Difference Between Bond Dipole and Molecular Dipole

The key difference between bond dipole and molecular dipole is that bond dipole refers to the creation of two polar ends in a particular chemical bond. Whereas, the molecular dipole refers to the creation of two polar ends in a particular chemical compound.

The terms bond dipole and molecular dipole are important in determining the chemical and physical properties of a chemical compound. A dipole is the state of having two ends with different polarity existing in the same chemical bond or chemical compound.

CONTENTS

1. Overview and Key Difference
2. What is a Bond Dipole 
3. What is a Molecular Dipole
4. Side by Side Comparison – Bond Dipole vs Molecular Dipole in Tabular Form
5. Summary

What is a Bond Dipole?

A bond dipole is the presence of two opposite charged ends in the same chemical bond. A bond dipole moment is an electric dipole moment where there is a positive charge and a negative charge in the same chemical bond. This charge separation occurs due to the polarity of the chemical bond. We can denote bond dipole as “”. We can give the bond dipole as a function of the partial electrical charge of the bond which is given by “δ”, and the distance between the charged ends given by “d” as follows:

= δd

A bond dipole is a vector that is parallel to the bond axis and its points from minus (negative charge) to plus (positive charge). However, in chemistry, we usually draw this vector pointing from plus to minus, in order to indicate the physical movement of electrons.

The SI unit for the measurement of a bond dipole is Coulomb-meter. Practically, this unit is not suitable because it gives too large values. Therefore, we can use the unit debye with the symbol “D” which uses the charge in (10^-10 x) statcuolombs and distance in angstroms.

For polyatomic molecules, the bond dipole is a characteristic for a particular chemical bond within the molecule. For diatomic molecules, there is only one bond between two atoms; thus, the bond dipole and the molecular dipole are the same.

What is Molecular Dipole?

Molecular dipole is the presence of two opposite charged ends in the same molecule. This type of dipole moment arises due to the net polarity of the molecule. The net polarity or the overall polarity of a molecule comes with the effect of the shape of the molecule and the polarity of each chemical bond.

Sometimes, a molecule having polar chemical bonds may not have a net polarity due to the shape that balances the net polarity to be zero. For example, a linear molecule with two polar bonds. Here, the dipole moment vectors of the two bonds cancel out. E.g. carbon dioxide.

What is the Difference Between Bond Dipole and Molecular Dipole?

The key difference between bond dipole and molecular dipole is that the term bond dipole refers to the creation of two polar ends in a particular chemical bond, whereas the term molecular dipole refers to the creation of two polar ends in a particular chemical compound. In other words, a bond dipole is given for a particular bond within a molecule, while molecular dipole is given for a particular chemical compound.

Below infographic summarizes the difference between bond dipole and molecular dipole.

Summary – Bond Dipole vs Molecular Dipole

Bond dipole and molecular dipole are two terms that are important in the determination of chemical and physical properties of chemical species. The key difference between bond dipole and molecular dipole is that the term bond dipole refers to the creation of two polar ends in a particular chemical bond whereas the term molecular dipole refers to the creation of two polar ends in a particular chemical compound.

Reference:

1. “Dipole Moments.” Chemistry LibreTexts, Libretexts, 30 Sept. 2019, Available here.
2. “Bond Dipole Moment.” Wikipedia, Wikimedia Foundation, 17 Dec. 2019, Available here.

Image Courtesy:

1. “Polarity boron trifluoride” By Emma Farmer (Serephine at English Wikipedia) – Created with Adobe Photoshop (Public Domain) via Commons Wikimedia
2. “Carbon-dioxide-3D-balls” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia