂

Solution

The dipole moment of a compound having regular geometry and the same type of atom is zero.

BF₃ has a triangular planar structure hence its resultant dipole moment is zero.

Hence, the correct answer is Option (2)

Example 3: Which of the following isomers will have the highest dipole moment?

1) Ortho form

2)Meta form

3)Para form

4)All have equal dipole moment

Solution

Because the dipole moment is a vector quantity and the resultant dipole moment is highest when the angle between them is minimum.

Hence, the option number (1) is correct.

Example 4: H₂O has a net dipole moment, while BeF₂ has zero dipole moment, because:

1)H₂O molecule is linear while BeF₂ is bent.

2) The BeF₂ molecule is linear while H₂O is bent.

3)Fluorine is more electronegative than oxygen.

4)Be is more electronegative than oxygen.

Solution

As we have learned,

The structure of H₂O and BeF₂ are:

By seeing the bond angle we can say that BeF₂ molecule is linear while H₂O is bent. Moreover, a water molecule is sp³ hybridized and BeF₂ is sp hybridized.
Hence, option number (2) is correct.

Example 5: Which one of the following pairs of molecules will have permanent dipole moments for both members?

1) SiF₄ and NO₂
2) NO₂ and CO₂
3) NO₂ and O₃
4) SiF₄ and CO₂

Solution

Both NO₂ and O₃ are bent molecules with asymmetry. Hence both will exhibit permanent dipole moment.

Hence, the answer is the option(3).

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Summary

The dipole moment points from the positive to the negative charge and is quantified by the product of the charge difference and the distance between the charges, expressed in Debye units (D). The presence and magnitude of a dipole moment are determined by the electronegativity of atoms and the molecule's geometry. Polar molecules have significant dipole moments, while nonpolar molecules have zero dipole moments. Understanding dipole moments is essential for predicting molecular behavior in electric fields, interactions with other molecules, and solubility in solvents.

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