Problem 128 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I List II (Compounds) (Structures) 1\. \(\mathrm{CS}_{2}\) (i) Bent 2\. \(\mathrm{SO}_{2}\) (ii) Linear 3\. \(\mathrm{BF}_{3}\) (iii) Trigonal planar 4\. \(\mathrm{NH}_{3}\) (iv) Tetrahedral (v) Trigonal pyramidal The correct matching is: \(1 \quad 2\) \(3 \quad 4\) (a) (i) (ii) (iv) (v) (b) (ii) (i) (iii) (v) (c) (i) (ii) (v) (iv) (d) (ii) (i) (v) (iv)

Step-by-Step Solution

Answer

(b) 1-(ii), 2-(i), 3-(iii), 4-(v)

Step 1: Understand Each Compound's Geometry

Firstly, analyze the molecular geometry of each compound. \( \mathrm{CS}_2 \) has a linear geometry. \( \mathrm{SO}_2 \) is a bent molecule due to the presence of a lone pair on the sulfur atom. \( \mathrm{BF}_3 \) is a trigonal planar molecule as it has three bonds and no lone pair on the central boron atom. \( \mathrm{NH}_3 \) is trigonal pyramidal due to the presence of a lone pair on the nitrogen atom.

Step 2: Match Each Compound to its Structure

Now, match the compounds from List I to the structures provided in List II based on the geometries identified. \( \mathrm{CS}_2 \) is linear, which corresponds to option (ii). \( \mathrm{SO}_2 \) is bent, which corresponds to option (i). \( \mathrm{BF}_3 \) is trigonal planar, which matches option (iii). \( \mathrm{NH}_3 \) is trigonal pyramidal, corresponding to option (v). Therefore, the correct matches are: 1-(ii), 2-(i), 3-(iii), 4-(v).

Step 3: Choose the Correct Answer Based on Matches

From the options provided, find the one that matches our findings. The matches are: \(CS_2\) with (ii), \(SO_2\) with (i), \(BF_3\) with (iii), and \(NH_3\) with (v). Option (b) provides the sequence (ii), (i), (iii), (v), which is the correct answer based on our matching.

Key Concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory is a key tool for predicting the geometry of molecules. The central idea is that electron pairs around an atom will arrange themselves as far apart as possible to minimize repulsion. This can apply to both bonding pairs and lone pairs of electrons.

• Bonding Pairs: These are the shared pairs of electrons in a molecule that connect atoms together, forming the molecule’s bonds. They contribute to the arrangement because they, too, need to keep repulsion to a minimum.
• Lone Pairs: Lone pairs are non-shared pairs of electrons that belong to a single atom. They can cause deviations from ideal geometry due to their greater repulsion compared to bonding pairs.

Understanding that lone pairs exude more repulsion than bonding pairs helps explain many molecular shapes. For instance, in \\(\mathrm{SO}_{2}\), the lone pair on sulfur pushes the bonding pairs closer, resulting in a bent shape. This theoretical framework helps to predict and explain the 3D arrangement of molecules and thus is fundamental to understanding molecular geometry.

Chemical Bonding

Chemical bonding refers to the interactions that account for the assembly and stability of atoms into molecules. It's essential for constructing the structures that VSEPR Theory helps categorize. There are different types of bonds depending on electron distribution and atom interaction.

• Covalent Bonds: These involve the sharing of electron pairs between atoms, which occurs in most non-metallic compounds. For instance, in \(\mathrm{NH}_{3}\), nitrogen shares electrons with three hydrogen atoms to form stable covalent bonds.
• Ionic Bonds: Formed when electrons are transferred from one atom to another, resulting in positively and negatively charged ions. These are common in compounds formed between metals and non-metals.

In molecules like \(\mathrm{BF}_{3}\), where the bonding architecture is trigonal planar, a clear connection between bonding and molecular structure is evident. Understanding the type and arrangement of chemical bonds is critical to predicting the molecular form.

Molecular Shape Identification

Identifying molecular shape is about recognizing the spatial arrangement of atoms in a molecule. This plays a crucial role in understanding molecule function and reactivity. Utilizing VSEPR and knowledge of chemical bonds, you can determine shapes that range from simple linear forms to more complex tetrahedral structures.

Some common molecular shapes include:

• Linear: Characterized by an angle of 180°, like in \(\mathrm{CS}_{2}\).
• Bent: Seen in molecules like \(\mathrm{SO}_{2}\), where lone pairs push back against bonded atoms, bending the structure.
• Trigonal Planar: Found in \(\mathrm{BF}_{3}\), with 120° angles forming a flat, triangular arrangement.
• Trigonal Pyramidal: As with \(\mathrm{NH}_{3}\), where one lone pair creates a three-sided pyramid shape around the central atom.

Recognizing these patterns helps in anticipating physical and chemical properties, allowing chemists to predict reactions and interactions between molecules.