Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

The electronegativity difference between \(\mathrm{N}\) and \(\mathrm{F}\) is greater than that between \(\mathrm{N}\) and \(\mathrm{H}\), yet the dipole moment of \(\mathrm{NH}_{3}(1.5 \mathrm{D})\) is larger than that of \(\mathrm{NF}_{3}\) \((0.2 \mathrm{D})\) This is because (a) in \(\mathrm{NH}_{3}\) as well as \(\mathrm{NF}_{3}\) the atomic dipole and bond dipole are in opposite directions (b) in \(\mathrm{NH}_{3}\) the atomic dipole and bond dipole are in the opposite directions whereas in \(\mathrm{NF}_{3}\) these are in the same direction (c) in \(\mathrm{NH}_{3}\) as well as in \(\mathrm{NF}_{3}\) the atomic dipole and bond dipole are in same direction (d) in \(\mathrm{NH}_{3}\) the atomic dipole and bond dipole and in the same direction whereas in \(\mathrm{NF}_{3}\) these are in opposite directions

Step-by-Step Solution

Answer

Option (d) is correct: In NH₃, the dipoles are aligned; in NF₃, they are opposite.

Step 1: Understand the Concept of Dipole Moment

The dipole moment is a measure of the separation of positive and negative charge in a molecule, affecting its polar nature. It depends on both the individual atomic dipoles and the overall molecular geometry.

Step 2: Compare Electronegativity Differences

The electronegativity difference affects the bond dipole. Nitrogen (N) versus Fluorine (F) has a greater electronegativity difference compared to Nitrogen (N) versus Hydrogen (H). This suggests stronger bond dipoles toward F than H.

Step 3: Analysis of Molecular Shape

NH₃ and NF₃ both have a trigonal pyramidal shape due to the lone pair on nitrogen. However, the direction of bond dipoles and the lone pair causes different net dipole moments.

Step 4: Direction of the Bond and Molecular Dipoles

For NH₃, the bond dipoles are directed towards nitrogen, and the lone pair also contributes in the same direction, leading to a strong net dipole moment. For NF₃, the highly electronegative fluorines direct the bond dipoles away from nitrogen, but the lone pair is not aligned to add to the net dipole effectively.

Step 5: Conclusion and Selection of Correct Option

In NH₃, the atomic dipole due to the lone pair and the bond dipoles are aligned, increasing the dipole moment. In NF₃, the lone pair's dipole partially cancels out the dipoles from the N-F bonds. Therefore, option (d) is correct.

Key Concepts

Electronegativity

Electronegativity is a chemical property that describes how strongly an atom can attract electrons towards itself in a chemical bond. It plays a crucial role in determining the bond's polarity within a molecule.
Fluorine is known to be the most electronegative element. When it bonds with nitrogen, the bond is highly polar due to the significant electronegativity difference. Meanwhile, nitrogen is also more electronegative than hydrogen, but the difference is smaller.
This variance affects the bond dipoles, which are the individual dipole moments of each bond within a molecule. The larger the electronegativity difference, the more polarized the bond will be, meaning a stronger bond dipole.
This concept is pivotal when comparing molecules like NH₃ and NF₃. Despite NF₃ having a greater electronegativity difference and potentially stronger bond dipoles, the overall molecular dipole moment is influenced by other factors such as molecular geometry.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule, impacting its physical and chemical properties, including dipole moment.
Both NH₃ and NF₃ have a trigonal pyramidal geometry. This shape is due to the presence of a lone pair of electrons on the nitrogen atom, which exerts a repulsive force pushing the bonds into this particular formation.
Despite having the same geometry, the orientation of dipoles in these molecules differs. In NH₃, the lone pair and bond dipoles align to enhance the net dipole moment. In contrast, in NF₃, the orientation of bond dipoles is such that the influence of the lone pair is diminished, leading to a lower net dipole moment.
Understanding molecular geometry helps predict how bond dipoles might cancel out or add up, affecting the overall molecular dipole moment.

Bond Dipole

A bond dipole occurs when there is a difference in electronegativity between two atoms in a bond, causing a displacement of electron density from one atom to another.
The direction of a bond dipole is from the less electronegative element towards the more electronegative element. In NH₃, the N-H bonds have dipoles directed towards nitrogen. In NF₃, the N-F bonds have dipoles directed towards fluorine.
However, the overall dipole moment of a molecule is not just a sum of bond dipoles. It also depends greatly on the spatial arrangement of these dipoles and any lone pairs. For instance, in NH₃, bond dipoles and the dipole due to the lone pair are in the same direction, leading to a greater net dipole moment. In NF₃, they are in opposing directions, which reduces the net dipole moment.
By understanding bond dipoles within the context of molecular structure, one can deduce the polarity and subsequent behavior of molecules in varied environments.