Fajans’ Rule

What is 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.  


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 which 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 covalent character. A cation which has inert gas like configuration has less power of polarizing as compared to the cation having pseudo inert gas like configuration. 


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 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 it 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 as smaller is the cation. Therefore the order is CsF<RbF<KF<NaF<LiF.


Ionic Characteristic

Covalent Characteristic

Large Cation

Small Cation

Small Anion

Large Anion

Small-charge

Large Charge

FAQ (Frequently Asked Questions)

Q1. Define Fajans’ Law?

Ans: 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.

Q2. What is Polarizing Power and Polarizability?

Ans: The cation’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.

Q3. How is Polarizability Increased?

Ans: 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.

Q4. Which Element has the Highest Polarizability?

Ans: 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.