Dipole Moment - Definition, Formula, and Solved Examples

Two small charges (equal and opposite in nature) when placed at small distances behave as a system and are called as Electric Dipole. Now, electric dipole movement is defined as the product of either charge with the distance between them. Electric dipole movement is helpful in determining the symmetry and 3-D orientation of any polar molecule. In this article, we will learn about the Dipole movement, its properties, Formula, and others in detail.

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What is Dipole Moment?

A pair of equal and opposite point charges q and –q separated by a distance 2a form an electric dipole and the electric dipole moment (p) is the product of the charge and the space between the charges (2a), is used to determine the strength of an electric dipole.

A direction in space is defined by the line joining the two charges. The direction from –q to q is commonly referred to as the dipole’s direction. The centre of the dipole is the location of the middle point of –q and q.

The electric dipole’s overall charge is zero. This does not imply that the electric dipole’s field is zero. Because the charges q and –q are separated by a certain distance, the electric fields produced by them do not exactly cancel out when put together. The fields attributable to q and –q almost cancel out over distances much greater than the spacing of the two charges producing a dipole (r >> 2a).

Dipole Moment Formula

Dipole moment is represented by the Greek letter ‘µ’ and is defined as the product of the magnitude of either charge and the distance between them, the formula for the calculation of dipole moment is,

Dipole Moment (µ) = Charge (Q) × distance of separation (r)

μ = δ.d

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

Unit and Dimensions of Dipole Moment

Dipole Moment is measured in Debye denoted by ‘D’.

1 D = 3.33564 × 10-30 C.m

where
C is Coulomb
m denotes a metre

Its dimensional formula is [M0L1T1I1].

Dipole Moment (μ) is a vector quantity, whose direction is measured from +q to -q charge.

The image given below shows the dipole moment of the HCl molecule.

Dipole moment of the HCl molecule

Dipole Moment of BeF2

The net Dipole Moment of the BeF2 Beryllium Fluoride molecule is zero. The bond angle between the BeF2 molecule is 180°, and the two dipole moments are opposite to each other and they cancel out each other. The image given below shows the dipole moment of the BeF2.

Dipole Moment of BeF2

Dipole Moment of H2O (Water)

The net Dipole Moment of the H2O water molecule is found to be 1.84 D. The bond angle in the water molecule is 104.5°, the water molecule has two oxygen-hydrogen bonds that can individually be treated as dipole and their individual bond moment of an oxygen-hydrogen bond is 1.5 D. The image given below shows the dipole moment of the H2O.

Dipole Moment of H2O (Water)

Dipoles in an External Electric Field

If an electric dipole is placed in an external electric field the electric dipole experiences some force called torque. It is represented by the Greek letter τ. The torque in any external electric field on the dipoles is given by,

τ = P x E
= PE Sin θ

where,
P is the Dipole Moment
E is the Applied External Field

Significance of Electric Dipole Moment

Electric Dipoles are not only studied in Physics but are also very important in Chemistry. The significance of Electric Dipole is by categorizing molecules.

Molecules on the basis of electric dipole moment are divided into two categories,

Polar Molecules: If any molecule has a net dipole moment then it is called a polar molecule. For example, HCl is a polar molecule. These molecules are randomly arranged in the absence of an external electric field. On applying an electric field, the polar molecules align themselves according to the direction of the electric field.
Non-Polar Molecules: If any molecule has a net dipole moment of zero then it is called a non-polar molecule. For example, BeF2 is a non-polar molecule.

Uses of Dipole Moment

Every compound is made up of bonds and bonds have polarity. The polar nature of any bond is studied using the Dipole movement of that compound. Molecules with high dipole movement have high polarity and molecules with no dipole movements are non-polar in nature. The various uses of dipole movements are,

Dipole moment tells us about the symmetry of the molecules. i.e. molecules with high dipole movement are generally non-linear and asymmetrical, whereas molecules which have zero dipole movement are symmetrical and linear.
Cis- and Trans-isomers of any compound can easily be distinguished using the dipole moment. Compounds with high dipole moment are generally trans-isomers and compounds with low dipole moment are cis-isomers.
Dipole movement helps us to find the percentage ionic character of a molecule.
Ortho, Para and Meta compounds are also distinguished using the dipole moment.

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FAQs on Dipole Moment

Question 1: What is a Dipole Moment?

Answer:

The product of charge on either end of the dipole with the distance between them is called the Dipole moment.

Question 2: What is the SI unit of the Dipole Moment?

Answer:

SI unit of the Dipole Moment is Coulomb-metre.

Question 3: Where is Dipole Moments used?

Answer:

There are various uses of Dipole Moment some of them are,

They are used to determine symmetry in a molecule.

They explain the various physical properties of the compounds, etc.