Problem 155 - A Complete Resource Book in Chemistry for JEE Main

Question

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

(d) All of these statements are correct according to Fajan's rule.

Step 1: Understanding Polarization

Polarization refers to the distortion of the electron cloud in an anion by a cation. This leads to a shift in electron density that enhances covalent character in a molecule.

Step 2: Analyzing Fajan's Rule

Fajan's rule provides insight into how ionic compounds exhibit covalent character based on certain factors like the charge on the cation, the size of the cation, and the size of the anion.

Step 3: Evaluating Greater Polarization and Covalent Character

According to Fajan's rule, greater polarization increases covalent character in a molecule, therefore this statement (a) is true.

Step 4: Evaluating Charge on Cation and Polarizing Power

Increasing the charge on the cation enhances its ability to attract and distort the electron cloud of the anion, thus increasing polarization, making statement (b) true.

Step 5: Evaluating Size of Cations and Anions

A smaller cation has a higher polarizing power, while a larger anion is easily polarized, both of which contribute to increased polarization as stated in (c).

Step 6: Conclusion Based on Fajan's Rule

Since all the mentioned statements (a, b, c) accurately describe factors that influence polarization as per Fajan's rule, statement (d) 'All of these' is correct.

Key Concepts

Polarization

Polarization occurs when a cation distorts the electron cloud of an anion. This distortion causes a shift in electron density. It's like when a magnet pulls on some metal filings, distorting their usual arrangement. The main outcome of polarization is an increase in covalent character within what we might typically consider ionic compounds.
When anion's electron cloud is pulled towards the cation, it reduces the purely ionic nature of the bond. Consequently, the bond takes on a more covalent nature. This essentially blurs the line between ionic and covalent bonds, making the simple classification less straightforward.

Covalent Character

The covalent character in a bond describes how shared the electrons are between the atoms. The more the anion's electron cloud is distorted, the more its electrons are shared with the cation. This increased sharing gives the bond greater covalent character.
Covalent character is maximized during strong polarization. Why? Because a heavily distorted electron cloud usually means electrons spend more time between both bonded atoms. This reduced electron separation typically results in the molecule behaving more like a covalent compound than an ionic one.

Anion and Cation Size

The size of ions plays a crucial role in polarization. Smaller cations often have significant charge densities, meaning they can pull the electron cloud of anions closer. Conversely, larger anions, having loosely held outer electrons, are more susceptible to being polarized.
Imagine having a small, dense magnet and a large metal object. The magnet represents the cation and the metal object the anion. The small magnet has a powerful pull relative to its size. Similarly, in smaller cations, the positive charge concentration extends further, influencing nearby anions significantly. Larger anions, with their spacious electron clouds, give in more easily to this influence.

Ionic Compounds

Ionic compounds are formed when there’s a transfer of electrons from a metal to a non-metal. This transfer culminates in the formation of oppositely charged ions. Usually, these compounds are recognized for their strong electrostatic attractions. However, Fajan’s rule portrays that many ionic bonds possess some covalent character.
In practice, no bond is entirely ionic. The electron cloud distortion in ionic compounds, characterized by polarization, thereby grants them attributes of covalent compounds too, which affects their properties such as solubility and melting points. Understanding this dual nature guides chemists in predicting compound behavior.

Polarizing Power

Polarizing power is essentially the ability of a cation to distort the electron cloud of an anion. Factors enhancing this power include the cation's high charge and small size. A small, highly charged cation exerts considerable influence over the electron cloud of an anion, leading to increased polarization.
For instance, consider a tiny cation with a 3+ charge. Such strength makes it highly effective in attracting and distorting electron clouds. Cations like aluminum (Al³⁺) show considerable polarizing power due to their size and charge, lending aluminum compounds some covalent characteristics. Recognizing polarizing power helps in discerning the nuanced bonding in complex compounds.