The Fajans’ Rule can be used to identify the covalent or ionic nature of a chemical connection. At the American Chemical Society in 1923, Kazimierz Fajans shared his findings on the partially covalent nature of a few ionic bonds.

When X-ray crystallography was developed, it enabled him to predict the presence or absence of ionic or covalent bonding by examining physical factors such as ionic and atomic radiuses. So, let’s learn about the Fajans’ rule application in detail.

What is the Fajans’ rule specifically, and how does it work?

If a substance contains an electron pair, it can be classified as either ionic or covalent, and this distinction is important. Now, the question is- which of the alkali chlorides is the most ionic?


The guidelines developed by Fajans are a useful tool when dealing with these types of situations. First, let us define a few key terms necessary to understand the Fajans’ Rule Application accurately.

  • Polarising power 

Polarisation extent refers to the amount of polarisation that a cation can induce in an anion. It varies in direct proportion to the amount of charged particles present in the atmosphere, whether for the better or the worse.

It is defined as the relationship between charge density and volume. The charge density polarisation power is measured in watts, and this is the unit of measurement. The polarising power of the electrons increases proportionally to the increase in charge density.

  • Polarisability

The width of an ion’s charge distribution represents the extent to which the ion can be polarised in the presence of an electron. This can also be described in terms of the ease with which an ion can become polarised.

Because of this, it is necessary to disrupt the spherical symmetry of an otherwise completely symmetric electron cloud to generate asymmetrical electron clouds in some way.

Fajans’ Rule Hypotheses

Fajan’s Rule Application is based on three components that work together:

  • Size of the ion: The covalent character of an ionic bond is determined by the ion’s size, which increases as the cation’s size decreases.
  • The charge of Cation: When a cation’s charge is increased, the covalent nature of the ionic connection becomes more apparent.
  • Electronic configuration: The difference in covalent character between two identically charged and sized cations is attributable to the cations’ electronic configuration. Transition element cations are more abundant in alkali and alkaline earth metals. They have an electronic configuration of (n-1)dn nso, suggesting that they are transition elements.


How does Fajans’ Rule Application work?

Rule 1:

To begin, the cation contributes to the solution’s polarisation. By substituting an alkaline solution, a smaller volume ionic cation is generated. The higher an ion’s charge density, the less space is required. The ion’s polarising power would be substantial due to its high charge density. The formation of more covalent bonds increases the molecule’s overall properties.

Rule 2:

The second rule discusses in depth an anion’s polarisability. As an ion’s effective nuclear charge drops, the anion’s size must rise to keep its valence electrons in their appropriate places. A cation may readily polarise the last electron of an anion, hence increasing the molecules covalently.

Rule 3:

The third rule deviates from the previous two rules’ universality. The following can be understood using a specific example.

Examine the following example to help you better understand Fajans’ rule:

Consider the chemical compound Aluminum Iodide as an example (AlI3). We are dealing with an ionic connection made by electron transport-

  • Its more extensive size results in a less nuclear solid charge. As a result, the iodine nucleus draws fewer electrons capable of bonding with other atoms.
  • On the other hand, the shared pair of electrons is attracted to aluminium, which possesses three positive charges and attracts the electrons.
  • AlI3 obtains covalent properties as a result of the inadequate charge separation caused by this process.

Consider fluoride aluminium as an example (AlF3)-

In this instance of Fajans’ Rule Application, electron transfer was used to form an ionic bond as in the previous one. This is despite the fact that fluorine’s small size causes it to attract a previously shared pair of electrons closer to it, resulting in sufficient charge separation to qualify as an ionic compound.

Fajans’ Rule Application in Specific Cases

Example 1: The metal halide complex exhibiting the greatest ionic and covalent concentrations are?


The covalent ions with the most excellent affinity are technically the minor metal ions and the biggest anion.

LiI is the most covalently bound element in this case. And ionicity is defined as the difference between the most significant cation and the smallest anion. As a result, CsF is the most ionic of the compounds.

Example 2: The following components should be arranged in ascending order of covalency.

  • NaF, NaCl, NaBr, NaI
  • LiF, NaF, KF, RbF, CsF


Due to their similarity, no comparison of the cations is necessary. Hence, by definition, the wider the diameter of an ion, the greater its covalency. As a result, NaF precedes NaCl, and so on until the alphabetical ordering is complete.

In this case, there is no difference between an anion and a cation. When one cation is less than another, it has a higher covalency value. This suggests that CsF precedes RbF, K, K, Na, and Li in the chronology of their occurrences.


You may sum up Fajans’ Rule by saying this:

Ionic Characteristic Covalent Characteristic
Large Cation Small Cation
Small Anion Large Anion
Small-charge Large Charge


In this short article, we will summarise and discuss Fajan’s Rule in greater detail. Fajans’ Rule’s application to determine whether a chemical bond is covalent or ionic is an essential concept in chemistry, and this concise explanation summarises it well. Make sure to heed Fajan’s recommendations! When you maintain a positive attitude and enjoy yourself, chemistry can be quite interesting!


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