Question

Statement I: Since fluorine is more electronegative than nitrogen, the net dipole moment of NF₃ is greater than NH₃.
Statement II: In NH₃, the orbital dipole due to the lone pair and the dipole moment of NH bonds are in opposite directions, but in NF₃, the orbital dipole due to the lone pair and the dipole moments of N-F bonds are in the same direction.
In light of the above statements, choose the most appropriate from the options given below:

• A. Statement I is true but Statement II is false.
• B. Both Statement I and Statement II are false.
• C. Both statement I and Statement II is are true.
• D. Statement I is false but Statement II is are true.

Answer 1

The Correct Option is C

Evaluate the given statements regarding the comparison of net dipole moments in NF₃ and NH₃ and the provided reason involving the direction of bond dipoles and the lone pair orbital dipole.

Concept Used:

The net dipole moment of a molecule is the vector sum of all individual bond dipoles and the dipole associated with any lone pair (often called the orbital dipole). For pyramidal molecules like NH₃ and NF₃, the direction of the net dipole moment depends on the relative magnitudes and directions of the N–H/N–F bond dipoles and the lone pair dipole. The bond dipole direction is from less electronegative to more electronegative atom.

Step-by-Step Solution:

Step 1: Analyze the known experimental result. The net dipole moment of NH₃ (≈ 1.47 D) is greater than that of NF₃ (≈ 0.23 D). This is a key experimental fact that any correct reasoning must align with.

Step 2: Analyze Statement I. It claims that because fluorine is more electronegative than nitrogen, the net dipole moment of NF₃ is greater than NH₃. This is false. While the N–F bond is highly polar, the net molecular dipole is a vector sum. The experimental data directly contradicts this claim, as NH₃ has the larger dipole moment.

Step 3: Understand the vector model for dipole moments in such molecules. In both NH₃ and NF₃, the lone pair on nitrogen contributes a significant dipole moment in a direction opposite to the resultant of the three N–X bond dipoles.

For NH₃: The N–H bond dipole points from H to N (since N is more electronegative than H). The resultant of the three N–H bond dipoles is in the same direction as the lone pair orbital dipole. Thus, they add up, leading to a large net dipole moment.

For NF₃: The N–F bond dipole points from N to F (since F is more electronegative than N). This direction is opposite to that in NH₃. The resultant of the three N–F bond dipoles is in the direction opposite to the lone pair orbital dipole. Thus, the bond dipoles and the lone pair dipole partially cancel each other, leading to a small net dipole moment.

Step 4: Analyze Statement II. It states: "In NH₃, the orbital dipole due to the lone pair and the dipole moment of NH bonds are in opposite directions, but in NF₃, the orbital dipole due to the lone pair and the dipole moments of N-F bonds are in the same direction." This description is incorrect. The accurate description is the reverse: In NH₃, they are in the same direction, and in NF₃, they are in opposite directions.

Thus, both Statement I and Statement II are false.