The Study on Fajan’s Rule and its Explanation, Applications, Examples and FAQs
The Fajans rule determines whether a chemical link is covalent or ionic. Kazimierz Fajans initially described the partial covalent properties of a few ionic bonds in 1923. He was able to anticipate ionic or covalent bonding given attributes like ionic and atomic radius using X-ray crystallography at the time.
So, for JEE aspirants, this guide on Fajans rule JEE can act as a sufficient revision piece.
What is fajan’s rule?
Certain substances are classified as ionic, whereas others are classified as covalent. Which alkali chloride is the most ionic among the alkali chlorides? We use Fajans’ rules as a tool to answer these kinds of queries.
Charge density polarising power The polarising power of a cation is proportional to its charge density. It is proportional to the density of charges. The ratio of charge to volume is known as charge density. It refers to how far a cation may polarise an anion.
The deformation of a spherically symmetric electron cloud into an unsymmetric cloud is known as polarisation. It’s the amount of polarisation that an ion can have. It’s also known as the ease with which an ion can polarise.
Fajans’ Rule Postulates
Three factors can be used to formulate the rule:
- Size of ion: Smaller cations have a larger polarising power, meaning they cause an anion’s electron charge cloud to polarise to a greater extent. As a result, the covalent character of the concerned compounds for a certain anion increases as the cationic size decreases. The cationic size [ M 2 ]of group-II alkaline metal elements, for example, steadily increases down the group from top to bottom. As a result, from top to bottom, the covalent character decreases. As a result, the covalent character of group-II metal halides reduces as one moves down the group.
- The charge of cation: Large-sized anions have high polarizability. That is, cations can easily deform their electron charge cloud. As a result, the covalent nature of the associated chemical grows as the anionic size increases. The size of halide ions, for example, gradually increases from top to bottom, and so the covalent nature of halogen compounds for a given cation increases from top to bottom. The covalent nature of several calcium halides, for example, increases from F – to I –.
- Electronic configuration: The degree of polarisation of an anion by a cation increases as the charge on the cation, anion, or both increases. Because the electrostatic forces that induce polarisation increase as the charge on the ions increases, the polarisation will also increase. As a result, as the charge on ions grows, so does the covalent character of the generating compounds.
Explanation of the fajan’s rule
- Rule 1:
The first rule discusses the cation’s polarising ability. We can state that the ion’s volume is less if the cation is smaller. If the volume is less, we can assume that the ion’s charge density is large.
The polarising power of the ion would be high since the charge density is large. The compound becomes more covalent as a result of this.
- Rule 2:
The anion’s polarizability is addressed in the second rule. The less effective the nuclear charge that holds the valence electron of the ion in place, the larger the anion. Because the final electron in big anions is weakly attached, a cation can easily polarise it, making the molecule more covalent.
- Rule 3:
The third rule is an exception. To illustrate this argument, consider the following scenario.
For example, we can’t use size as a criterion to determine whether the compound is more covalent than HgCl2 and Calcium Chloride. This is since Hg2 and Ca2 are nearly similar in size. The third rule is used to explain this.
Hg2 has the electronic structure 6s0 5d10. Because the d-orbital is entirely occupied, the element does not contain 8 electrons or an octet. This configuration is called pseudo-octet.
We know that d orbitals are bad at shielding. Thus the anion (Cl–) would be more polarised since the d orbital is bad at shielding, making HgCl2 more covalent than CaCl2 due to the noble gas configuration of the Ca2 ion.
To answer the first question, which alkali chloride is the most covalent among the alkali chlorides?
We must compare the cations because the anion is the same. The smaller the cation, the higher the covalency, according to Fajans’ criteria. As a result, LiCl has the highest covalent bonding.
Explanation through Example
Let’s look at a detailed illustration of Fajans’ rule:
Take a look at Aluminum Iodide (AlI3)
This is an ionic bond that was established through electron transfer.
Because iodine is larger, it has a lower effective nuclear charge. As a result, bonding electrons are attracted to the Iodine nucleus less.
The three positive charges on aluminium, on the other hand, attract the shared pair of electrons towards it.
As a result, there is insufficient charge separation to be ionic, and AlI3 develops a covalent nature.
Take Aluminum Fluoride, for example (AlF3)
This is an ionic bond that was also produced by electron transfer. However, because fluorine is smaller, it attracts the shared pair of electrons closer to itself, resulting in enough charge separation to make it ionic.
Example of fajan’s rule
- Among the metal halides, which compound should theoretically be the most ionic and covalent?
Technically, the smallest metal ion and the largest anion should be the most covalent. As a result, LiI has the highest covalent value. The most ionic cation and anion should be the largest and smallest. As a result, CsF should be the most ionic compound. Technically, the smallest metal ion and the largest anion should be the most covalent.
- Arrange the following items in ascending order of covalency:
- NaF, NaCl, NaBr, NaI
- LiF, NaF,KF,RbF,CsF
- Compare the anions because the cation is the same. The greater the size of the anions, the higher the covalency. As a result, the order is: NaF < NaCl < NaBr < NaI
- Because the anion is the same in this case, we compare it to cations. The higher the covalency, the smaller the cation. As a result, the following is the order: CsF RbF KF NaF LiF.
In this manner, the features and aspects of Fajan’s rule can be understood through examples. You can get to know more by browsing through Vedantu and going through the different concepts associated with this rule.