Question

Select the incorrect statement from the following.

• A. \(\text{NH}_3\) and \(\text{NF}_3\) both are polar molecules
• B. \(\text{NH}_3\) and \(\text{NF}_3\) both have pyramidal shape
• C. In \(\text{NH}_3\) and \(\text{NF}_3\) the orbital dipole due to lone pair is in the same direction as that of resultant dipole moment of other three bonds
• D. Dipole moment of \(\text{NH}_3\) is greater than \(\text{NF}_3\)

Hint:

In NH3 the lone-pair moment adds to the bond moments, but in NF3 it opposes them.

Answer 1

The Correct Option is C

Step 1: Compare the two dipole directions:
Draw each molecule as a pyramid with N at the top. The lone pair of N points upward, away from the three outer atoms.
In $\text{NH}_3$ the bond dipoles point from H towards N (N is more electronegative), so their resultant also points up, along the lone pair.
In $\text{NF}_3$ the bond dipoles point from N towards F (F is more electronegative), so their resultant points down, against the lone pair.

Step 2: Test each statement:
(A) Both are polar because neither is symmetric. True.
(B) Both are pyramidal with one lone pair on N. True.
(D) $\text{NH}_3$ has the larger moment (about 1.46 D against about 0.24 D) because the contributions add in one and subtract in the other. True.
(C) The same-direction claim holds only for $\text{NH}_3$. For $\text{NF}_3$ the directions are opposite. So (C) is false.

Final Answer:
The incorrect statement is (C). \[ \boxed{\text{(C)}} \]