Which one among the following is the correct arrangement of molecules in increasing order of their dipole moment ?
[amp_mcq option1=”NF₃ < NH₃ < BF₃" option2="BF₃ < NH₃ < NF₃" option3="BF₃ < NF₃ < NH₃" option4="NF₃ < BF₃ < NH₃" correct="option3"]
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UPSC Geoscientist – 2024
– **BF₃**: Boron trifluoride has a central Boron atom bonded to three Fluorine atoms. Boron is sp² hybridized, and the molecule is trigonal planar. The B-F bonds are polar, but the three bond dipoles are symmetrically arranged at 120° to each other in a plane. Their vector sum is zero, so BF₃ has a zero net dipole moment.
– **NF₃**: Nitrogen trifluoride has a central Nitrogen atom bonded to three Fluorine atoms and one lone pair. Nitrogen is sp³ hybridized, and the molecule is trigonal pyramidal. N-F bonds are polar (F is more electronegative). The bond dipoles point towards the F atoms, away from N. The lone pair contributes a dipole moment pointing away from N. The N-F bond dipoles and the lone pair dipole are in opposite directions along the molecular axis, partially canceling each other. This results in a small net dipole moment for NF₃.
– **NH₃**: Ammonia has a central Nitrogen atom bonded to three Hydrogen atoms and one lone pair. Nitrogen is sp³ hybridized, and the molecule is trigonal pyramidal. N-H bonds are polar (N is more electronegative). The bond dipoles point towards the N atom. The lone pair contributes a dipole moment pointing away from N. Both the N-H bond dipoles and the lone pair dipole point in the same general direction (upwards along the molecular axis), reinforcing each other. This results in a significant net dipole moment for NH₃.
Comparing NF₃ and NH₃, the dipole moment of NH₃ is significantly larger than that of NF₃.
Therefore, the order of increasing dipole moment is BF₃ (0) < NF₃ < NH₃.