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

(c) \(\mathrm{NaCl}<\mathrm{LiCl}<\mathrm{BeCl}_{2}\)

Step 1: Understanding the Concept of Covalent Character

The covalent character in a compound is influenced by the difference in electronegativity between the atoms and the polarization ability of the cation. Smaller cations with high charge density (like those with a charge of +2) have a greater polarizing power, leading to a greater covalent character.

Step 2: Analyzing the Compounds

In the given options, we have the compounds \(\mathrm{LiCl}\), \(\mathrm{NaCl}\), and \(\mathrm{BeCl}_{2}\). \(\mathrm{LiCl}\) and \(\mathrm{NaCl}\) are chlorides of group 1 metals, while \(\mathrm{BeCl}_{2}\) is a chloride of a group 2 metal.

Step 3: Comparing Cation Polarizing Power

Among the cations \(\mathrm{Li^+}\), \(\mathrm{Na^+}\), and \(\mathrm{Be^{2+}}\), the beryllium ion \(\mathrm{Be^{2+}}\) has the highest charge and smallest size, giving it the greatest polarizing power. Hence, \(\mathrm{BeCl}_{2}\) has the highest covalent character, followed by \(\mathrm{LiCl}\), and finally \(\mathrm{NaCl}\).

Step 4: Reviewing the Given Options

Looking at the options, increase in covalent character follows the order: \(\mathrm{NaCl}\), \(\mathrm{LiCl}\), then \(\mathrm{BeCl}_{2}\). This corresponds to option (c).

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry. It explains the tendency of an atom to attract electrons towards itself when it forms a chemical bond. Typically, electronegativity values are assigned to elements in the periodic table. Fluorine, for example, is the most electronegative element. When there is a large difference in electronegativity between two bonded atoms, the bond tends to be polar, meaning one atom will attract electrons more strongly. In the case of ionic bonds, this difference is crucial because it dictates how electrons are shared, which influences the bond's ionic or covalent nature.

• The higher the electronegativity difference, the more ionic the bond.
• A smaller difference results in a more covalent bond.

When analyzing covalent character in compounds like the chlorides mentioned in the exercise, understanding electronegativity helps clarify why these bonds behave the way they do.

Polarizing Power

Polarizing power refers to the ability of a cation to distort the electric field of an anion. This distortion affects how electron clouds in a compound interact and is a vital factor in determining covalent character. A smaller, highly charged cation generally exhibits a higher polarizing power. This leads to increased electron cloud distortion and higher covalent character.
For instance, in the exercise,

• The \( ext{Be}^{2+}\) cation has a higher polarizing power due to its small size and +2 charge.
• This strong polarizing power explains why \( ext{BeCl}_2\) has a high covalent character compared to \( ext{LiCl}\) and \( ext{NaCl}\).

Thus, polarizing power is crucial in explaining why some compounds seem to have more covalent characteristics despite initially appearing ionic based on electronegativity differences.

Cations

Cations are positively charged ions resulting from the loss of electrons. They play a significant role in determining the properties of ionic compounds. The size and charge of a cation are critical factors that influence its behavior.

• Smaller cations generally have a higher charge density, leading to greater polarizing power.
• This is why \( ext{Li}^+\) cations, being smaller than \( ext{Na}^+\) cations, show differences in covalent character despite both being in Group 1.

In Group 2, the comparison becomes even more pronounced with beryllium ions \( ext{Be}^{2+}\) demonstrating enhanced polarizing power compared to other lower charge cations because of a smaller ionic size and higher positive charge. Hence, the nature of a cation helps predict the likelihood of covalent character.

Group 1 and Group 2 Elements

Elements in Group 1 and Group 2 of the periodic table, also known as the alkali and alkaline earth metals, have distinct properties. They are typically found as cations in compounds because they readily lose electrons to achieve noble gas configurations.

• Group 1 elements form +1 cations, like \( ext{Li}^+\) and \( ext{Na}^+\).
• Group 2 elements form +2 cations, like \( ext{Be}^{2+}\).

The difference in charge greatly influences the chemical properties and reactivity of their compounds. For instance, Group 2 elements like beryllium exert more polarizing power compared to Group 1, altering the covalent vs. ionic nature of their bonds. As such, compounds like \( ext{BeCl}_2\) exhibit different characteristics than typical Group 1 chlorides due to this variance in charge.