Given below are two statements — Assertion (A) and Reason (R): Assertion (A): NH3 and NF3 molecules have pyramidal…
Chemical Bonding · Class 11 · JEE Main Previous Year Question
Given below are two statements — Assertion (A) and Reason (R):
Assertion (A): and molecules have pyramidal shape with a lone pair on nitrogen. The resultant dipole moment of is greater than that of .
Reason (R): In , 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.
Choose the correct answer:
- a✓
Both (A) and (R) are true and (R) is the correct explanation of (A)
- b
(A) is false but (R) is true
- c
Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
- d
(A) is true but (R) is false
Both (A) and (R) are true and (R) is the correct explanation of (A)
🧠 In the bond dipoles of N–H and the lone pair dipole point in the same direction (both toward N); in they point in opposite directions.
🗺️ Evaluate Assertion (A):
- : μ ≈ 1.47 D; : μ ≈ 0.24 D
- has a greater net dipole moment → A is TRUE ✓
Evaluate Reason (R):
In : N is the negative end of N–H bonds (N more electronegative than H), and the lone pair also contributes a dipole pointing away from the bond axis — both reinforce each other → large net μ ✓
In : F is the negative end of N–F bonds, so bond dipoles point away from N; but the lone pair dipole points toward N — they partially cancel → small net μ ✓
R correctly explains A ✓
⚡ The key insight: electronegativity of the attached atoms (H vs F) reverses the direction of bond dipoles relative to the lone pair, drastically changing the net dipole moment.
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