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

The Correct Option is A

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).