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Dipole Moment occurs in a system in which there is a separation of positive and negative charges. It can arise in both ionic bonds as well as in covalent bonds. Dipole moments arise because of the difference in electronegativity between two chemically bonded atoms.
If a molecule contains unevenly distributed polar bonds, there will be an uneven charge distribution across the entire molecule. This uneven distribution of charge will make the molecule a polar molecule.
Read More: Chemical Bonding and Molecular Structure
Key Terms: Dipole Moment, Covalent Bonds, Molecules, Electronegativity, Electrons, Charge, Bond Dipole Moment
What is Dipole Moment?
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Dipole Moment can be defined as the measurement of net polarity in a molecule.
- A dipole moment arises in any system wherein there is a separation of charge.
- The dipole moment can arise between two ions in ionic bonds and between two atoms in covalent bonds.
- The reason behind the occurrence of dipole moment between two chemically bonded atoms is the difference between their electronegativity.
- The larger the difference in electronegativity, the more the dipole moment.
Also Read: Chemical Bonding and Molecular Structure Important Questions
Bond Dipole Moment
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Bond Dipole Moment helps find the measurement of the polar character of a chemical bond in a molecule, between two atoms. The polar character of a compound is the separation of positive and negative charges. It also involves the concept of electric dipole moment in a system.
The bond dipole moment is a vector quantity because it has both magnitude and direction.
| Example: The dipole moment that arises in an HCl (hydrochloric acid) molecule can be expressed as:  Here, |
Representation of Dipole Moment
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The dipole moment is represented by the arrow symbol with a cross (+) on one side. The arrow side denotes the negative sign, while the + side denotes the positive sign.
Important Points about Dipole Moment
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- The dipole moment of a single bond in a polyatomic molecule is called the bond dipole moment, which is different from the dipole moment of the molecule.
- Dipole Moment is a vector quantity, that is, it has a certain magnitude and direction.
- As a vector, dipole moment can also be zero, because two opposite bond dipoles can cancel each other.
- Dipole moment is represented by a slight change in the arrow symbol.
- Dipole moment is represented by a cross in the positive center and an arrow in the negative centre. This arrow symbolizes the change of electron density in the molecule.
- In a polyatomic molecule, the dipole moment of the molecule is the vector sum of all bound dipoles present in the molecule.
Dipole Moment Formula
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Dipole moment formula can be expressed as the product of electron charge on a molecule and the intermolecular distance between atoms in the molecule. It is given by the equation:
| Dipole moment (μ) = Charge (Q) × Intermolecular distance between atom (d) Thus, (μ) = (Q) × (d) |
Dipole Moment is measured in Debye units, represented by D.
- D = 3.33564 x 10-30 cm
- C = Coulomb
- m = meters
The bond dipole moment arises between a chemical bond of two atoms of different electronegativities and can be expressed as:
μ = δ.d
Where
- μ refers to the bond dipole moment.
- δ refers to the magnitude of the partial charges δ+ and δ–.
- d refers to the distance between δ+ and δ–.
The bond dipole moment, represented by the symbol μ, is a vector quantity, and its direction is parallel to the bond axis. The arrows drawn to represent dipole moments begin at the positive charge and end at the negative charge.
When two atoms with different electronegativities interact with each other, the electrons move from their initial positions and come closer to the more electronegative atom. Bond Dipole moment can be used to represent this movement of electrons.
Examples of Dipole Moment
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There are several examples of dipole moment. Some of them include:
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Dipole Moment of HCl Molecule
In the diatomic HCl molecule, the dipole moment of the HCl molecule is the same as the dipole moment of the HCl bond, which is 1.03 D.
Dipole Moment of HCI
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Dipole Moment of BeF2 Molecule
In the beryllium fluoride molecule, the dipole moment is zero.
BeF2 is linear in shape. In it, there are two independent bond dipole moments that cancel each other out, resulting in a total dipole moment of zero. This is because the bond dipole moments in the BeF2 molecules are equal in magnitude and opposite in direction.
Dipole Moment of BeF2
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Dipole Moment of CO2 Molecule
In a CO2 (carbon dioxide) molecule, the dipole moment is zero.
Due to the linear structure of the molecule, the dipole moment of the C = O (2.3D) bond on one side of the molecule is cancelled by the molecule on the other side. This eventually results in the zero dipole moment.
Dipole Moment of CO2
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Dipole Moment of H2O Molecule
In the triatomic water molecule (H2O), the dipole moment is 1.84D. The dipole moment of the H2O molecule is not zero due to its curved structure. In the H2O molecule, the two OH bonds are inclined at an angle of 104.5 degrees and have two lone pairs on oxygen atoms.
Dipole Moment of H2O
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Dipole Moment of BH3 and NH3 Molecule
In tetra-atomic boron hydride (BH3), the dipole moment is zero, and in ammonia (NH3), the dipole moment is 1.49D. This is because BH3 has a symmetrical structure and all the three BH bonds are placed at an angle of 120 degrees to each other. Since all 3 bonds are on the same plane, the dipole moments cancel each other out, and the total dipole moment becomes zero.
On the other hand, NH3 has a pyramidal structure with 3 NH bonds and a pair of electrons on the nitrogen atom. It results in the dipole moment of 1.49D.
Dipole Moment of NH3 and BH3
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Dipole Moment of NH3 and NF3 Molecule
Both NH3 and NF3 molecules have 3 NH bonds and a lone pair on the nitrogen atom, but the dipole moment of NF3 is smaller than that of NH3 because the dipoles are in a pair.
The nitrogen atoms in NH3 and NF3 molecules are different. Fluorine is more electronegative than nitrogen, and it attracts all the common electrons of nitrogen in a direction opposite to the total dipole moment. Hydrogen, due to the NH bond, pulls all the common electrons of hydrogen to a common dipole moment in one direction. As a result, the net dipole moment of NH3 increases.
Dipole Moment of NH3 and NF3
Read More: Molecular Orbital Theory
Things to Remember
- Dipole Moment occurs in a system in which there is a separation of charge.
- It occurs due to the difference in electronegativity between two chemically bonded atoms.
- Dipole moment occurs in ionic bonds as well as in covalent bonds.
- Bond dipole moment is defined as a measure of the polarity of a chemical bond between two atoms in a molecule. It is a vector quantity as it has both magnitude and direction.
- Dipole moment, denoted by the Greek letter ‘µ’, can be expressed as Dipole Moment (µ) = Charge (Q) * Distance of Separation (r)
Also Read: Nuclear Bond
Previous Years’ Questions (PYQs)
- The unit of dipole moment is…
- Dipole moment is shown by…
- The molecule which has zero dipole moment is… [JCECE 2010]
- The correct decreasing order of dipole moment in CH3Cl, CH3Br… [DUET 2011]
- If a molecule MX3 has zero dipole moment, the sigma bonding orbitals… [JEE Advanced 1981]
- The dipole moment of H2S is 0.95 D. If the bond angle is…[KEAM]
- Molecule having least dipole moment is…
- How many σ bonds and π bonds are present in… [BITSAT 2018]
- The pair of species that has the same bond order in the following is… [NEET 2013]
- Which one is not paramagnetic among the following… [NEET 1998]
- Linear combination of two hybridised orbitals belonging to the two atoms… [NEET 1990]
- The number of antibonding electron pairs in… [NEET 1998]
- Which one shows maximum hydrogen bonding… [NEET 1990]
- H2O has a net dipole moment while BeF2 has zero dipole moment because… [NEET 1989]
- Which of the following molecule has highest dipole moment…
Sample Questions
Ques: Give the mathematical expression of the dipole moment. (2 Marks)
Ans: Dipole moment is expressed as: Dipole moment (μ) = Charge (Q) × Intermolecular distance between an atom (d)
(μ) = (Q) × (d)
Dipole moment is usually expressed in Debye units (D).
Ques: Define Bond dipole moment. (1 Mark)
Ans: Bond dipole moment can be defined as the dipole moment possessed by a single bond and is found present within a polyatomic molecule.
Ques: Although both CO2 and H2O are triatomic molecules, the shape of the H2O molecule is bent while that of CO2 is linear. Explain this on the basis of the dipole moment. (2 Marks)
Ans: In CO2, there are two C=O bonds. Each C=O bond is a polar bond.
The bond dipoles of two C=O bonds cancel the moment of each other resulting in the net dipole moment of CO2 molecule zero. This is possible only if CO2 is a linear molecule (O=C=O).
Whereas, H2O molecule has a net dipole moment (1.84 D). The H2O molecule has a bent structure because the O—H bonds are at an angle of 104.5° and do not cancel the bond moments of each other.
Ques: Why does NH3 have a higher dipole moment than NF3 though both are pyramidal? Explain with the structure. (3 Marks)
Ans: Both NH3 and NF3 molecules have 3 NH bonds and a lone pair on the nitrogen atom, but the dipole moment of NF3 is smaller than that of NH3 because the dipoles are in a pair.
The nitrogen atoms in NH3 and NF3 molecules are different. Fluorine is more electronegative than nitrogen, and it attracts all the common electrons of nitrogen in a direction opposite to the total dipole moment. Hydrogen, due to the NH bond, pulls all the common electrons of hydrogen to a common dipole moment in one direction. As a result, the net dipole moment of NH3 increases.
Ques: Define dipole moment. What are the units of the dipole moment? (2 Marks)
Ans: Dipole moment is defined as the product of the magnitude of the positive or negative charge and the distance between the charges. In a polar molecule, one atom is positively charged and the other has a negative charge.
Thus, the molecule has two poles with an equal magnitude of the charges. These types of molecules are known as dipolar molecules and possess dipole moments.
µ (dipole moment) = q x d
The SI unit of the dipole moment is coulomb meter (m) or Debye.
Ques. Dipole moment values help in predicting the shapes of covalent molecules. Explain. (1 Mark)
Ans. The dipole moment values are quite helpful in determining the general shapes of molecules. For molecules with zero dipole moment, shapes will be either linear or symmetrical. For Example, BeF2, CO2, etc. These molecules possess dipole moments therefore their shape will not be symmetrical.
Ques. Is Dipole moment a vector or scalar quantity? (1 Mark)
Ans. Dipole moment is a vector quantity since it has a magnitude and different directions.
Ques. How is the largest dipole moment determined? (2 Marks)
Ans. Depending upon the difference in the electronegativity of two atoms, a dipole moment is seen to take place. The larger the difference in the electronegativity, the larger will be the bond’s dipole moment.
Ques. Why is the dipole moment of CO2, BF3, and CCl4 zero? (1 Mark)
Ans. Dipole moments of CO2, BF3, and CCl4 are zero because their molecules have symmetrical shapes and thus the dipoles get cancelled and the net dipole moment becomes zero.
Ques. Why does Hf have the largest dipole moment? (2 Marks)
Ans. Hf has the largest dipole moment due to the fact that the molecules, H and F have a large electronegativity difference between them. In other words, the larger the difference in the electronegativity, the larger will be the bond’s dipole moment and polarity.
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