Problem 157 - 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. In which of the halides, there is maximum polarization ? (a) \(\mathrm{AlBr}_{3}\) (b) \(\mathrm{AlF}_{3}\) (c) \(\mathrm{AlI}_{3}\) (d) \(\mathrm{AlCl}_{3}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{AlI}_3\) shows maximum polarization.

Step 1: Identify the Cation and Anion

For each given compound, identify the cation and anion. All these compounds have aluminum (Al) as the cation. The anions are the halide ions: - In \(\mathrm{AlBr}_3\), the anion is \(\mathrm{Br^-}\) (bromide).- In \(\mathrm{AlF}_3\), the anion is \(\mathrm{F^-}\) (fluoride).- In \(\mathrm{AlI}_3\), the anion is \(\mathrm{I^-}\) (iodide).- In \(\mathrm{AlCl}_3\), the anion is \(\mathrm{Cl^-}\) (chloride).

Step 2: Understand Polarization Mechanics

Polarization occurs when a cation exerts an electric field that distorts the electron cloud of the anion. This is stronger when the cation has a high charge, small size, and is capable of polarizing easily distortable anions (larger anions with more electrons that are less tightly bound).

Step 3: Evaluate Anion Distortion

The larger the anion, the more easily its electron cloud can be distorted, or polarized. Among halides, the sizes follow the order: \( \mathrm{I^-} > \mathrm{Br^-} > \mathrm{Cl^-} > \mathrm{F^-} \).Hence, \(\mathrm{I^-} \) is the most easily polarized anion.

Step 4: Determine Polarization Impact

Anions with larger radii and lower effective nuclear charge (more loosely held outer electrons) will experience greater polarization by the cation. Therefore, \(\mathrm{I^-}\) being the largest, will experience the maximum polarization by the \(\mathrm{Al}^{3+}\) cation.

Step 5: Select the Halide with Maximum Polarization

Given that \(\mathrm{I^-}\) experiences maximum polarization, \(\mathrm{AlI}_3\) will have the most covalent character based on polarization effects due to \(\mathrm{Al}^{3+}\).

Key Concepts

Polarizing power

Polarizing power is the ability of a cation to distort the electron cloud of an anion. It plays a crucial role in determining the nature of bonds in compounds. When a cation with high polarizing power approaches an anion, it pulls on the anion's electrons, altering the distance between them. This behavior imparts some covalent character to what might traditionally be considered ionic bonds.
Key factors influencing polarizing power include:

• Charge of the Cation: A higher positive charge on the cation results in a stronger electric field, increasing its polarizing power.
• Size of the Cation: Smaller cations can pack closely to anions, enhancing their ability to polarize the anion's electron cloud.

Understanding polarizing power aids in predicting the behavior and properties of various chemical compounds.

Cations and anions

Cations and anions are oppositely charged ions that attract one another due to electrostatic forces. These opposites are essential players in the realm of ionic compounds. Cations possess a positive charge, resulting from the loss of electrons, and are typically metals. Anions, on the other hand, bear a negative charge due to the gain of electrons and are often non-metals or polyatomic ions.
Key nuances include:

• Cations: Positively charged, usually smaller compared to anions because they lose some of their outer shell electrons.
• Anions: Negatively charged, generally larger as they gain electrons, which increases electron-electron repulsion within the ion, expanding its radius.

Understanding how cations and anions interact via electrostatic forces helps in predicting the formation and characteristics of different compounds.

Covalent character

Covalent character refers to the degree of electron sharing between atoms within a bond. Even in ionic compounds, polarization can impart covalent characteristics by distorting the anion's electron cloud, resulting in partial charge sharing. The transfer of electrons becomes more of a sharing situation, making the bond less purely ionic.
Influencing Factors:

• Polarization: Increased polarization by a cation leads to larger covalent character.
• Anion Size: Larger anions are more polarizable, allowing for greater electron cloud distortion.
• Cation Size and Charge: Small, highly charged cations can sufficiently polarize the anion, enhancing covalent characteristics.

This concept is essential in predicting the nature of bonds and understanding the properties of substances.

Halides

Halides are a group of anions derived from halogen atoms that have gained an additional electron. These include fluoride, chloride, bromide, and iodide, each with unique sizes and polarizabilities. The behavior of halides in chemical bonding largely depends on their interaction with cations.
Halides are crucial for understanding chemical properties:

• Fluoride (1^-): Smallest size, strong ionic nature, minimal polarization.
• Chloride (1^-): Moderate size, standard ionic behavior.
• Bromide (1^-): Larger, exhibits more polarization compared to fluoride and chloride.
• Iodide (1^-): Largest, most polarizable, maximum covalent character when paired with highly charged cations like Al³⁺.

Analyzing halides helps determine which compounds harbor greater covalent or ionic characteristics.

Anion distortion

Anion distortion occurs when a cation influences the shape of an anion’s electron cloud. The ease with which an anion is distorted is directly related to its size and polarizability. Larger anions are more easily distorted because they have more loosely held electrons that respond quickly to external electric fields exerted by cations.
Why it Matters:

• Anion distortion is key to increasing covalent character in a compound.
• The degree of distortion influences the physical and chemical properties of a substance.
• Helps predict the reactivity and formation of complex ions in solution.

Understanding this concept assists in visualizing how compounds may behave under different conditions and explains certain anomalies in compound behavior.

Chemical bonding

Chemical bonding involves the interactions that hold atoms together in molecules and compounds. It includes various types characterized by electron sharing and transfer, such as ionic, covalent, and metallic bonds. The nature of chemical bonds affects key properties such as melting point, boiling point, and solubility.

• Ionic Bonds: Formed from the transfer of electrons, producing electrically charged ions. Common in compounds like salts.
• Covalent Bonds: Involve the sharing of electrons between atoms, forming molecules with pronounced directional properties.

Importance of Bond Hybridization: This is the process of mixing atomic orbitals before the formation of covalent bonds, creating stronger, more optimal electron pair sharing configurations.
Analyzing chemical bonding provides insights into the stability, reactivity, and structure of substances in chemistry.