Problem 44 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following species has the highest dipole moment? (a) \(\mathrm{PH}_{3}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{SbH}_{3}\) (d) \(\mathrm{AsH}_{3}\)

Step-by-Step Solution

Answer

ext{NH}_3 has the highest dipole moment.

Step 1: Understanding Dipole Moment

Molecules have a dipole moment when there is an uneven distribution of electrons between atoms, causing a separation of charge. This is influenced by differences in electronegativity and molecular geometry. A higher dipole moment reflects a greater extent of charge separation.

Step 2: Assessing Electronegativity

Examine the electronegativity of the central atoms involved: - Nitrogen ( ext{N}) is the most electronegative, followed by Phosphorus ( ext{P}), Arsenic ( ext{As}), and Antimony ( ext{Sb}). This suggests that more electronegative central atoms will form more polar bonds with hydrogen, affecting the dipole moment.

Step 3: Evaluating Molecular Geometry

Each molecule in the question has a trigonal pyramidal shape due to the lone pair on the central atom. This geometry allows a dipole moment to exist, as there is an asymmetrical distribution of charge.

Step 4: Comparing the Molecules

Given that all have similar shapes, the determining factor for the dipole moment is the electronegativity of the central atom. - ext{NH}_3 has Nitrogen as its central atom, which is the most electronegative, creating a stronger polar bond with hydrogen compared to ext{PH}_3, ext{SbH}_3, or ext{AsH}_3.

Step 5: Concluding the Answer

Therefore, ext{NH}_3 has the highest dipole moment because it has the most electronegative central atom among the choices, resulting in the greatest separation of charge.

Key Concepts

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons when part of a compound.
Atoms with higher electronegativity tend to draw electrons closer, creating a more polar bond. This polar bond leads to an uneven distribution of electron density between the atoms involved.

• Nitrogen (\(N\)) is highly electronegative, while atoms like Phosphorus (\(P\)), Arsenic (\(As\)), and Antimony (\(Sb\)) have progressively lower electronegativity.
• The higher the electronegativity difference between bonded atoms, the greater the potential dipole moment.

In our case, the nitrogen atom in \(\mathrm{NH}_{3}\) is the most electronegative among the given species, leading to the strongest polarity and thus, the highest dipole moment.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. Geometry plays a crucial role in determining the dipole moment.
When a molecule has an uneven spatial distribution of atoms or lone pairs, it can lead to an asymmetrical charge distribution.

• All the compounds in our exercise (\(\mathrm{PH}_{3}\), \(\mathrm{NH}_{3}\), \(\mathrm{SbH}_{3}\), and \(\mathrm{AsH}_{3}\)) involve a central atom bonded with hydrogen atoms.
• These molecules all feature a trigonal pyramidal geometry, caused by a lone electron pair on the central atom.
• This lone pair results in a shape where the distribution of charge is not symmetric, allowing for a net dipole moment.

In essence, geometry determines the extent and direction of an existing dipole moment, aiding in the polarization of a molecule.

Charge Separation

Charge separation is the degree to which opposite charges are distanced within a molecule. The greater this separation, the larger the dipole moment.
This concept is rooted in the idea that differences in electronegativity will pull electrons toward more electronegative elements, creating partial positive and negative charges within the molecule.

• In \(\mathrm{NH}_{3}\), electrons are more tightly pulled towards nitrogen compared to its counterparts, leading to a more pronounced separation of charges.
• The separation is vital because it leads to a dipole moment vector pointing towards the more electronegative atom, signifying the molecule's polarity.

Ultimately, in the case of \(\mathrm{NH}_{3}\), it exhibits the greatest charge separation, resulting in a higher dipole moment than those of \(\mathrm{PH}_{3}\), \(\mathrm{SbH}_{3}\), and \(\mathrm{AsH}_{3}\), where such separation is less marked.