Problem 191 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of bond angles (smallest first) in \(\mathrm{H}_{2} \mathrm{~S}, \mathrm{NH}_{3}, \mathrm{BF}_{3}\) and \(\mathrm{SiH}_{4}\) is [2004] (a) \(\mathrm{H}_{2} \mathrm{~S}<\mathrm{SiH}_{4}<\mathrm{NH}_{3}<\mathrm{BF}_{3}\) (b) \(\mathrm{NH}_{3}<\mathrm{H}_{2} \mathrm{~S}<\mathrm{SiH}_{4}<\mathrm{BF}_{3}\) (c) \(\mathrm{H}_{2} \mathrm{~S}<\mathrm{NH}_{3}<\mathrm{SiH}_{4}<\mathrm{BF}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{~S}<\mathrm{NH}_{3}<\mathrm{BF}_{3}<\mathrm{SiH}_{4}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{H}_2\mathrm{S}<\mathrm{NH}_3<\mathrm{SiH}_4<\mathrm{BF}_3\)

Step 1: Determine bond angles for H2S

Hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)) has a bent shape due to the presence of two lone pairs on the sulfur atom. This results in a bond angle that is typically around \(92.1^\circ\).

Step 2: Determine bond angles for NH3

Ammonia (\(\mathrm{NH}_3\)) has a trigonal pyramidal shape with one lone pair on the nitrogen atom, leading to a bond angle of approximately \(107.3^\circ\).

Step 3: Determine bond angles for BF3

Boron trifluoride (\(\mathrm{BF}_3\)) is trigonal planar with bond angles of approximately \(120^\circ\) due to the absence of lone pairs on the boron atom.

Step 4: Determine bond angles for SiH4

Silane (\(\mathrm{SiH}_4\)) is tetrahedral with a standard bond angle of \(109.5^\circ\) because there are no lone pairs.

Step 5: Order bond angles from smallest to largest

Based on the bond angles: \(\mathrm{H}_2\mathrm{S}\approx 92.1^\circ\), \(\mathrm{NH}_3\approx 107.3^\circ\), \(\mathrm{SiH}_4\approx 109.5^\circ\), and \(\mathrm{BF}_3\approx 120^\circ\), arrange them in ascending order: \(\mathrm{H}_2\mathrm{S} < \mathrm{NH}_3 < \mathrm{SiH}_4 < \mathrm{BF}_3\).

Key Concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This arrangement significantly affects the molecule's properties, including bond angles, reactivity, phase of matter, color, magnetism, and biological activity. Understanding molecular geometry is crucial because it forms the foundation upon which much of chemistry is built.

• Linear Geometry
• Trigonal Planar
• Tetrahedral
• Bent Shape
• Trigonal Pyramidal

Each of these shapes comes about because of the electrons surrounding a central atom. The electrons, both bonding and non-bonding, repel each other, and this causes the molecule to adopt a shape that minimizes this repulsion.

VSEPR Theory

The VSEPR (Valence Shell Electron Pair Repulsion) Theory helps us understand and predict the shapes of molecules. The main idea behind VSEPR Theory is that electron pairs around a central atom stay as far apart as possible to minimize repulsion. This concept is key in determining the molecular geometry of a molecule.

• Electron pairs include bonding pairs and lone pairs.
• Lone pairs repel more strongly than bonding pairs.
• The number of domains of electron density (either a pair or a single unpaired electron) around the central atom dictates the shape.

By using these principles, we can understand why certain molecules with similar numbers of electron domains have different shapes and thus different bond angles.

Bent Shape

A bent shape occurs in molecules where the central atom has two bonded atoms and at least one lone pair of electrons. The classic molecules with this geometry are water ( H2O ) and hydrogen sulfide ( H2S ). The bent shape is due to the repulsive forces exerted by the lone pairs that cause the bonded atoms to be pushed together, creating a smaller bond angle compared to a linear shape.

Properties of Bent Shape:

• The typical bond angle is less than 120° and varies based on the number and strength of the repulsions.
• The shape is non-linear due to the lone pairs on the central atom.
• Lone pairs are located in positions that allow them to repel the bonded atoms towards each other.

In hydrogen sulfide ( H2S ), the bond angle is approximately 92.1°, which is smaller due to the two lone pairs on sulfur.

Trigonal Pyramidal

The trigonal pyramidal shape arises when there is one lone pair on the central atom with three bonded atoms. This results in a pyramidal structure with the lone pair creating a stronger repulsion, slightly pushing the bonded atoms closer together. Ammonia ( NH3 ) is the classic example of a molecule with a trigonal pyramidal geometry.

Characteristics of Trigonal Pyramidal:

• The bond angle is slightly less than the tetrahedral angle, often around 107° to 109.5°.
• The lone pair affects the geometry by exerting more repulsion on the bonding pairs.
• The shape gives the appearance of a three-sided pyramid.

In NH3 , the bond angle is about 107.3°, affected by the lone pair on nitrogen.

Trigonal Planar

Trigonal planar geometry is found in molecules where the central atom is connected to three peripheral atoms with no lone pairs present. This geometry results in a flat, triangular shape where all bond angles are equal. Boron trifluoride ( BF3 ) is a typical example of a trigonal planar molecule.

Characteristics of Trigonal Planar:

• The bond angles are all 120°.
• There are no lone pairs on the central atom to distort the angle.
• The molecule lies entirely within a single plane.

This shape is highly symmetrical and is crucial for predicting the physical properties and reactivity of compounds with a trigonal planar configuration.

Tetrahedral

A tetrahedral geometry occurs when a central atom is surrounded by four bonded atoms and no lone pairs. This arrangement allows for equal spacing in three-dimensional space, reducing repulsion and resulting in symmetric distribution. Silane ( SiH4 ) is an example of a molecule with a tetrahedral shape.

Features of Tetrahedral Geometry:

• The bond angles in a tetrahedral molecule are typically 109.5°.
• This arrangement achieves a symmetric, balanced shape with minimal repulsion.
• It is the most common geometry for carbon atoms in organic molecules.

The tetrahedral shape is fundamental in organic chemistry, forming the basis for understanding complex molecular structures.