NCERT Line-by-Line
1. TITLE
NCERT says 'orbital dipole is in the same direction' — Here's how NEET tests it
2. THE NCERT STATEMENT
NCERT EXACT LINE:
"This is because, in case of $NH_3$ the orbital dipole due to lone pair is in the same direction as the resultant dipole moment of the N–H bonds, whereas in $NF_3$ the orbital dipole is in the direction opposite to the resultant dipole moment of the three N–F bonds."
— NCERT Class 11, Chapter 4, Page 112
3. WHY THIS LINE MATTERS
- Tested Frequently: This is the definitive "anomaly" question in Chemical Bonding. It challenges the assumption that "higher electronegativity difference = higher dipole moment."
- The Tricky Part: Fluorine is more electronegative than Nitrogen, which usually implies a stronger polar bond. However, the Dipole Moment is a vector quantity, meaning direction matters as much as magnitude.
- Student Mistake: Students often guess $NF_3$ has a higher dipole moment because Fluorine is the most electronegative element, ignoring the geometry and vector cancellation described in this specific line.
4. MCQ (Directly from this NCERT line)
According to NCERT, the dipole moment of $NH_3$ is significantly greater than that of $NF_3$ because:
A) Fluorine is more electronegative than Nitrogen, causing a stronger pull on electrons.
B) In $NH_3$, the orbital dipole involves a sigma bond, while $NF_3$ involves pi bonding.
C) In $NH_3$, the orbital dipole is in the same direction as the resultant bond moments, whereas in $NF_3$ they oppose each other.
D) The orbital dipole in $NH_3$ is in the direction opposite to the resultant dipole moment of the N–H bonds.
5. ANSWER + LINE BREAKDOWN
Correct Answer: C) In $NH_3$, the orbital dipole is in the same direction as the resultant bond moments, whereas in $NF_3$ they oppose each other.
Break down the NCERT line:
* "Orbital dipole due to lone pair": The lone pair on Nitrogen creates its own dipole moment (pointing "up" away from the atom).
* "Same direction... in NH3": In $NH_3$, Nitrogen pulls electrons from Hydrogen. The N–H bond dipoles point towards Nitrogen (up). This adds to the lone pair dipole.
* "Direction opposite... in NF3": In $NF_3$, Fluorine pulls electrons from Nitrogen. The N–F bond dipoles point towards Fluorine (down). This pulls against the lone pair dipole, cancelling out much of the net moment.
6. HOW NEET TWISTS THIS
1. As an Assertion-Reason Question:
* Assertion: The dipole moment of $NH_3$ ($4.90 \times 10^{-30}$ C m) is greater than that of $NF_3$ ($0.80 \times 10^{-30}$ C m).
* Reason: In $NF_3$, the orbital dipole is in the direction opposite to the resultant dipole moment of the three N–F bonds.
* (Answer: Both True, and Reason is the correct explanation).
2. As a "True/False" Statement:
* "The orbital dipole because of the lone pair increases the effect of the resultant N–F bond moments in $NF_3$."
* (Answer: False. NCERT explicitly says it decreases the effect).
3. As a Diagram-Based Question:
* NEET may show the vector diagrams of $NH_3$ and $NF_3$ (Fig 4.10 in NCERT) and ask you to identify which vector sum represents $NF_3$. You must look for the arrows pointing away from the central Nitrogen atom (towards Fluorine) opposing the lone pair.
7. MEMORY LOCK
MEMORIZE:
* $NH_3$ = Vectors ADD (Lone pair & Bond pairs go UP).
* $NF_3$ = Vectors SUBTRACT (Lone pair goes UP, Bond pairs go DOWN).
Mnemonic:
"Hydrogen Helps (Adds), Fluorine Fights (Cancels)."
8. RELATED NCERT LINES
Also remember these from the same page (Page 111-112):
* "Dipole moment ($\mu$) = charge (Q) $\times$ distance of separation (r)".
* "The dipole moment in case of $BeF_2$ is zero... because the two equal bond dipoles point in opposite directions and cancel the effect of each other".
* "In tetra-atomic molecule, for example in $BF_3$, the dipole moment is zero".
9. NCERT REFERENCE
NCERT Class 11, Chapter 4 (Chemical Bonding), Page 112, Paragraph under "4.3.6 Polarity of Bonds".
10. CTA
All important NCERT lines from this chapter → Chemical Bonding & Molecular Structure Guide