Difference Between Bond Moment and Dipole Moment

Key Difference – Bond Moment vs Dipole Moment
 

The terms bond moment and dipole moment are based on similar principles but are different based on the application. The bond moment is also known as bond dipole moment. It is the polarity of a chemical bond that is located within a certain molecule. Dipole moment, on the other hand, is any kind of electrical separation (separation of charges). The key difference between bond moment and dipole moment is that bond moment occurs in a covalent chemical bond whereas dipole moment occurs between two ions in an ionic bond or between two atoms in a covalent bond.

CONTENTS

1. Overview and Key Difference
2. What is Bond Moment 
3. What is Dipole Moment
4. Similarities Between Bond Moment and Dipole Moment
5. Side by Side Comparison – Bond Moment vs Dipole Moment in Tabular Form
6. Summary

What is Bond Moment?

Bond moment is the separation of electrical charges in a covalent chemical bond that is present within a certain chemical compound. Hence, it gives the polarity of a chemical bond. The bond moment occurs when there is a positive and negative charge separation in a chemical bond. The bond dipole moment is denoted by the symbol “μ”.

μ  =  δd

in which, δ is the value of charge and d is the distance between two atoms in the covalent bond. In the creation of a bond dipole moment, the electrical charges are separated as partial charges δ+ and δ-. This charge separation occurs in chemical bonds when two atoms participating in the bond formation have different electronegativity values. Since the electronegativity of an atom is the affinity for electrons, more electronegative atoms attract bond electrons towards them. Then, the atom with a less electronegative value gets a partial positive charge because the density of electrons around that atom is less. Correspondingly, the atom having high electronegativity gets a partial negative charge.

The SI unit for the measurement of bond dipole moment is Coulomb-meter (C m). A diatomic molecule has only one covalent bond. Therefore, the bond dipole moment of a diatomic molecule is similar to the molecular dipole moment. The molecular dipole moment for diatomic molecules containing two similar atoms is zero, i.e., the molecular dipole moment of Cl₂ is zero.  But highly ionic compounds such as KBr have a high bond moment and molecular moment. For complex polyatomic molecules, there are many covalent bonds. Then the molecular dipole moment is determined by all individual bond dipole moments.

What is Dipole Moment?

Dipole moment is the separation of electrical charges. The charge separation may take place between two ions in an ionic bond or two atoms in a covalent chemical bond. A dipole moment arises due to differences between electronegativity values of different atoms that form chemical bonds. The greater the difference between electronegativity values, greater the dipole moment. The dipole moment measures the polarity of a molecule. The dipole moment of a molecule is calculated using the following equation.

μ   =   Σq.r

where μ is the dipole moment, q is the magnitude of charge and r is the position of charge. Here, μ and r are vectors, which are quantities having a direction as well as a magnitude.

But sometimes, the term dipole moment is used to name the measure of the magnetic polarity of a system of charges. The magnetic dipole moment determines the torque that a magnet experiences in an external magnetic field. (Torque=moment; a rotational force).

What are the Similarities Bond Moment and Dipole Moment?

• Both are used to measure the polarity (that is created due to charge separation)
• Both are measured in the unit Debye (D) which is equal to coulomb-meter (C m).

What is the Difference Between Bond Moment and Dipole Moment?

Summary – Bond Moment vs Dipole Moment

Bond moment and dipole moment are related terms when it comes to the polarity of chemical bonds in molecules or ionic compounds. The key difference between bond moment and dipole moment is that bond moment occurs in a covalent chemical bond whereas dipole moment occurs between two ions in an ionic bond or between two atoms in a covalent bond.

Reference:

1. “Dipole Moments.” Chemistry LibreTexts, Libretexts, 16 Jan. 2018, Available here.
2. “Bond dipole moment.” Wikipedia, Wikimedia Foundation, 11 Mar. 2018, Available here.

Image Courtesy:

1. “Polarity boron trifluoride” By Emma Farmer (Serephine at English Wikipedia) – Created with Adobe Photoshop by Emma Farmer (Public Domain) via Commons Wikimedia
2. “Propane-dipole-moment-2D” By Ben Mills – Own work (Public Domain) via Commons Wikimedia