Problem 64 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following statements is true? (a) The dipole moment of \(\mathrm{NF}_{3}\) is more than \(\mathrm{NH}_{3}\) (b) The dipole moment of \(\mathrm{NF}_{3}\) is less than \(\mathrm{NH}_{3}\) (c) The dipole moment of \(\mathrm{NH}_{3}\) is zero (d) The dipole moment of \(\mathrm{NF}_{3}\) is equal to \(\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The correct statement is (b): The dipole moment of \(\mathrm{NF}_{3}\) is less than \(\mathrm{NH}_{3}\).

Step 1: Understanding Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule. It depends on both the magnitude of the charges and the distance between them. It is a vector quantity, meaning it has both direction and magnitude.

Step 2: Structure of NH3 and NF3

Ammonia (\(\mathrm{NH}_{3}\)) has a trigonal pyramidal shape, with nitrogen at the center and hydrogen atoms forming the base. The lone pair of electrons on nitrogen pushes the hydrogen atoms downwards. In \(\mathrm{NF}_{3}\), nitrogen is also in the center with a lone pair, forming the same trigonal pyramidal shape, but with fluorine atoms.

Step 3: Direction of Dipole Moment

In \(\mathrm{NH}_{3}\), the dipole moment points from the nitrogen atom towards the hydrogen atoms since nitrogen is more electronegative than hydrogen. This results in a net dipole moment directed upwards. In \(\mathrm{NF}_{3}\), the dipole moment vectors are directed from the nitrogen towards the fluorine atoms, but since fluorine is more electronegative than nitrogen, the net dipole moment is directed downwards.

Step 4: Comparison of Dipole Moments

For \(\mathrm{NH}_{3}\), the upward-directed dipole moment results in a relatively high net dipole moment because the vectors add in the same direction as the lone pair. However, in \(\mathrm{NF}_{3}\), the strong electronegativity of fluorine causes the dipole moments of the bonds to oppose the lone pair, reducing the net dipole moment.

Step 5: Determining the Correct Statement

Based on the analysis, \(\mathrm{NH}_{3}\) has a larger dipole moment than \(\mathrm{NF}_{3}\) due to the addition of its molecular dipole vectors aligning with the lone pair of electrons, resulting in option (b) being correct: "The dipole moment of \(\mathrm{NF}_{3}\) is less than \(\mathrm{NH}_{3}\)."

Key Concepts

Electronegativity

Electronegativity is a key concept in understanding molecular interactions. It is the ability of an atom to attract shared electrons towards itself in a chemical bond. Different elements have varying electronegativity values, with fluorine being the most electronegative element on the periodic table.
In molecules like \(F_3\) and \(H_3\), electronegativity plays a crucial role in determining the direction and magnitude of their dipole moments.
This is because the difference in electronegativity between bonded atoms leads to a polar bond.
For instance, ammonia (\(NH_3\)) comprises nitrogen and hydrogen atoms, where nitrogen is more electronegative than hydrogen. This difference causes a partial negative charge on nitrogen and a partial positive charge on each hydrogen, resulting in a net dipole moment directed upwards.

• Electronegativity increases across a period from left to right.
• It generally decreases as you move down a group due to increasing atomic size.

Similarly, in \(F_3\), nitrogen is less electronegative than fluorine, leading to a different dipole moment behavior. Ultimately, understanding electronegativity helps explain why \(H_3\) has a larger dipole moment than \(F_3\), as the direction of bond polarity contributes significantly to the overall molecular dipole moment.

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule, dictated by electron pair repulsion and the number of atoms attached to a central atom.
Understanding molecular geometry is critical for predicting the behavior and properties of molecules, including polarity and reactivity.
For ammonia (\(NH_3\) and nitrogen trifluoride (\(NF_3\)), both compounds exhibit a trigonal pyramidal shape.
This geometry arises due to the presence of three bonding pairs and one lone pair of electrons around the central nitrogen atom.

• Electron pairs, both bonding and non-bonding, repel each other.
• This repulsion affects the positioning of the atoms to minimize energy and maximize stability.

The lone pair on the nitrogen atom in both molecules influences the overall molecular shape. It pushes the bonded atoms (hydrogen or fluorine) downward, affecting the net dipole moment. As a result, despite the same geometric arrangement, the differences in the type of atoms bonded to nitrogen lead to different dipole behaviors, making molecular geometry essential in distinguishing \(H_3\) from \(F_3\) concerning their polar characteristics.

Trigonal Pyramidal Shape

The trigonal pyramidal shape is characteristic of molecules having a central atom surrounded by three atoms and a lone pair. This shape is a result of the VSEPR (Valence Shell Electron Pair Repulsion) theory, which predicts the arrangement of electron pairs around a central atom to minimize repulsion.
In both ammonia (\(NH_3\) and nitrogen trifluoride (\(NF_3\)), the central nitrogen atom exhibits this trigonal pyramidal shape.
The lone pair of electrons plays a crucial role in shaping these molecules as it exerts a greater repulsive force than bonded pairs, pushing the other atoms downward.

• This results in a pyramid-like structure with the lone pair at the apex.
• Overall dipole moments result from both the geometry and electronegativity differences.

The geometrical structure affects the distribution of charge within the molecule. In \(H_3\), the axial symmetry caused by hydrogen atoms and nitrogen's lone pair means that the dipole moments align with the lone pair's direction, enhancing the net dipole.
Conversely, in \(F_3\), the stronger electronegative fluorine atoms oppose the lone pair's influence, thus diminishing the net dipole compared to \(H_3\). Understanding the trigonal pyramidal shape helps explain the distinct dipole moment differences between these two compounds.