Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): NH$_3$ and NF$_3$ molecule have pyramidal shape with a lone pair of electrons on nitrogen atom. The resultant dipole moment of NH$_3$ is greater than that of NF$_3$.
Reason (R): In NH$_3$, the orbital dipole due to lone pair is in the same direction as the resultant dipole moment of the N$-$H bonds. F is the most electronegative element.
In the light of the above statements, choose the correct answer from the options given below:

• A. Both (A) and (R) are true and (R) is the correct explanation of (A)
• B. (A) is false but (R) is true
• C. (A) is true but (R) is false
• D. Both (A) and (R) are true but (R) is NOT the correct explanation of (A)

Answer 1

Answer: (A)

Both (A) and (R) are true and (R) is the correct explanation of (A)

Answer 2

The analysis of the molecules $\text{NH}_3$ and $\text{NF}_3$ is as follows:
Step 1: Structure and dipole moment of $\text{NH}_3$
$\text{NH}_3$ has a pyramidal shape due to the presence of one lone pair on the nitrogen atom.
- The dipole moments of the $\text{N–H}$ bonds and the lone pair point in the same direction, leading to a higher resultant dipole moment.
Step 2: Structure and dipole moment of $\text{NF}_3$
$\text{NF}_3$ also has a pyramidal shape, but the $\text{N–F}$ bonds are highly electronegative.
- The dipole moment of the lone pair on nitrogen is opposite to the resultant dipole moment of the $\text{N–F}$ bonds, which reduces the overall dipole moment.
Step 3: Comparison of dipole moments
- The dipole moment of $\text{NH}_3$ is approximately $1.47 \, \text{D}$, while that of $\text{NF}_3$ is approximately $0.80 \, \text{D}$.
- This confirms that $\text{NH}_3$ has a greater dipole moment than $\text{NF}_3$.
Step 4: Validating the statements
- Assertion (A): True, because $\text{NH}_3$ has a higher dipole moment than $\text{NF}_3$.
- Reason (R): True, as the lone pair’s dipole in $\text{NH}_3$ aligns with the bond dipoles, while in $\text{NF}_3$, it opposes them.
- $(R)$ is the correct explanation of $(A)$.
Final Answer: (1).