Concept of Dipole Moment- Definition, Representation, In Regular Geometry, In Geometrical Isomers, % Ionic Character, Practice Problems, FAQs

You must wonder when you know Peter Debye, the Dutch chemist who received the Nobel prize in 1936 for his work on X-ray diffraction and dipole moments. To honour him, the magnitude of the dipole moment is represented in Debye units. Don't you find this fascinating?

Why do we study this concept?

As we know neither every covalent bond is 100% covalent nor every ionic bond is 100% ionic.
We study dipole moment to find % ionic character in a covalent molecule. It gives an idea about the polar and non-polar nature of a molecule.

Table of contents:

What is dipole moment?
Direction and Representation of Dipole Moment
Dipole Moment for Diatomic and Polyatomic Molecules
Dipole Moment in Regular Geometries
Dipole Moments in Geometrical Isomers
Percentage Ionic Character
Practice problems
Frequently asked questions-FAQs

What is dipole moment?

It is the product of the magnitude of the charge and the distance between the centres of the positive and negative charges. It is denoted by the Greek letter μ.

Mathematically, it is expressed as
Dipole moment (μ) = Charge (q) × Distance of separation (d)
Dipole moment is usually expressed in Debye (D).
Magnitude of charge on an electron =

Direction and representation of dipole moment

Dipole moment is a vector quantity, i.e., it has magnitude as well as direction. It is represented by a crossed arrow ( )

Dipole Moment for Diatomic and Polyatomic Molecules

For diatomic molecules: More the electronegativity difference between the bonded atoms, more is the dipole moment. In homonuclear molecules, there is no difference in electronegativity between bonded atoms and hence they have zero dipole moment.
For example, the value of the dipole moment for , etc., is zero.
In heteronuclear molecules, polar diatomic molecule possesses only one polar bond, so the dipole moment of such molecule depends only upon the dipole moment of this polar bond.
For example, the value of the dipole moment for is 1.78 D.
For polyatomic molecules: In this case, the dipole moment not only depends upon the individual dipole moments of bonds known as bond dipoles but also on the spatial arrangement of various bonds in that molecule. In such a case, the net dipole moment is the vector sum of the dipole moments of all the bonds.
Let P and Q be the two individual dipole moments of the bonds
in a polyatomic molecule, with the bond angle θ, the magnitude
of resultant dipole moment (R) can be given by:

Example: The resultant dipole moment of water is 1.85 D, ignoring the effects of lone pair. Calculate, the dipole moment of each bond. (given that bond angle in = 104°, cos 104° = -0.25).
Solution: We all know Dipole moment is a vector quantity. Let us suppose the dipole of bond be D
Using the formula , we can calculate easily the dipole moment of each bond. Since bonds are equivalent in nature ,therefore dipole =
Putting these values in equation, we get
1.85 =

∴
Hence, the dipole moment of each bond comes out to be 1.51 D

Dipole Moment in Regular Geometries

When the molecule is symmetrical, i.e., when all the atoms attached to the central atom are the same and there are no lone pairs, then the net dipole moment of the molecule will be zero.
For example: , , etc.
Some of the symmetrical structures are shown below.

Dipole Moment in Geometrical isomers

Cis and trans isomers are the types of geometrical isomers.

Dipole moment in the cis isomer of dichloroethene
In the cis isomer of dichloroethene, the two dipole moments of the polar bonds, are in the same direction. Thus, the net dipole moment is non-zero and the molecule is polar. The net dipole moment is shown in the figure.
Dipole moment in the trans isomer of dichloroethene
In the trans isomer of dichloroethene, the two dipole moments of the polar bonds, are equal and in the opposite directions. Thus, the net dipole moment is zero and the molecule is nonpolar. The net dipole moment along is shown in the figure:

Percentage Ionic Character

Percentage ionic character is the measure of a compound’s ionic character. To determine a bond's percentage ionic character, the dipole moment and electronegativity values can be used.
Percentage ionic character can be calculated by the following method:

From the data of the dipole moment:

μObserved = Experimental value of dipole moment
μTheoretical = Value of dipole moment when the compound is assumed as purely ionic
Example:

molecule,= 1.03 D and bond length = 1.275 Å. percentage ionic character can be calculated as follow:
Let q be the magnitude of charge on the or -atom in molecule and be the charge on the or -atom when the molecule is assumed to be purely ionic. Therefore,
= 1.03 D,
Dipole moment (μ) = Charge (e) × Distance of separation (d)
∴=

1D =

Hence, % ionic character = 16.81%

Practice problems

Q 1. Explain the Order of Dipole Moment in Hydrogen Halides.
Answer: Out of charge and internuclear distance, generally charge is the dominating factor on which dipole moment depends. Also, the electronegativity of halides decreases down the group. Thus, the electronegativity difference between the hydrogen and the halogen decreases in the same order. Therefore, the order of dipole moment will be as follows:

Q 2. Why does have a higher dipole moment than ?
Answer:

The dipole moment vector of the lone pair will be away from in both the molecules. In case of , two vectors (lone pair dipole moment and sum of dipole moment vectors through bonds) will add up, whereas in the case of , these two vectors will be against each other where net magnitude reduces. Thus, has a higher dipole moment than.

Q 3. Why does cis-isomer of dichloroethene have more boiling point than trans-isomer?
Answer: In cis-isomer are in the same direction. Thus, the net dipole moment is non-zero. In the trans-isomers two dipole moments of the polar bonds, are equal and in the opposite directions. Thus, the net dipole moment is zero. When the dipole moment of a compound is more, more is the intermolecular force of attraction,Thus the boiling point of the cis-isomer of the compound is more than that of the trans-isomer.

Q 4. Molecule with zero dipole moment is
A.
B.
C.
D.
Answer: , , all are polar molecules giving the resultant non-zero dipole moment where as is a non-polar molecule having linear structure which gives net dipole moment value equal to zero.

Frequently asked questions-FAQs

Q 1. What is an electric dipole?
Answer: Due to asymmetrical electron cloud distribution, partial charges are developed in the polar covalently bonded molecules and they act as electric dipoles.

Q 2. Does a lone pair contribute to the dipole moment?
Answer: Yes, lone pairs on the central atom contribute to the dipole moment. However, its contribution cannot be quantified, as the size of lone pairs is not known but its effect has to be considered.

Q 3. Dipole is represented by . what do head and tail represent?
Answer: The tail of the arrow is on the positive centre and the head of the arrow points towards the negative centre, i.e., a dipole moment has the same direction as that of the shift of electron density in the molecule.

Q 4. What is polarizability?
Answer: Polarizability is the ease with which an atom's or molecule's electron cloud can be altered.