Molecular Polarity Analysis

The polarity of a molecule depends on its molecular geometry and the polarity of its bonds. A molecule is polar if it has a net dipole moment.

$BF_3$ (Boron Trifluoride) Analysis

• Geometry: Trigonal planar. Boron is at the center with three Fluorine atoms arranged symmetrically.
• Bond Polarity: The B-F bonds are polar because Fluorine is more electronegative than Boron. The bond dipoles point from Boron towards each Fluorine atom.
• Net Dipole Moment: Due to the symmetrical trigonal planar geometry, the three B-F bond dipoles cancel each other out vectorially.
• Conclusion: $BF_3$ has a net dipole moment of zero and is a nonpolar molecule.

$NF_3$ (Nitrogen Trifluoride) Analysis

• Geometry: Trigonal pyramidal. Nitrogen is at the apex with three Fluorine atoms forming the base. There is one lone pair of electrons on the Nitrogen atom.
• Bond Polarity: The N-F bonds are polar because Fluorine is significantly more electronegative than Nitrogen. The bond dipoles point from Nitrogen towards each Fluorine atom.
• Net Dipole Moment: The lone pair on the Nitrogen atom points upwards, while the bond dipoles point downwards towards the Fluorine atoms. These effects partially oppose each other. The resulting net dipole moment is small and points downwards, making the molecule polar.

$NH_3$ (Ammonia) Analysis

• Geometry: Trigonal pyramidal. Nitrogen is at the apex with three Hydrogen atoms forming the base. There is one lone pair of electrons on the Nitrogen atom.
• Bond Polarity: The N-H bonds are polar because Nitrogen is more electronegative than Hydrogen. The bond dipoles point from Hydrogen towards Nitrogen.
• Net Dipole Moment: The lone pair on the Nitrogen atom points upwards, and the N-H bond dipoles also point upwards towards the Nitrogen atom. The lone pair reinforces the bond dipoles, leading to a significant net dipole moment pointing towards the Nitrogen atom.
• Conclusion: $NH_3$ has a substantial net dipole moment and is a polar molecule.

Ordering the Polarity

• $BF_3$ is nonpolar (dipole moment = 0).
• $NF_3$ is polar, but has a smaller dipole moment compared to $NH_3$ because the lone pair and bond dipoles partially oppose each other.
• $NH_3$ is polar and has a larger dipole moment because the lone pair reinforces the bond dipoles.

Therefore, the order of polarity from least polar to most polar is:

$BF_3 < NF_3 < NH_3$