Why `BF_3` is non - polar and `NF_3` is polar although `BF_3 and NF_3` both molecules are covalent ?

Options

A. In uncombined state boron is metal and nitrogen is gas
B. B - F bond has no dipole moment whereas N - F bond has dipole moment
C. The size of boron atom is smaller than nitrogen
D. `BF_3` is planar whereas `NF_3` is pyramidal

Answer

To understand why BF₃ is non-polar and NF₃ is polar despite both being covalent molecules, we need to analyze their molecular structures and the concept of dipole moments.

### Step-by-Step Solution:

1. **Molecular Geometry of BF₃**:
- BF₃ (Boron Trifluoride) has a trigonal planar geometry.
- The boron atom is at the center with three fluorine atoms symmetrically arranged around it at 120-degree angles.

**Hint**: Remember that molecular geometry plays a crucial role in determining the polarity of a molecule.

2. **Electronegativity and Dipole Moments in BF₃**:
- Fluorine is more electronegative than boron, which means each B-F bond is polar, with the dipole moment pointing from boron to fluorine.
- However, because of the symmetrical arrangement of the three dipole moments (120 degrees apart), they cancel each other out.

**Hint**: Consider how vector addition works with dipole moments. Symmetry can lead to cancellation.

3. **Resultant Dipole Moment of BF₃**:
- Since the dipole moments cancel each other out due to symmetry, the resultant dipole moment of BF₃ is zero.
- Therefore, BF₃ is classified as a non-polar molecule.

**Hint**: A molecule is non-polar if the vector sum of its dipole moments equals zero.

4. **Molecular Geometry of NF₃**:
- NF₃ (Nitrogen Trifluoride) has a trigonal pyramidal geometry due to the presence of a lone pair of electrons on the nitrogen atom.
- The three fluorine atoms are arranged around the nitrogen atom, but the lone pair pushes the fluorine atoms down, creating an asymmetrical shape.

**Hint**: The presence of lone pairs can affect molecular geometry and polarity.

5. **Electronegativity and Dipole Moments in NF₃**:
- Similar to BF₃, each N-F bond is polar because fluorine is more electronegative than nitrogen.
- However, in NF₃, the dipole moments do not cancel out due to the pyramidal shape. The lone pair contributes to a net dipole moment pointing towards the nitrogen.

**Hint**: Non-cancellation of dipole moments due to asymmetry leads to a polar molecule.

6. **Resultant Dipole Moment of NF₃**:
- The dipole moments from the three N-F bonds add up to give a net dipole moment that is not zero.
- Thus, NF₃ is classified as a polar molecule.

**Hint**: A molecule is polar if it has a net dipole moment that does not equal zero.

### Conclusion:
- BF₃ is non-polar because its symmetrical trigonal planar structure causes the dipole moments to cancel out.
- NF₃ is polar due to its asymmetrical trigonal pyramidal structure, which results in a net dipole moment.