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Fajans’ Rule

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Last updated date: 17th Jul 2024
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What is Fajans’ Rule?

In Chemistry, it's important to know how atoms connect with each other. If you're preparing for the JEE Main exam in India, you'll be tested on these basic ideas. One such idea is Fajans' rule, a handy tool that helps predict if a bond between atoms will be purely ionic or have some covalent character. So, JEE aspirants, let's dive into the interesting topic of Fajans' rule!

Fajans’ rule in inorganic chemistry, formulated by Kazimierz Fajan in 1923, is used to predict whether a chemical bond will be covalent or ionic. This depends on the charge and the size of the cations and anions. In the following table, they can be summarized:

Ionic

Covalent

Low positive charge

High positive charge

Large cation

Small cation

Small anion

Large anion

The sodium chloride with a low positive charge (+1), a fairly large cation and the relatively small anion is ionic. However, aluminium iodide with a high positive charge and a large anion are covalent. Polarization will be increased by:

  • The small size of cation and high charge 

  • Ionic potential and Armstrong Z+/r+(polarising power)

  • The large size of anion and high charge; the polarizability of an anion is related to the deformability of its electron cloud

  • An incomplete valence shell electron configuration

Noble gas configuration of the cation procedure better shielding and less polarization power. Example- Hg2+(r+=102pm) is more polarizing than Ca2+(r+=100pm). 

 

The size of the charge in an ionic bond depends on the number of electrons transferred. For example, an aluminium atom with a+3 charge has a relatively larger positive charge. Then, this positive charge exerts an attractive force on the electron cloud of the other ion which has accepted the electrons from the aluminum positive ion.

Types of Chemical Bonds

The chemical bonds in chemistry are divided into two types, that is, ionic and covalent bonds. Though, in reality, ionic bonds and covalent bonds represent two extreme forms of bonds and most of the bonds are neither purely covalent nor purely ionic. Ionic bond refers to those bonds that transfer electrons from one atom to the other completely. The covalent bonds refer to those bonds that involve sharing of electrons equally. Thus, in nature bonds generally have characters of both types of bonds wherein, one bond denotes more of its characteristics to a particular bond. 

 

Ionic compounds generally have high melting and boiling points and also show electrical conductivity in the liquid state. These compounds are soluble in polar solvents like water and are sparingly soluble in nonpolar solvents like carbon tetrachloride and benzene. In molten state though, some ionic compounds are fairly soluble in nonpolar solvents and may show electric conductivity and fewer melting points as well.

 

The change in the characteristics of covalent bonds clearly shows that some compounds may exhibit a less covalent nature for the ionic compounds. Similarly, some covalent compounds show a less ionic nature than others. These observations were explained by the concept of polarization of anion which later led to the formulation of Fajan’s rule in 1923. Fajan’s rule made it possible to decide the nature of chemical bonds.

What is Fajans’ Law?

Whether a chemical bond will be converted into covalent or ionic is predicted by the Fajans’ law. Few of the ionic bonds have partial covalent characteristics which were 1st discussed in 1923 by Kazimierz Fajans. With the help of x-ray and crystallography, he was able to predict ionic or covalent bonding with the attributes like atomic or ionic radius. 

 

The variation in effect can be illustrated by two contrasting examples. In the case of aluminium iodide, a bond that is ionic but with lots of covalent character is present. In All3 the aluminium bonding gains a +3 charge. On the cloud of iodine, it pulls the large charge of the electrons. Considering the iodine atom we can see that it is relatively large. Thus the outer shell electrons are relatively well-shielded from the nuclear charge. 

 

The aluminium ion charge will tug on the electron clouds of iodine, in this case, drawing it closer to itself. Near the aluminium atom as the electron clouds of the iodide, the negative charge of the electron cloud cancels out the positive charge of the aluminium cation. This produces an ionic bond with the help of a covalent character. A cation that has inert gas like configuration has less power of polarizing as compared to the cation having pseudo inert gas like configuration. 

Fajans' Rule with an Example: 

Aluminum Iodide $(AlI_3)$ and Aluminum Fluoride $(AlF_3)$.


In $AlI_3$, the electrons are transferred, creating an ionic bond. Because iodine is larger, it has a weaker pull on the bonding electrons. On the other hand, aluminum has a stronger pull due to its positive charge, but it's not enough for a complete ionic bond. This results in a somewhat covalent character in $AlI_3$.


Now, in $AlF_3$, the ionic bond is also formed by electron transfer. However, fluorine, being smaller, attracts the shared electrons more strongly. This creates enough charge separation, making $AlF_3$ distinctly ionic.

What are the postulates of Fajan’s Rule?

Fajan’s rule is based on 3 main factors or postulates which are the size of ion, electronic configuration, and the charge of cation. The details of these 3 factors or postulates are given below:

  1. Size of the ion: Greater covalent character of the ionic bond is facilitated by the smaller size of the cation and the larger size of the anion.

  2. The charge of cation: the covalent character of an ionic bond increases as the charge of the cation increases.

  3. Electronic configuration: if the two cations have the same charge and size, then in that case those cations that have electronic configuration of (n-1)dn nso, are known to possess a greater covalent character than the cations which have an electronic configuration of ns2 np6, which is a common electronic configuration found in alkali metals and alkali earth metals.

What are the factors that can affect the polarization of anion?

Fajan’s rule is stated on the basis of the concept of polarization which states that the covalent nature of an anion increases with an increase in its polarization. The two main factors that affect the extent of polarization of an anion are the polarizing power of the cation and the polarizability of the anion. These two factors are discussed in detail below:

  1. Polarizing power of cation: It is defined as the ability of a cation to polarize an anion. The polarizing power of cation increases with an increase in charge density, charge on cation, and size of cation and decreases with an increase in the size of the anion. 

  2. The polarizability of an anion: it is defined as the capability of an anion to undergo polarization, that is, how easily an anion undergoes distortion because of the presence of a cation. The polarizability of an anion increases with an increase in the size of the anion and the negative charge on the anion.

Explanation of Fajans’ Rule

Here is the explanation of Fajans’ rules:

 

Rule 1: It speaks about the polarising power of the cation. In the case when the cation is smaller, we can say that the volume of ions is less. We can conclude that the charge density of the ion will be high if the volume is less. The polarizing power of the ion will be high since the charge density is high, this makes the compound to be more covalent.

 

Rule 2: The second rule tells us about the polarizability of the anion. The effective nuclear charge in the case of a large anion that holds the valence electron of the ion in its place is less. Since in the large aions the electron is loosely bonded, it can easily be polarized by an anion. It can easily be polarized by a cation thereby making the compound more and more covalent.  

 

Rule 3: It is a special case so we will understand it by taking an example:

If we are interested in finding more covalent compounds amongst calcium chloride and Hgcl2  we cannot use the factor size to conclude. This is because Hg2+ and Ca2+ are of almost equal size and to understand.

Examples and Applications of Fajans’ Law

Understanding the Fajan’s Law with the help of examples.

 

Theoretically which compound should be the most ironic and most covalent amongst the metal halides?

 

The largest anion and the smallest metal ion should technically be more covalent that's why Lil is the most covalent. The smallest anion and the largest cation should be the most ionic therefore CsF should be the most ionic.

 

These are arranged in the increasing order of covalency.

Naf, NaCl, NaBr, Nal

LiF, NaF, KF, RbF,CsF

 

The cation is the same as compared to the anions. Larger the size amongst the anion more would be the covalency. Therefore the order will be as: NaF< NaCl< NaBr< Nal.

 

The anion is the same here as compared to cation. More is the covalency and smaller is the cation. Therefore the order is CsF<RbF<KF<NaF<LiF.


Summary of Fajans’ Rule


Condition

Bond Character

Example

Low charge cation, large cation, small anion

Ionic

NaCl

High charge cation, small cation, large anion

Covalent character

AlI₃

Cation with (n-1)dn ns⁰ configuration

More covalent character

Transition metal compounds



This table simplifies Fajans' Rule to help you understand different bond characters in various examples.


The Three Key Players in Fajans’ Rule are:

  1. Charge of the Cation: Imagine a strongman with multiple arms trying to grab a feather. The stronger the pull (charge), the more likely the feather (electrons) will be pulled towards the strongman (cation), creating a stronger connection.

  2. Size of the Cation: Now, picture the strongman clenching his fist around the feather. If the strongman (cation) is smaller, the positive charge is concentrated in a smaller area, making it even better at pulling electrons – like a tighter grip on the feather.

  3. Size of the Anion: Think of the anion as the size of the feather. A larger anion provides more "electron real estate" for the cation to tug on, making it easier for the strongman to grab hold. So, smaller anions tend to lead to more covalent character.


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Conclusion

Learn Fajans' Rule and its important aspects to handle bond-related questions in your JEE Main exams. Practice, visualise, and be aware of exceptions. By embracing these strategies, you'll be well-prepared to conquer the challenges posed by chemical bonds and perform exceptionally well in your exams.

FAQs on Fajans’ Rule

1. Define Fajans’ Law?

In the inorganic branch of chemistry, Fajans’ rule, formulated by Kazimierz Fajan in 1923, is used for the prediction purpose, whether a chemical bond will be covalent or ionic. It depends on the charge on the cation and the relative size of the cation and anion. Fajan’s rule is applied in cases where we need to predict which compound is most ionic amongst a group of compounds. Kazimierz Fajans in 1923 used X-ray crystallography to predict ionic or covalent bonding based on the atomic radius and ionic radius.

2. What is Polarizing Power and Polarizability?

The nation's ability to distort the power and tendency of an anion is known as polarization power. The tendency of an anion to become polarized by cation is known as polarizability. Polarizing power can also be defined as the capability of cation to distort the electron density of an anion towards itself, that is, the capability of the cation to attract the electron density of anion towards itself and this tendency by which anion is polarized by a cation is known as polarizability.

3. How is Polarizability Increased?

Polarizability generally increases as the volume occupied by the electrons increases. This occurs because large atoms have more loosely held electrons in contrast to smaller atoms with tightly bound electrons. Hence, in the periodic table row, polarizability decreases from left to right. Thus, ions which are smaller in size are less polarizable and the ions which have a greater size are more polarizable. Also, atoms which have a negative charge are more polarizable than atoms that have a positive charge.

4. Which Element has the Highest Polarizability?

The polarizability of any element refers to the measure of easiness with which an electron cloud is distorted by an electric field. Electrons which have larger sizes and are negatively charged are more polarizable than the other elements. Therefore, ions that have small sizes and high positive charges have very low polarizability. Sb is the most polarizable element. Its electron cloud is most easily distorted by another incoming atom. This is because it is the largest and the least electronegative among the choices.

5. Explain Fajan’s rule with the help of aluminium iodide as an example.

Aluminium iodide forms an ionic bond due to the transfer of electrons. Since, iodine is a bigger atom and therefore, has a lesser effective nuclear charge due to which the bonding electrons attract less towards the iodine nucleus. The aluminium atoms contain three positive charges and thus, attract the shared pair of electrons towards itself. This causes an insufficient charge separation which diminishes the chances of it being an ionic bond and thus, results in the development of covalent character.