fajans' rule

Fajans’ rule is used to predict if the chemical bond will be an ionic or a covalent bond. There are a few ionic bonds that have some particle covalent characteristics, which Kazimierz Fajan discussed in the year 1923.

Using the x-ray crystallography, Fajan could predict whether it is an ionic or covalent bond with the ionic or the atomic radius attributes. In exams like IIT JEE Fajan’s Rule and its various applications need to be understood.

Fajans’ rule explained

Certain compounds are ionic, and others are covalent. However, if you had to answer which alkali chlorides are the most ionic, you will need Fajans’ rule. To understand Fajans’ rule, it is first essential to comprehend polarising power and polarizability.

Polarising power is the extent to which the cation can polarise the anion. This is proportional to the charge density. The charge density is the ratio of the charge to the volume. If the charge density is more, then the polarising power of that particular cation is more.

Polarisability is the extent to which the ion can be polarised. It is also the ease with which you can polarise the ion. Polarisation distorts the electron cloud that is spherically symmetrical to form an unsymmetrical cloud.

Fajans’ rule – Postulates

Fajans’ rule can be stated based on the following three factors.

  • The ion size – If the cation size is small and the anion size is large, then the covalent character of the ionic bond is greater.
  • If the charge of the cation is more significant, then the covalent character of this ionic bond is also greater.
  • For the cations with the same size and charge, the one with an (n-1)dn nso found in the transition elements has a higher covalent characteristic than the cation with an electric configuration ns2 np6. The latter is usually found in alkaline and alkali earth metals.

Rule 1

The first rule talks about the cation polarising power. If the cation is small then the ion volume is less. If the volume is less, then the ions charge density is high. Because the charge density is high, it causes the ion’s polarising power to be high. This makes the compound to be more covalent.

Rule 2

The second rule talks about the anions’ polarising power. If the anion is large, then the nuclear charge that holds the ions valence electron is less. The last electron is loosely attached to the large anions. Thus it can get polarised by the cation easily. This makes the compound more covalent.

Rule 3

The third rule is a little tricky. So let us understand it with an example.

If you wish to find which is the more covalent compound between calcium chloride and HgCl2, then size cannot be used as a factor to differentiate them. This is because both the Ca2 and the Hg2 are of the same size. This is where the third rule comes into the picture.

Hg2 has an electronic configuration of 6s0 5d10. This is a pseudo octet configuration because of the d-orbital, which is filled fully, but the element does not contain 8 electrons, neither does it have an octet.

The d orbital is not good when it comes to shielding. The Cl– anion is more polarised. This makes HgCl2 more covalent than CaCl2 because of the noble gas configuration in the Ca2 ion.

So, which of the alkali chlorides is the most covalent? When the anions are the same, you need to compare the cations. If the cation is small, then the covalency is higher.

Example of Fajans’ rule

Let us consider aluminium iodide or AlI3.

This is an ionic bond that has been formed by electron transfer. Iodine is bigger and thus has a less effective nuclear charge. This causes the bonding electrons to get lesser attracted to the iodine nucleus.

Aluminium has three positive charges that attract the electrons pairs that are shared towards itself. This causes insufficient charge separation for it to stay ionic. Thus it results in developing into a covalent character which is found in AlI3.

Application of Fajans’ rule

Fajans’ rule is applied to predict the nature of the bond. It makes it possible to understand if the bond is covalent or ionic.


Chemical bonds are divided into ionic and covalent. The ionic bond denotes the complete electron transfer from one to the other atom. The covalent bond is equal to electron sharing. However, in reality, neither is purely covalent nor purely ionic. The nature of the chemical bond is such that the compound lies somewhere in between them.

It is possible to have more or less of the covalent nature for an ionic compound and more or less ionic nature in the covalent compounds. This is explained by anion polarization, which led to Fajans’ rules formulation, which assists in deciding the nature of any chemical bond.

FAQ Fajans’ rule

Among the metal halides, which compound should be more ionic and more covalent?

Technically, the small metal ion and the biggest anion should be the one that is the most covalent. Thus the answer is Lil. The smallest anion and the largest cation should be most ionic. CsF is thus the most ionic.

Arrange the following based on the increasing covalence order.

NaF, NaCl, NaBr, NaI

Here the cation is the same, so you will have to compare only the anion. Among the anions, the larger is the size of the anion; the more is its covalency. Thus the correct order is

NaF < NaCl < NaBr < NaI

Arrange the following in the decreasing order of covalency.

LiF, NaF, KF, RbF, CsF

The anions are the same here. So we will have to compare the cations—the cations, when smaller, have more covalency.

LiF> NaF> KF> RbF> CsF

Why is Aluminium Fluoride (AlF3) an ionic bond?

Aluminium Fluoride (AlF3) has an ionic bond that is formed by electron transfer. Here the fluorine is small and thus attracts the electrons shared pair more towards itself. This causes sufficient separation of charge, which in turn makes it ionic.

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