What is the Difference Between Bond Moment and Dipole Moment

Bond moment and dipole moment are two terms that describe the polarity within a molecule. Polarity occurs when atoms share electrons unequally. It creates one slight positive end and one slight negative end. This uneven sharing depends on the electronegativity of atoms.

What is the difference between bond moment and dipole moment? Bond moment refers to the polarity of a single bond between two atoms, whereas dipole moment refers to the overall polarity of a molecule.

Key Areas Covered

1. What is Bond Moment  
      – Definition, Features 
2. What is Dipole Moment
      – Definition, Features 
3. Similarities Between Bond Moment and Dipole Moment
      – Outline of Common Features
4. Difference Between Bond Moment and Dipole Moment
      – Comparison of Key Differences
5. FAQ: Bond Moment and Dipole Moment
      – Frequently Asked Questions

Key Terms

Bond Moment, Dipole Moment

What is Bond Moment

Bond moment is a measure of the polarity within a single chemical bond. It occurs because atoms share electrons unequally due to the difference in electronegativity. The more electronegative atom pulls the shared electrons closer. This creates a slight negative change around it. The other atom gets a positive charge. This uneven pull is the bond moment, represented by an arrow pointing toward the electronegative atom.

The strength of the bond moment depends on the electronegativity difference. The bigger difference creates a larger bond movement. If the bonds are arranged symmetrically, their individual pulls might cancel out, resulting in a zero overall polarity for the molecule.

What is Dipole Moment

Dipole moment is a measure of polarity in a molecule. The reason for the dipole moment is the uneven electron sharing between the atoms within a molecule. The greater the difference in electronegativity between atoms and the greater the distance between the partial charges, the larger the dipole moment. Consider water as an example. Hydrogen is less electronegative than oxygen. Hence, oxygen pulls the shared electrons closer to itself, creating a slight negative charge on the oxygen atom and a slight positive charge on the hydrogen atom. This results in a bent molecule with a polar dipole moment.

Dipole moment plays a significant role in diverse aspects of chemistry. It influences the interaction of molecules with each other. Hence, it affects properties such as intermolecular forces, solubility, and even melting point. Polar molecules can attract each other through dipole-dipole interactions, leading to higher boiling points and better solubility in polar solvents like water.

Similarities Between Bond Moment and Dipole Moment

1. Bond moment and dipole moment arise due to differences in electronegativity between atoms in a molecule.
2. Both are vector quantities.
3. Both are measured in Debye (D)

Difference Between Bond Moment and Dipole Moment

Definition

• Bond moment refers to the polarity of a single bond between two atoms, whereas dipole moment refers to the overall polarity of a molecule.

Polarity

• Bond moment focuses on the polarity of a single bond within a molecule, while dipole moment focuses on the overall polarity of the entire molecule.

Representation

• Bond moment represents the individual vector describing the polarity of a single bond, whereas dipole moment represents the net vector resulting from the sum of all individual bond moments in the molecule.

Dependence

• Bond moment is independent of the molecule’s overall geometry, while dipole moment is highly dependent on the molecule’s geometry.

Predicting

• Bond moment is easier to predict based on the electronegativity difference of the bonded atoms. However, dipole moment requires considering all bond moments and their directions within the molecule.

FAQ: Bond Moment and Dipole Moment

1. What is the relationship between bond length and dipole moment?

Bond length can influence dipole moment, but it’s not the sole factor. Generally, as the bond length increases, the dipole moment can also increase.

2. How do you know if a bond has a dipole moment?

There are two main ways to determine if a bond has a dipole moment. It can be determined by the difference in electronegativity and Lewis’s structure and molecular geometry.

3. Does H₂O have a dipole moment?

Yes, H₂O (water) does have a dipole moment. The dipole moment of water is approximately 1.85 Debye (D).

4. Does CO₂ have a dipole moment?

No, CO₂ (carbon dioxide) does not have a dipole moment, even though the individual C-O bonds are polar.

5. What is the difference between a dipole and a polar bond?

A polar bond emerges when two atoms share electrons in a covalent bond unevenly. This results in one atom holding a slight negative charge while the other holds a slight positive charge. A dipole, on the other hand, consists of two equal but opposite charges or magnetic poles positioned close together.

Conclusion

Bond moment and dipole moment are two terms that describe the polarity within a molecule. Bond moment focuses on a single bond’s polarity, while dipole moment considers the entire molecule. This is the basic difference between bond moment and dipole moment.

Reference:

1. Smyth, P, Charles. “Bond Moment and Electronegativity.” ACS Publications.
2. “Dipole Moment.” Science Direct.

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. “Water-elpot-transparent-3D-balls” (Public Domain) via Commons Wikimedia