Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following compounds will the bond angle be maximum? (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{NH}_{4}^{+}\) (c) \(\mathrm{PCl}_{3}\) (d) \(\mathrm{SCl}_{2}\)

Step-by-Step Solution

Answer

The maximum bond angle is in 60;NH_{4}^{+}7 with 60;109.5^{3circ}7.

Step 1: Identify the Electron Geometry

To determine the bond angle, we first need to understand the electron geometry of each compound. - 60;NH_{3}7 is pyramidal due to one lone pair and three bonds, leading to 60;sp^37 hybridization. - 60;NH_{4}^{+}7 is tetrahedral with no lone pairs, also having 60;sp^37 hybridization. - 60;PCl_{3}7 is also pyramidal with one lone pair and three bonds. - 60;SCl_{2}7 is bent, as it has two lone pairs and two bond pairs.

Step 2: Determine the Ideal Bond Angles

Now, let's consider the theoretical bond angles for different geometries: - Tetrahedral structures, like 60;NH_{4}^{+}7, have a bond angle of 60;109.5^{3circ}7. - Pyramidal structures, as in 60;NH_{3}7 and 60;PCl_{3}7, typically have slightly less than 60;109.5^{3circ}7 due to lone pair repulsion, approximately 60;107^{3circ}7. - Bent structures, like 60;SCl_{2}7, have even smaller angles, usually around 60;104.5^{3circ}7.

Step 3: Compare Theoretical Angles

By comparing the ideal bond angles: - 60;NH_{4}^{+}7 with 60;109.5^{3circ}7. - 60;NH_{3}7 and 60;PCl_{3}7 both with 60;107^{3circ}7. - 60;SCl_{2}7 with 60;104.5^{3circ}7. It is clear that 60;NH_{4}^{+}7 has the largest bond angle.

Key Concepts

Electron Geometry

Electron geometry is a foundational concept in understanding molecular shapes and bond angles. It refers to the spatial arrangement of all electron pairs (including bonding pairs and lone pairs) around the central atom in a molecule. This geometry plays a pivotal role in defining the overall molecular shape.

• For molecules with two areas of electron density, the geometry is linear.
• Three areas lead to a trigonal planar geometry.
• Four areas of electron density create a tetrahedral geometry, which is relevant in our current discussion.

Identifying electron geometry allows us to predict molecular shapes, bond angles, and reactivity. It guides how atoms bond in molecules, influencing the molecule's physical and chemical properties.

Hybridization

Hybridization is the process of mixing atomic orbitals into new hybrid orbitals. This process helps in forming covalent bonds in molecules and influences molecule geometry.
In cases of sp, sp2, and sp3 hybridization, different geometries arise:

• sp hybridization results in a linear shape.
• sp2 leads to a trigonal planar structure.
• sp3 hybridization is characteristic of tetrahedral geometry, creating angles of approximately 109.5°.

For instance, in ammonia (\(\text{NH}_3\)), the nitrogen is sp3 hybridized, but the presence of a lone pair slightly reduces the bond angle from the ideal 109.5° to about 107°.

Tetrahedral Structure

A tetrahedral structure is a common molecular geometry formed when a central atom is surrounded by four bonding pairs of electrons. This arrangement allows the atoms to be equidistant from each other, minimizing electron pair repulsion, and results in bond angles of approximately 109.5°.

• In methane (\(\text{CH}_4\)), for example, the hydrogen atoms are positioned at the vertices of an imaginary tetrahedron.
• This structure is prevalent in various organic compounds.

The concept of tetrahedral geometry is crucial for understanding how molecular shapes influence physical and chemical properties. It is often taught using models to help visualize and comprehend the three-dimensional arrangements.

Pyramidal Structure

A pyramidal structure is typical in molecules with one lone pair and three bonding pairs. This arrangement leads to a slight distortion from the tetrahedral ideal angle due to lone pair-bond pair repulsion.
The nitrogen in ammonia (\(\text{NH}_3\)) provides a good example of a pyramidal shape.

• The lone pair takes up more space, pushing the three hydrogen atoms closer together.
• This decreases the H-N-H bond angle to approximately 107°, slightly less than the tetrahedral 109.5°.

Pyramidal structures significantly affect intermolecular interactions and can influence boiling and melting points due to the presence of lone pairs that affect molecular polarity.

Bent Structure

The bent structure occurs in molecules where there are two bond pairs and one or more lone pairs on the central atom. Due to the lone pair's greater repulsion strength, the bond angles in these structures are even smaller compared to pyramidal structures.
A classic example is sulfur dichloride (\(\text{SCl}_2\)):

• The lone pairs push the bond pairs together, forming an angle around 104.5°.
• This smaller angle compared to the tetrahedral angle arises from lone pair-lone pair and lone pair-bond pair repulsion.

Bent structures are significant in chemistry because they lead to polar molecules, which have unique chemical and physical properties due to their shape and electron distribution.