Fajans Rule, Meaning, Postulates, Polarization Power in JEE

Fajans Rule : The covalent or ionic nature of a chemical bond may be predicted using the Fajans rule. Partial covalent properties of certain ionic bonds were initially reported by Kazimierz Fajans in 1923. Using X-ray crystallography, he was able to anticipate ionic or covalent 

What is Fajans Rule?

Fajan's Rule And Polarizability

Fajan's Rule developed by Polish chemist Kazimierz Fajans in the early 20th century, Fajan Rule provides valuable insights into the prediction of ionic character and covalent character in chemical bonds. Fajan's Rule is particularly relevant when dealing with polar covalent compounds and ionic compounds that possess covalent characteristics. The rule is based on the idea that the polarization of an ionic bond increases with the size and charge of the ions involved.

Positive ions attract the outermost electron of an anion and repels its nucleus, causing distortion or polarization of the anion. And anion is called Polarized.

Polarization Power

It is generally represented by ϕ. The ability of a cation to polarize the anion is referred to as polarizing power. It is directly proportional to the charge density, which is directly related to the charge on cation, while inversely related to the size of anion. The polarizing power increases with increase in the size of cation i.e. smaller cations are very effective in the polarization of anion. However, the polarizing power increases with increase in the charge on cation.

Polarizability of Anion

• It is the tendency of an anion to undergo polarization. It indicates the easiness with which an anion undergoes distortion in the presence of a cation.

• It is directly proportional to the size as well as the negative charge on the anion.

• The larger anions can undergo distortion very easily than the smaller ones.

• It is also important to note that the anions with greater negative charge also undergo polarization easily.

• Oxyanions are generally less polarizable because charge is present on O atom which is very small, and we need to consider only the element which acquires the charge (and not the complete anion)

Important Points To Note

1. Smaller size and increase in charge density on cation, increases its polarizing power.

2. Larger anions hold its valence electron loosely — more polarization occurs.

Fajan’s Rule For covalent character

Fajan’s Rule For covalent character : This rule is for covalent character of an ionic bond. Covalent character of an ionic bond is favored by:

i) Small positive ion

ii) Large negative ion

iii) large charge on ions (Cations & Anions).

• Thus, for a fixed cation, larger the size of anion, more the magnitude of the charge, more is covalent character.

Fajan’s Rule Example

Covalent character of sodium halides follows the order: NaI > NaBr > NaCl > NaF.

• For fixed anion, the smaller the size of cation, more is the magnitude of the charge, more is the covalent character.

Example:

BeCl ₂ > MgCl ₂ > CaCl ₂ > SrCl ₂ > BaCl ₂

• • Charge on ions (Cations & Anions):

• Charge on cation increases.

• Covalent character increases

• Ionic character decreases (M.P. decreases)

• Charge increases

• Covalent character increases

AlN > Al ₂ O ₃ > AlF ₃

Determine the covalent And ionic character of molecule

From left (larger size) to right (smaller size) in a period ϕ increases so covalent character increases.

• Covalent character increases

From top to bottom in a group ϕ decreases so covalent character decreases.

Applications of Fajans’ Rule

(a) To compare the covalent and ionic character of molecule.

(b) To compare the nature of oxide.

(c) To compare the electrical conductivity of ionic compounds.

(d) Tendency of the formation of complex compounds.

(e) To compare the thermal stability of metal salts.

(f) To compare the intensity of colour of compounds.

(g) To compare the solubility of heavier metal halide in water.

3 Important Factor About Main Postulates of Fajans Rule

1. Ion size: Anion size increases with covalent bond strength; smaller cations have a higher covalent character.
2. The cation charge: The covalent nature of the ionic bond increases with the cation's charge.
3. Electronic configuration: Among cations of the same charge and size, the one with the electronic configuration of (n-1)dn nso, found in transition elements, is more covalently bonded than the one with the configuration of ns2 np6, found in alkali or alkaline earth metals.

What is Polarising Power?

Explanation of Fajans Rule

Main Factors in Fajans Rule

1. Charge of the Cation: Consider a multi-armed strongman attempting to grab a feather. The feather, or electrons, are more likely to be drawn towards the strongman, or cation, and form a stronger bond, the greater the pull (charge).
2. Size of the Cation: Now imagine the strongman curling the feather in his fist. Stronger cations draw electrons more forcefully than weaker ones because their concentrated positive charge is more tightly bound, like a stronger grip on a feather.
3. Anion Size: Imagine that an anion is the same size as a feather. A bigger anion gives the cation more "electron real estate" to pull on, which helps the strongman's grip. Smaller anions hence often result in greater covalent character.

Fajans Rule Examples

1. Compare the anions because the cation is the same. The greater the size among the anions, the higher the covalency. As a result, the sequence is NaF < NaCl < NaBr < NaI.
2. Since the anion is the same in this case, we may compare it to cations. The covalency increases with the size of the cation. As a result, the sequence is as follows: CsF < RbF < KF < NaF < LiF.