Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is the correct statement? (a) Boric acid is a protonic acid (b) Beryllium exhibits coordination number of six. (c) Chlorides of both beryllium and aluminium have bridged chloride structures in solid phase. (d) \(\mathrm{B}_{2} \mathrm{H}_{6} \cdot 2 \mathrm{NH}_{3}\) is known as 'inorganic benzene'

Step-by-Step Solution

Answer

Statement (c) is correct.

Step 1: Understanding the Question

We need to determine which one of the given statements is correct by checking the facts about boric acid, beryllium, aluminium, and diborane.

Step 1: Evaluate Boric Acid Statement

Boric acid, \(\text{H}_3\text{BO}_3\), is not a protonic acid. Instead, it acts as a Lewis acid because it accepts a pair of electrons rather than donating a proton. Thus, statement (a) is incorrect.

Step 2: Analyze Beryllium Coordination

Beryllium usually exhibits a coordination number of four due to its small size and the properties of its compounds. Therefore, statement (b) claiming beryllium exhibits a coordination number of six is incorrect.

Step 3: Review Chloride Structures

Both beryllium chloride \(\text{BeCl}_2\) and aluminum chloride \(\text{AlCl}_3\) are known to have bridged chloride structures in their solid phase, known as polymeric structures. Hence, statement (c) is correct.

Step 4: Evaluate Diborane Compound

\(\mathrm{B}_2\mathrm{H}_6 \cdot 2\mathrm{NH}_3\) is not called 'inorganic benzene'; that term is used for \(B_3N_3H_6\), which resembles benzene in structural characteristics.

Key Concepts

Lewis Acid

In the world of chemistry, "Lewis Acid" refers to a molecule or ion that can accept an electron pair. This characteristic sets them apart from traditional acids, which donate protons. Lewis acids act as potent electron pair acceptors.
For instance:

• Boron compounds like boric acid ( \( ext{H}_3 ext{BO}_3\)) are classic examples, as they accept electrons.
• Aluminum chloride ( \( ext{AlCl}_3\)) is another, especially in reactions where it can take electrons to form a more stable structure.

Understanding this concept helps in predicting how molecules will interact. When a Lewis acid meets a "Lewis Base"—a molecule donating an electron pair—a bond forms. This principle explains many reaction mechanisms in inorganic chemistry, offering insight into complex chemical behavior.

Coordination Number

The "Coordination Number" is a term used to describe the number of atoms directly bonded to a central atom in a molecule or complex. It provides critical information about the geometry and bonding in chemical compounds.
Consider these illustrations:

• For many transition metals, the typical coordination number can be six, leading to an octahedral shape.
• Beryllium, which often has a coordination number of four due to its small size and two available bonds with its electrons in its ground state, forms tetrahedral structures.

Coordination numbers depend on several factors, such as the size and electron configuration of the central atom and the ligands surrounding it. Understanding them is crucial for grasping molecular structure and dynamics in both simple and complex chemical systems.

Polymeric Structures

"Polymeric Structures" are fascinating because they involve repeating units bonded in large chains or networks. In inorganic chemistry, this concept often applies to the solid-state forms of certain compounds, forming intricate lattice structures.
For example:

• Beryllium chloride ( \( ext{BeCl}_2\)) and aluminum chloride ( \( ext{AlCl}_3\)) both form bridged chloride structures. Here, chlorine atoms bridge between metal centers, extending into a polymeric chain.
• These structures are stabilized by strong forces between repeated units, forming a solid network.

Polymeric structures have unique properties like increased melting points and solubility patterns. Understanding these properties gives insights into material science and the development of new materials with specific desired properties.

Solid Phase Chemistry

"Solid Phase Chemistry" concerns the study of materials in their solid state. It explores the arrangement of atoms and the forces holding them together, which dictate the material properties.
Here are some key points:

• Solids can be crystalline, where atoms are in a repeating pattern, or amorphous, with no long-range order.
• Incrystalline solids, the bonding and structure result in specific attributes, such as hardness, conductivity, and melting points.

For compounds like \( ext{BeCl}_2\) and \( ext{AlCl}_3\), their solid-state involves polymeric structures with bridged chlorides. The understanding of solid-phase chemistry is essential for predicting material behavior under different conditions and for technological applications like semiconductor technology and catalyst development.