Fajan’s Rule in Chemistry is a rule of Chemical Bonding Named after the Polish chemist Kazimierz Fajan. Fajan’s Rule provides insights into the dynamics of ionic bonding and helps explain why certain ionic compounds exhibit covalent characteristics.
In this article, we will start with an explanation of Fajan’s Rule in chemistry. We’ll then explore its foundational principles, learn the significance of polarizability and polarizing power, and discuss its applications, limitations, and then solve some sample questions. This article helps you understand the world of Fajan’s Rule, making it a valuable resource for your class 11 studies.
Table of Content
What is Fajan’s Rule?
Fajan’s Rule is a principle that helps us understand the nature of chemical bonds, specifically ionic bonds. It explains why some ionic bonds exhibit characteristics of covalent bonding. This Rule takes into account the charge of the cation (positively charged ion) and the relative sizes of the cation and anion (negatively charged ion).
Distortion Of Electron
Postulates of Fajan’s Rule
Fajan rule stands on three fundamental postulates:
- Size of the ion: The size of the cation and anion matters. Smaller cations and larger anions are more likely to produce an ionic compound with covalent characteristics.
- Charge of the cation: The charge of the cation plays an essential role. The greater cationic charge leads to higher covalent character in the ionic bond.
- Electronic configuration: Cations with (n-1)d10 ns2 electronic configuration, often found in transition elements, exhibit greater covalent character when they have the same charge and size compared to cations with ns2 np6 electronic configurations, typically found in alkali or alkaline earth metals.
What Is Polarizability and Polarizing Power?
In Fajan’s Rule, two key concepts come into play: polarizability and polarizing power. These concepts are essential for understanding the covalent character of ionic compounds.
Polarizability refers to how easily the electric field of a cation distorts the electron cloud of an anion. Polarizing power measures a cation’s ability to distort the electron cloud of an anion, indicating how strongly the cation can attract the anion’s electrons towards itself. These concepts play a significant role in understanding the covalent character of ionic compounds.
Let’s understand an ionic bond before digging deeper into the Fajan Rule.
What is an Ionic Bond?
The ionic bond is a bond that forms when electrons are transferred from one atom to another, leading to the formation of ions. In this process, one atom donates electrons to become a positively charged cation, and another atom accepts electrons and becomes a negatively charged ion. The electrostatic force of attraction among these oppositely charged ions creates an ionic bond.
Relationship Between Ionic Size and Ionic Character
The ionic character also depends on the charge of the ions and the size of the ions. As we had discussed above, according to the Fajan Rule, in an ionic compound, the smaller cations with higher positive charges have a strong ability to distort the electron cloud of the anion. This distortion of the electron leads to an increase in covalent character in the ionic Compound. As covalent character increases, ionic character decreases in the Compound.
In other words, the interaction between smaller and highly charged cations and larger anions increases the covalent character in the ionic Compound, which eventually decreases the ionic character of the ionic Compound.
Explanation of Fajan’s Rule
Fajan’s Rule depends on the polarising power of cations and the polarizability of the anion. More minor, highly charged cations distort relatively larger anions’ electron clouds. This results in the formation of ionic Compounds with covalent properties. This distortion occurs as the positive charge cation pulls the electron cloud of anion closer to itself, giving rise to a covalent character within an ionic bond.
Examples Of Fajan’s Rule
Let’s consider a few examples of the Fajan Rule to understand the topic better.
Example 1: AlCl3
In this Compound, (Al3+) acts as the small highly charged cation, while chloride (Cl–) serves as a relatively large anion. Due to the small size and more charge on Al3+, it possesses strong polarising power. This power causes the distortion of the electron cloud of chloride, leading to significant covalent character in the Al-Cl bond.
Example 2: (Na+), (Mg2+), (Al3+) Which one of them have the most covalent character?
According to the Fajan rule, cations with more charge will tend to exhibit more covalent character so that the sequence will go as (Al3+) > (Mg2+) > (Na+). So, (Al3+) will show the most covalent character.
Application Of Fajan’s Rule
Fajan’s Rule finds practical application in predicting the behaviour of various ionic compounds, especially in terms of solubility and melting points. It offers insights into why some ionic compounds exhibit properties resembling covalent bonding. However, it’s essential to recognize the limitations of this Rule and understand that real-world scenarios often involve a blend of ionic and covalent bonding.
Limitations of Fajan’s Rule
While Fajan’s Rule is a powerful rule, it’s essential to acknowledge its limitations. It provides a simplified view of chemical bonding and may only partially explain the behaviour of some compounds. Various factors and interactions can influence the nature of chemical bonds, and Fajan’s Rule serves as a foundational guideline rather than an absolute determinant.
Also, Check
Sample Questions on Fajan’s Rule
Question 1: Define Fajan’s Rule, Polarizing Power, and Polarizability
Answer:
Fajan’s Rule predicts that smaller cations, larger anions, and cations with higher charges contribute to a more covalent nature in ionic compounds. Polarizing power measures a cation’s ability to distort the electron cloud of an anion, while polarizability refers to how easily the electric field of a cation distorts the electron cloud of an anion.
Question 2: Explain the distortion of electrons in the Fajan Rule?
Answer:
Smaller, highly charged cations can distort the electron clouds of relatively larger anions, leading to the formation of ionic compounds with covalent properties.
Question 3: What are the three factors that determine the covalent character of an ionic bond according to Fajans’ Rule?
Answer:
The three factors that determine the covalent character of an ionic bond are The size of the ions, the charge of the cation, and the electronic configuration of the cations.
Question 4: Why do small cations with high charges have high polarizing power?
Answer:
Small cations with high charges have a strong ability to distort the electron cloud of anions, leading to higher polarizing power.
Question 5: Give two examples of ionic compounds with a high covalent character due to Fajans’ Rule.
Answer:
As per Fajan’s Rule, two ionic compounds with high covalent character are AlCl3, LiCl
Question 6: Check the ionic and covalent character of each molecule: NaCl, LiCl, MgCl2, AlCl3
Answer:
The covalent and ionic character of these four molecules will be as follows:
Covalent Character: AlCl3 > MgCl2 > LiCl > NaCl
Ionic Character: NaCl > LiCl > MgCl2 > AlCl3
Question 7: Check the ionic and covalent character of each molecule: H2O, NH3, CH4, CO2
Answer:
The Covalent and Ionic character of these molecules are as follows:
Covalent Character: H2O > NH3 > CH4 > CO2
Ionic Character: CO2 > CH4 > NH3 > H2O
Fajan’s Rule – FAQs
1. State Fajan’s Rule.
Fajan’s Rule predicts that smaller cations, larger anions, and cations with higher charges contribute to a more covalent nature in ionic compounds.
2. According to Fajan’s Rule, Covalent Bond is Favoured by?
According to Fajan’s Rule, a covalent bond is favoured by smaller cations, larger anions, and cations with higher charges.
3. Can Fajan’s Rule predict the behaviour of all Ionic Compounds Accurately?
Fajan’s Rule provides a simplified view of chemical bonding and may only partially explain the behaviour of some compounds. Real-world scenarios often involve a blend of ionic and covalent bonding, so the Rule has limitations.
4. What is Fajan’s Rule of Solubility and Melting Point?
Fajan’s Rule aids in predicting the behaviour of ionic compounds by offering insights into their solubility and melting points. Compounds with higher covalent character are likelier to have lower melting points and be more soluble in polar solvents.
5. Which is most Ionic as per Fajan’s Rule? – NaCl, MgCl2, AlCl3, PbCl4
As per Fajan’s Rule, if a cation has greater charge then it has greater covalent character, hence NaCl with Na having lowest +1 charge will be most ionic
6. What is the Practical Application of Fajan’s Rule in Chemistry?
Fajan’s Rule finds practical application in various areas of chemistry, particularly in predicting the behaviour of ionic compounds and understanding why some exhibit covalent characteristics. This knowledge is valuable in fields like materials science and chemical engineering.
7. How can Fajan’s Rule applied to solve Problems related to Chemical Bonding?
To apply Fajan’s Rule effectively, students should focus on assessing the size of ions, their charges, and electronic configurations. By considering these factors, students can make educated predictions about the nature of chemical bonds in different compounds.
8. Is Fajan’s Rule for class 11 different from Fajan’s Rule for BSc 1st year?
No, Fajan’s Rule remains consistent across different educational levels and is fundamental in studying chemical bonding.