Explain why BCl3 is not polar, but NCl3 is polar.
Explain why BCl3 is not polar, but NCl3 is polar.
The Correct Answer and Explanation is:
BCl₃ is not polar, but NCl₃ is polar due to differences in molecular geometry and symmetry.
Polarity Basics
Molecular polarity depends on two factors:
- Electronegativity difference between atoms (which creates dipole moments).
- Molecular geometry, which determines whether dipole moments cancel out or not.
Boron Trichloride (BCl₃)
- Lewis Structure: Boron (B) is bonded to three chlorine (Cl) atoms.
- Molecular Geometry: Trigonal planar (due to three bonding pairs and no lone pairs on boron).
- Bond Angles: 120°.
- Symmetry: Highly symmetrical.
Each B–Cl bond is polar because Cl is more electronegative than B. However, in the trigonal planar shape, these three identical polar bonds are arranged symmetrically around the central boron atom. As a result, their dipole moments cancel out perfectly, leading to a nonpolar molecule overall.
Nitrogen Trichloride (NCl₃)
- Lewis Structure: Nitrogen (N) is bonded to three Cl atoms and has one lone pair.
- Molecular Geometry: Trigonal pyramidal (due to three bonding pairs and one lone pair on nitrogen).
- Bond Angles: Approximately 107°.
- Asymmetry: The lone pair on nitrogen causes the molecule to be asymmetrical.
While each N–Cl bond is polar, the presence of the lone pair on nitrogen distorts the shape, preventing the dipole moments from canceling out. This results in a net dipole moment pointing away from the lone pair toward the chlorine atoms. Therefore, NCl₃ is a polar molecule.
Conclusion
BCl₃ is nonpolar because its symmetric trigonal planar shape allows dipole moments to cancel out. In contrast, NCl₃ is polar due to its trigonal pyramidal shape caused by a lone pair on nitrogen, which introduces asymmetry and prevents dipole cancellation. Thus, molecular geometry plays a critical role in determining molecular polarity.
