Fajans Rule in Chemistry - Concept, Law & Importance
Fajans rule is a fundamental principle in chemical bonding used to predict whether a chemical bond will be ionic or covalent. It describes how a cation can polarize an anion, leading to a degree of electron sharing. Understanding this rule helps students determine the covalent character in supposedly ionic compounds based on atomic properties.
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Understanding Fajans Rule Chemistry for Class 11
In fajans rule chemistry, we learn that no bond is 100% ionic. The fajans rule definition states that the covalent character of an ionic bond depends on the ability of the cation to distort the electron cloud of the anion. This process is known as polarization. When using fajans rule class 11 concepts, we look at the polarizing power of the cation and the polarizability of the anion.
The Postulates of Fajans Rule:
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Size of the Cation: Smaller cations have a higher charge density, leading to greater polarizing power and more covalent character.
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Size of the Anion: Larger anions are more easily polarized because their outer electrons are further from the nucleus.
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Charge of Ions: Higher positive or negative charges on the ions increase the covalent character.
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Electronic Configuration: Cations with a "pseudo-noble gas" configuration (18 electrons in the outermost shell) are more polarizing than those with a noble gas configuration (8 electrons).
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Application and Importance of Fajans Rule
The importance of fajans rule lies in its ability to explain the physical properties of compounds, such as melting points, boiling points, and solubility.
Practical Examples and Comparison:
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LiCl vs. KCl: Li+ is smaller than K+. According to fajans rule, LiCl has more covalent character than KCl.
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AgCl vs. NaCl: Even though Ag+ and Na+ are similar in size, Ag+ has a pseudo-noble gas configuration, making AgCl more covalent and less soluble in water compared to NaCl.6
Summary Table for Covalent Character:
|
Feature |
Factor for High Covalent Character |
|
Cation Size |
Small |
|
Anion Size |
Large |
|
Ionic Charge |
High (e.g., +3 > +2 > +1) |
|
Configuration |
Pseudo-noble gas (d10) |
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PW JEE Study Material for 2026 Exam Preparation
PW JEE Study Material for the 2026 exam is specially prepared to help students build strong concepts and improve their problem-solving skills. It includes detailed theory, easy-to-understand NCERT-aligned explanations, previous years’ questions, and consistent practice exercises. The well-planned structure ensures effective preparation and boosts confidence for both JEE Main and JEE Advanced.
Fajans Rule in Chemistry FAQs
1. What is the basic definition of Fajans rule?
Fajans rule helps predict the covalent nature of an ionic bond by looking at the polarizing power of the cation and the polarizability of the anion.7
2. How does the size of a cation affect the Fajans rule?
According to fajans rule chemistry, a smaller cation has a higher concentration of positive charge, which allows it to pull the anion's electrons more effectively, increasing covalent character.8
3. Why is AlCl3 more covalent than MgCl2?
Al3+ has a higher charge (+3) than Mg2+ (+2).9 Higher charge leads to greater polarization of the Cl- anion, making AlCl3 more covalent.10
4. Where can I find detailed Fajans rule class 11 notes?
You can find comprehensive notes and practice questions in the IIT JEE Books section of the PW Store.
5. Does Fajans rule apply to all ionic compounds?
Yes, it is used as a general rule to estimate the degree of covalent character in any ionic compound, which helps in predicting properties like solubility and melting points.
