Question

Boron Trifluoride$\left({\mathrm{BF}}_3\right)$is a non polar molecule whereas ammonia $\left({\mathrm{NH}}_3\right)$is a polar molecule. The difference in polarity is related to the fact that

• A. ${\mathrm{BF}}_3$ has no hydrogen bonding and ${\mathrm{NH}}_3$ does
• B. ${\mathrm{BF}}_3$ is triangular planar and ${\mathrm{NH}}_3$ is pyramidal
• C. ${\mathrm{BF}}_3$ is Lewis base and ${\mathrm{NH}}_3$ is Lewis acid
• D. None of the above.

Answer 1

The correct option is B

${\mathrm{BF}}_3$ is triangular planar and ${\mathrm{NH}}_3$ is pyramidal

Explanation for correct option

(B)${\mathrm{BF}}_3$ is triangular planar and ${\mathrm{NH}}_3$ is pyramidal

Boron trifluoride

• They arrange themselves in trigonal planar geometry.
• The dipole moment in this molecule is zero as shown by the diagram given below:-

• So, the Boron Trifluoride$\left({\mathrm{BF}}_3\right)$is a nonpolar molecule.

Ammonia

• In case of ${\mathrm{NH}}_3$ the orbital dipole due to lone pair is in the same direction as the resultant dipole moment of the $\text{N – H}$ bonds.
• Due to one lone pair, it has a pyramidal shape with no symmetry.
• So, it is a polar molecule.

Explanation for incorrect option

• Due to the shape of (${\mathrm{BF}}_3$ is triangular planar and ${\mathrm{NH}}_3$ is pyramidal), Boron Trifluoride$\left({\mathrm{BF}}_3\right)$is a nonpolar molecule whereas ammonia $\left({\mathrm{NH}}_3\right)$is a polar molecule.
• Hence, options (A), (C), and (D) are incorrect.

Hence, option (B) is correct. The difference in polarity is related to the fact that ${\mathrm{BF}}_3$ is triangular planar and ${\mathrm{NH}}_3$ is pyramidal