Problem 97 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct sequence of increasing covalent character is represented by (a) \(\mathrm{LiCl}<\mathrm{NaCl}<\mathrm{BeCl}_{2}\) (b) \(\mathrm{BeCl}_{2}<\mathrm{LiCl}<\mathrm{NaCl}\) (c) \(\mathrm{NaCl}<\mathrm{LiCl}<\mathrm{BeCl}_{2}\) (d) \(\mathrm{BeCl}_{2}<\mathrm{NaCl}<\mathrm{LiCl}\)

Step-by-Step Solution

Answer

The correct sequence is (c) NaCl < LiCl < BeCl₂.

Step 1: Understanding Covalent Character

Covalent character is influenced by the difference in electronegativity between the atoms as well as the polarization capabilities of the atoms (Fajan's rules). According to Fajan's rules, a smaller cation and a larger anion will have more covalent character.

Step 2: Assessing the Cations in Compounds

The cations involved are Li, Na, and Be. Be has the smallest size and the highest ability to polarize the electron cloud. Thus, Be will form the most covalent bond compared to Li and Na.

Step 3: Arranging Based on Covalent Character

Considering the polarization effect, the order of increasing covalent character considering the cations is NaCl (least) < LiCl < BeCl₂ (most).

Step 4: Matching to Given Options

Compare the determined order NaCl < LiCl < BeCl₂ with the options provided: (a) LiCl < NaCl < BeCl₂ (b) BeCl₂ < LiCl < NaCl (c) NaCl < LiCl < BeCl₂ (d) BeCl₂ < NaCl < LiCl. Option (c) exactly matches the determined order.

Key Concepts

Fajan's Rules

Fajan's Rules help us understand why some ionic compounds have covalent character. These rules were formulated by Kazimierz Fajans in 1923 and are crucial for predicting the behavior of chemical bonds.

• Ionic vs Covalent Bonds: Typically, ionic compounds have bonds between a metal and a non-metal. However, they can exhibit covalent character, meaning the electrons aren't completely transferred but shared to some extent.
• Cation Size: A smaller cation can approach an anion more closely, leading to better overlap of their valence electrons. This overlap increases covalent character because the electron cloud is shared more.
• Anion Size: Larger anions are more easily polarized. When an anion's electron cloud is distorted by a nearby cation, it results in increased covalent character.
• Electronegativity Difference: The greater the difference in electronegativity between the cation and anion, the more ionic the bond. Smaller differences due to polarization lead to more covalent character.

Fajan's Rules are significant in understanding complex chemical behavior and predicting how substances will interact chemically.

Polarization

Polarization plays a key role in determining the covalent character of a compound.

• Definition: Polarization refers to the distortion of an anion's electron cloud by a neighboring cation.
• Polarizing Power: A cation with a smaller radius and higher charge density can cause significant distortion in an anion's electron cloud. This distortion leads to a shift from purely ionic to more covalent bonding.
• Polarizability of Anions: Larger anions with more diffuse electron clouds are easily distorted. A more polarizable anion results in bonds with higher covalent character.

For example, in the compounds analyzed like LiCl, NaCl, and BeCl₂, beryllium ions (Be²⁺) exhibit strong polarizing power due to their small size and high charge, leading to significant polarization of the chloride ion's cloud. This explains the greater covalent character of BeCl₂ compared to NaCl and LiCl.

Electronegativity

Electronegativity is the ability of an atom to attract shared electrons towards itself in a chemical bond.

• Fundamental Aspect: Higher electronegativity in an atom means it pulls electrons more strongly, affecting bond character.
• Relevance to Covalent Character: When atoms in a bond have similar electronegativities, electrons are shared more equally, increasing covalent character. Big differences would denote more ionic character.
• Periodic Trend: Electronegativity generally increases across a period from left to right and decreases down a group in the periodic table.

In the context of the original exercise, lithium (Li), sodium (Na), and beryllium (Be) cations have varying abilities to attract electrons in their combinations with chlorine (Cl). Among them, Be demonstrates the unique ability to polarize more and thus forms compounds with higher covalent character. Electronegativity differences in these bonds influence their behavior, leading to varied levels of ionic and covalent character.