Which of the following arrangements of molecules correct on the basis of their dipole moments?
Concept: Dipole moment is a measure of the polarity of a molecule. It depends on the polarity of individual bonds and the molecular geometry. If a molecule has polar bonds but a symmetrical geometry, the individual bond dipoles may cancel out, resulting in a zero net dipole moment. If the geometry is asymmetrical, or if the bond dipoles do not cancel, the molecule will have a net dipole moment.
Why (C) is correct:
1. BF\u2083: Boron trifluoride (BF\u2083) has a trigonal planar geometry. Although the B-F bonds are polar, the three bond dipoles are oriented at 120\u00b0 to each other and cancel each other out due to the symmetrical geometry. Therefore, BF\u2083 has a zero net dipole moment (\u00b5 = 0 D).
2. NH\u2083: Ammonia (NH\u2083) has a trigonal pyramidal geometry. The N-H bonds are polar, and nitrogen also has a lone pair of electrons. Both the bond dipoles and the lone pair dipole moment point in the same general direction (towards the nitrogen atom and away from the hydrogen atoms), adding up to give a significant net dipole moment (\u00b5 \u2248 1.47 D).
3. NF\u2083: Nitrogen trifluoride (NF\u2083) also has a trigonal pyramidal geometry, similar to NH\u2083. However, in NF\u2083, the N-F bonds are polar, but the bond dipoles point towards the more electronegative fluorine atoms, while the lone pair dipole points away from the nitrogen atom. These dipoles are oriented in opposing directions, leading to partial cancellation. As a result, NF\u2083 has a smaller net dipole moment (\u00b5 \u2248 0.23 D) compared to NH\u2083.
Comparing the dipole moments: NH\u2083 (\u2248 1.47 D) > NF\u2083 (\u2248 0.23 D) > BF\u2083 (0 D).
Option Analysis:
- A) BF\u2083 > NF\u2083 > NH\u2083: Incorrect, as BF\u2083 has zero dipole moment and NH\u2083 has the highest.
- B) NF\u2083 > BF\u2083 > NH\u2083: Incorrect, as BF\u2083 has zero dipole moment and NH\u2083 has the highest.
- C) NH\u2083 > NF\u2083 > BF\u2083: Correct, this order reflects the actual dipole moments.
- D) NH\u2083 > BF\u2083 > NF\u2083: Incorrect, as BF\u2083 has zero dipole moment, which is less than NF\u2083.
Correct Answer: (C)