Problem 156 - A Complete Resource Book in Chemistry for JEE Main

Question

When anions and cations approach each other, the valence shell of anions are pulled towards cation nucleus and thus, shape of anion is deformed. The phenomenon of deformation of anion by a cation is known as polarization and the ability of the cation to polarize the anion is called as polarizing power of cation. Due to polarization, sharing of electrons occurs between two ions to some extent and the bond shows some covalent character. Considering \(\mathrm{BeCl}_{2}, \mathrm{MgCl}_{2}, \mathrm{CaCl}_{2}\) and \(\mathrm{BaCl}_{2}\), predict which of the following statement is true? (a) Covalent character increases as the atomic number of the metal atom increases (b) \(\mathrm{BeCl}_{2}\) is least ionic out of the given chlorides. (c) \(\mathrm{BeCl}_{2}\) has the highest melting point among the given chlorides. (d) All are highly ionic compound.

Step-by-Step Solution

Answer

The correct statement is (b) BeCl₂ is least ionic out of the given chlorides.

Step 1: Understand Polarizing Power

Polarizing power refers to the ability of a cation to distort an anion. Smaller cations and those with higher charges have more polarizing power, leading to greater covalent character in compounds.

Step 2: Analyze BeCl_2, MgCl_2, CaCl_2, and BaCl_2

Beryllium (Be) is smaller in size and has a higher charge density compared to magnesium (Mg), calcium (Ca), and barium (Ba). Therefore, BeCl₂ has more covalent character due to the higher polarizing power of Be²⁺.

Step 3: Evaluate Statements

Now, let's evaluate the given statements: (a) Suggests covalent character increases with atomic number, which is incorrect as beryllium has the smallest atomic number but the highest covalent character. (b) Suggests BeCl₂ is least ionic, which aligns with its high covalent character. (c) Suggests BeCl₂ has the highest melting point. However, covalent nature often implies lower melting points. (d) States that all are highly ionic, which contradicts BeCl₂'s covalent nature.

Step 4: Determine the Correct Statement

Based on the understanding of polarizing power and covalent character, the statement that aligns with the chemical nature of BeCl₂ is (b), which claims it is the least ionic among the chlo-rides.

Key Concepts

Polarizing Power

When discussing ionic compounds, one crucial concept is the 'polarizing power' of cations. Essentially, polarizing power is the ability of a cation to distort or pull the electron cloud of an adjacent anion towards itself. This occurs because the positive charge of the cation can influence the negatively charged electrons of the anion. Cations with greater polarizing power can cause significant changes to the shape or structure of an anion. Several factors affect this power:

• Charge: Cations with higher positive charges have increased polarizing power because they generate a stronger electric field.
• Size: Smaller cations have higher polarizing power due to their higher charge density—that is, they pack more charge into a smaller volume.

In the context of \( \text{BeCl}_2 \), the beryllium ions \( \text{Be}^{2+} \) are small and have a relatively high charge density, giving them significant polarizing power compared to other cations such as \( \text{Mg}^{2+} \), \( \text{Ca}^{2+} \), or \( \text{Ba}^{2+} \). This leads \( \text{BeCl}_2 \) to exhibit more covalent character.

Covalent Character

The polarizing effect a cation has on an anion induces certain covalent characteristics in an otherwise ionic bond. When an anion is polarized, its electron cloud gets distorted towards the cation, and this distortion allows more sharing of electrons, similar to what occurs in a covalent bond. Here's how different factors contribute to covalent character:

• High Polarizing Power of Cations: As we discussed, the higher the polarizing power, the more distortion occurs, introducing more sharing of electrons as seen in covalent bonds.
• Anion Size: Larger anions are more easily polarizable compared to smaller ones, thus they tend to gain more covalent character when paired with a polarizing cation.

In compounds like \( \text{BeCl}_2 \), where beryllium is highly polarizing, the covalent character is pronounced. This is evident in its lower melting point compared to more ionic chlorides such as \( \text{MgCl}_2 \), \( \text{CaCl}_2 \), and \( \text{BaCl}_2 \). It provides a nice illustration of how covalent character tends to decrease as the atomic number increases and ionic character increases.

Anion Deformation

Anion deformation is a direct consequence of polarizing power, where the electron cloud of an anion is pulled or dragged towards the cation. When this happens, the shape of the anion is no longer spherical but is distorted towards the cation; this is referred to as 'deformation.'Anion deformation plays a key role in determining the type of bonding that will occur between ions. It turns the typically clear-cut ionic bond into a bond with mixed characteristics due to some degree of 'electron sharing'. Understanding this helps us grasp why some ionic compounds display unexpected behaviors, like lower melting points, which are generally associated with covalent compounds.For example, in \( \text{BeCl}_2 \), the small size and high charge density of \( \text{Be}^{2+} \) lead to significant anion deformation of \( \text{Cl}^- \) ions. This results in a strong covalent character and explains why \( \text{BeCl}_2 \) behaves differently from its counterparts, \( \text{MgCl}_2 \), \( \text{CaCl}_2 \), and \( \text{BaCl}_2 \), which have more ionic characteristics due to their lesser ability to deform the anion.