Fajans’ criteria are used in inorganic chemistry to evaluate whether a chemical connection is covalent or ionic. They were created by Kazimierz Fajans in 1923. They are based on the cation’s charge and the cation and anion’s relative sizes. The following table summarizes their findings:

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Thus, sodium chloride is ionic, with a low positive charge (+1), a large cation (~1 Å) and a tiny anion (0.2 Å), but aluminium iodide (AlI3) is covalent, with a high positive charge (+3) and a large anion.

According to Fajans’ rule, an ionic bond is formed by a compound with a low positive charge, a large cation, and a small anion. In contrast, a covalent bond is formed by a compound with a high positive charge, a small cation, and a large anion. With high charges, small cations have more polarising power.

Fajans’ rule of Polarization

There are three factors that can be used to formulate the rule:

1. Size of the ion: Larger is the size of ion weaker will be force of attraction and hence smaller will be lattice energy, ionic character increases.

Order of covalent character :   LiCl>NaCl>KCl>RbCl>CsCl

2. The charge of Cation: Higher will be charge on ion more will be the force of attraction so higher will be lattice energy and  the covalent property of the ionic bond increases as the charge of the cation increases.

Order of covalent character : NaCl< MgCl₂< AlCl₃< SiCl₄

3. Electronic  Configuration: The element with (n-1)d and  n so found in transition elements has a higher covalent character than the one with ns2 np6 electronic configuration found in alkali or alkaline earth metals.

Explanations of Fajans’ rules

 • The reduced the size of the cation and the larger the size of the anion, the larger the covalent character of an ionic bond.

• The larger the charge on the cation, the larger the covalent character of the ionic bond.

• For cations of the equal size and charge, the one, with electronic configuration (n-1)dx ns0, typical of transition metals, is further polarising than the one with a noble gas configuration, ns2 np6, standard of alkaline earth and alkali

metal cations.

• The cation polarises the anion, pull the electronic charge toward itself and thereby increasing the electronic charge among the two. This is precisely what occurs in a covalent bond, i.e., build-up of electron charge density(d) among the nuclei. The polarising power of the cation, the polarisability of the anion & the extent of distortion (polarisation) of anion are the factors, which determine the percent covalent character of the ionic bond.

Fajans’ rules and its applications with examples

Application of Fajans’ Rule with example

Covalent nature of alkali metal halides :

In a given group of the periodic table, we know that the size of cations rises from top to bottom. As a result, the cation size of alkali metals grows, as seen below.

Li+ < Na+ < K+ < Rb+ < Cs+

According to Fazan’s rule-1, the covalent nature of alkali metal halides having the same halide ion diminishes from Lithium halide to Cesium halide.

The decreasing order of covalent nature of alkali metal chlorides, for example, is:

RbCl > CsCl > LiCl > NaCl > KCl > RbCl > CsCl

It’s also worth noting that the solubility of these compounds drops in the same order in non-polar liquids with low dielectric constants.

Due to the decrease in covalent character, the melting points are reversed (or increase in ionic nature).

The trend in covalent nature depends on the size of the anion when the alkali metal ion is fixed while the halide is modified.

The order of covalent character for Lithium halides with various halide ions, for example, is indicated below.

LiF < LiCl < LiBr < LiI

The size of the anion grows from fluoride to iodide ion in the given scenario, and hence the covalent nature increases.

However, as the polarization of the anion increases, the stability of these compounds diminishes from LiF to LiI.

When exposed to air, lithium iodide turns yellow due to the oxidation of iodide to iodine. This occurs because of the significant polarization of the larger Iodide anion in the presence of the smaller lithium-ion.

Application of Fajans’ Rule-2 with examples

The covalent nature of Na, Mg, and Al chlorides is in this sequence.

NaCl < MgCl₂ < AlCl₃

The rise in positive charge on cations, such as Na+, Mg2+, and Al3+, is the cause.

Application of Fajans’ Rule-3 with examples

The effective nuclear charge is properly insulated by the inner s and p electrons when the cations have octet configuration or inert gas configuration (ns2np6) in their outer shells. As a result, their polarizing potential is lower than projected. As a result, the covalent nature of the ionic molecule containing these cations is reduced.

When there are electrons in the d-sublevel, however, the nuclear attraction is not sufficiently protected. As a result, cations having a pseudo-octet configuration, such as ns2np6nd10, have a lot of polarizing power.

As a result, main group metal compounds have a higher ionic content, whereas transition metal compounds have a higher covalent content.

The inert gas configuration (3s23p6) in the Ca2+ ion, for example, makes CaCl₂ more ionic. Because of the pseudo-inert gas state in the Zn2+  ion, the ZnCl₂ is more covalent (3s23p63d10).

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Conclusion

From the above article, we can conclude that the largest anion and the smallest metal ion should be most covalent, Lil is the most covalent. The most ionic anion and cation should be the smallest anion and largest cation, hence CsF should be the most ionic.

These are listed in order of increasing covalency.

NaF, NaCl, NaBr, and Nal are all sodium salts.

LiF, NaF, KF, RbF, and CsF are all types of halides.

When compared to anions, the cation is the same. The covalency increases as the size of the anion increases. As a result, the order will be: NaF< NaCl< NaBr< Nal.

In this case, the anion is the same as the cation. The higher the covalency, the smaller the cation. As a result, the order is CsF<RbF<KF<NaF<LiF. Related Links: