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

Assess the provided statements comparing the net dipole moments of NF₃ and NH₃, considering the reasons related to bond dipole direction and lone pair orbital dipole.

Concept Used:

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

Step-by-Step Solution:

Step 1: Note the experimental result: NH₃ has a net dipole moment of approximately 1.47 D, which is greater than NF₃'s approximately 0.23 D. This fact must be consistent with any correct reasoning.

Step 2: Evaluate Statement I: "Because fluorine is more electronegative than nitrogen, the net dipole moment of NF₃ is greater than NH₃." This statement is false. Although the N–F bond is highly polar, the net molecular dipole is a vector sum. The experimental data refutes this claim, as NH₃ exhibits a larger dipole moment.

Step 3: Analyze the vector model for dipole moments in these molecules. In both NH₃ and NF₃, the nitrogen lone pair contributes a dipole moment opposing the resultant of the three N–X bond dipoles.

For NH₃: The N–H bond dipole is directed from H to N (as N is more electronegative than H). The resultant of the three N–H bond dipoles aligns with the lone pair orbital dipole. Consequently, they add together, resulting in a substantial net dipole moment.

For NF₃: The N–F bond dipole is directed from N to F (as F is more electronegative than N). This direction is opposite to that in NH₃. The resultant of the three N–F bond dipoles is oriented opposite to the lone pair orbital dipole. Therefore, the bond dipoles and the lone pair dipole partially cancel, yielding a small net dipole moment.

Step 4: Evaluate 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." This description is incorrect. The correct description is the reverse: In NH₃, they are in the same direction, and in NF₃, they are in opposite directions.

Therefore, both Statement I and Statement II are false.