Study Guides/Chemistry/Dipole Moment: NF3 vs NH3
Study Guide · Chemistry

Why is the Dipole Moment of NF3 Less Than NH3?

In Class 11 Chemistry (Chemical Bonding), a classic and frequently asked question is: Why is the dipole moment of Ammonia (NH₃) greater than Nitrogen trifluoride (NF₃)? This is counter-intuitive because Fluorine is much more electronegative than Hydrogen.

Question (Click to Flip)

Why is the dipole moment of NH3 greater than NF3?

Answer

In NH3, the N-H bond dipoles and the lone pair dipole point in the same direction, adding up to a high net dipole. In NF3, the highly electronegative Fluorines pull the bond dipoles in the opposite direction of the lone pair dipole, partially canceling the net dipole.

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Key Facts

Dipole Moment is a vector quantity (magnitude + direction).

NH₃ (Ammonia): Bond dipoles and lone pair dipole point in the SAME direction (Addition).

NF₃: Bond dipoles and lone pair dipole point in OPPOSITE directions (Cancellation).

Result: NH₃ (1.47 D) > NF₃ (0.23 D).

The Structures

Both NH₃ and NF₃ have a pyramidal shape with a central Nitrogen atom. Nitrogen has one lone pair of electrons sitting at the top of the pyramid. Dipole moment is a vector quantity, meaning direction matters!

The Case of NH₃ (Ammonia)

  • In NH₃, Nitrogen is more electronegative than Hydrogen.
  • Therefore, the N-H bond dipoles point UPWARD (from H towards N).
  • The lone pair dipole also points UPWARD (away from the nucleus).
  • Result: All vectors point in the same direction. They add up, resulting in a high net dipole moment (1.47 D).

The Case of NF₃ (Nitrogen Trifluoride)

  • In NF₃, Fluorine is more electronegative than Nitrogen.
  • Therefore, the N-F bond dipoles point DOWNWARD (from N towards F).
  • However, the lone pair dipole still points UPWARD.
  • Result: The bond dipoles and the lone pair dipole point in opposite directions. They cancel each other out partially, resulting in a low net dipole moment (0.23 D).

Questions and Answers

Why is the dipole moment of NH3 greater than NF3?+

In NH3, the N-H bond dipoles and the lone pair dipole point in the same direction, adding up to a high net dipole. In NF3, the highly electronegative Fluorines pull the bond dipoles in the opposite direction of the lone pair dipole, partially canceling the net dipole.

What is the shape of NH3 and NF3 molecules?+

Both NH3 and NF3 have a trigonal pyramidal shape due to the presence of 3 bond pairs and 1 lone pair on the central Nitrogen atom.

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