Problem 155 - 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. The magnitude of polarization depends upon a number of factors. These factors were suggested by Fajan and are known as Fajan's rule. (a) Greater is the polarization in a molecule, more is covalent character. (b) As the charge on cation increases, its tendency to polarize the anion increases (c) As the size of the cation decreases or size of the anion increases, the polarization increases. (d) All of these.

Step-by-Step Solution

Answer

The correct answer is (d) All of these.

Step 1: Understand the Problem

The task requires us to identify how polarization, induced by a cation on an anion, affects the bonding characteristics of the compound and to determine the factors influencing this polarization.

Step 2: Recall Fajan's Rules

Fajan's rules help understand the factors affecting ionic and covalent characteristics in a compound. Key rules are: (a) Greater polarization results in more covalent character, (b) Smaller and highly charged cations have greater polarizing power, (c) Larger anions are more easily polarized.

Step 3: Evaluate Option (a)

Option (a) states that greater polarization leads to more covalent character. This is consistent with Fajan's rules because as polarization increases, the electron cloud distortion results in more sharing of electrons, resembling covalent character.

Step 4: Evaluate Option (b)

Option (b) indicates that an increase in cation charge enhances its ability to polarize an anion. According to Fajan's rules, higher charged cations exert more attractive force on anions, increasing polarization.

Step 5: Evaluate Option (c)

Option (c) suggests that smaller cation size or larger anion size increases polarization. Smaller cations can get closer to anions and exert a stronger electric field, while larger anions have more easily distorted electron clouds, conforming to Fajan's rules.

Step 6: Evaluate Option (d)

Option (d) states 'All of these,' which implies all previous options are true. Since all individual evaluations (a, b, and c) align with Fajan's rules, option (d) is the correct choice.

Key Concepts

Polarization

Polarization is a fascinating concept in the realm of chemistry that involves the distortion of an anion's electron cloud by a nearby cation. Imagine the anion as a cloud of electron charge; when a cation with a positive charge approaches, it attracts and pulls on this cloud. This shift, or compression of the electron cloud towards the cation, leads to what we call polarization.

The significance of polarization lies in its ability to alter the properties of a chemical bond. Normally, an ionic bond prompts electron transfer, creating attraction between oppositely charged ions. However, when polarization occurs, some of the ionic nature is masked by the sharing of electrons, introducing partial covalent character into the bond.

• This is a key aspect of Fajan's rules, which predict the degree of polarization and resulting bond characteristics.

Understanding polarization is crucial because it informs us about bond strength, solubility, and reactivity in various compounds.

Polarizing Power

Polarizing power refers to the ability of a cation to induce polarization in an anion. Think of it as the strength or influence a cation has over the electron cloud of an anion.

Several factors determine the polarizing power of a cation:

• The charge on the cation: A higher positive charge increases the cation's polarizing power. More charge means more force to pull on the electron cloud of the anion.
• The size of the cation: Smaller cations exhibit stronger polarizing power. Their compact nature allows them to get closer to the anion, which enhances the interaction between the ions.

The ability to predict polarizing power helps chemists anticipate how ionic a particular compound is, followed by how pronounced its covalent characteristics might be. This can impact both physical properties and reactivity, helping to design better materials and chemical processes.

Ionic and Covalent Character

In the study of chemical bonds, the terms ionic and covalent describe the nature of electron sharing between atoms. An ionic bond is understood as the complete transfer of electrons between a cation and an anion.

However, due to polarization, no bond is purely ionic; there is often some degree of electron sharing, which adds a layer of covalent character to the bond.

• Covalent character refers to the sharing aspect and how closely bonded the atoms become as opposed to being fully separated by their charges.
• Greater polarization increases the covalent character, meaning the bonds start to exhibit properties more similar to covalent bonds, including altered solubility and melting points.

Understanding this concept empowers students and chemists to predict and explain variations in chemical behavior based on bond types, thus aiding in the correct application of compounds in practical scenarios.

Anion Deformation

Anion deformation is associated with the reshaping or distortion of an anion when influenced by the polarizing power of a cation. This deformation is an integral part of the polarization process, which occurs at the atomic level.

Larger anions are more susceptible to deformation because they possess bigger, more easily distorted electron clouds:

• With larger electron clouds, the outer electrons are far from the nucleus, making them easier to pull or distort.
• As their shape changes, these anions contribute to the development of covalent character within what is often considered an ionic bond.

The concept of anion deformation isn't just academic; it highlights how flexible and dynamic molecular structures can be, impacting material properties like hardness and conductivity. Observing how anions change shape allows us to better understand and predict the behavior of substances in different environmental conditions.