Examples of Fajans’ Rule

Ionic and covalent bonding are two extreme types of chemical bonds; however, practically all bonds are intermediate, which can be explained by polarising (or deforming) the shape of ions. Even though most metal–nonmetal pairings exhibit the features of ionic compounds, there are a few exceptions that occur when the anion’s outermost electrons are strongly attracted to the cation, and the bond develops a significant degree of covalency; that is, the anion’s electron density is bent toward the cation.

Polarisation is defined as a deviation from the ideal anion’s spherical shape. A polarisable atom or ion has orbitals with similar energies; large, heavy atoms and ions tend to be more polarisable. The capacity of an atom to be deformed by an electric field (such as that of a neighbouring ion) is called polarisability. When unfilled atomic orbitals are near the highest-energy filled orbitals, an atom or ion (most typically, an anion) is highly polarisable as its electron distribution can be easily altered. 

Fajans’ Rule

Kasimir Fajans, a chemist, devised the following principles to summarise the information:

Factors that favour ion polarisation and thus an increase in covalency include:

1. If a cation is tiny and has a high positive charge, it will be more polarising.

2. If an anion is big and has a negative charge, it will be easier to polarise.

3. Covalency is favoured by large charges on either ion or both ions as a large charge causes more polarisation.

4. Cations that do not have a noble gas configuration encourage polarisation.

The influence of one ion on the other determines the sort of bond between A+ and B–. The positive ion attracts the electrons of the negative ion while repelling the nucleus, causing the negative ion to deform or polarise. The negative ion will polarise the positive ion as well. Still, because anions are typically large and cations are small, the influence of a large ion on a small one will be much less evident. 

If the degree of polarisation is more minor, the connection is still predominantly ionic. When polarisation is high, electrons flow from the negative ion to the positive, resulting in a large concentration of electrons between the two nuclei and a high degree of covalent bonding.

Examples of Fajans’ rule

Example 1: Which among F– ion or an I– ion would be more polarisable?

Answer: The fact that polarisable anions are often big and strongly charged can be used to our advantage. An F– ion is a single-charged, tiny particle. An I– ion has the same charge as a proton but is much larger. As a result, an I- ion is more likely to be polarisable.

Example 2: Compare sodium oxides and sulphides with copper oxides and sulphides (I). Even though these cations have a similar radius, sodium oxide and sodium sulphide react with water like conventional ionic compounds, but copper(I) oxide and copper(I) sulphide are almost entirely insoluble in water. Explain.

Answer: By Fajans’ rule, the cation in the non-noble gas arrangement has a higher tendency toward covalency. According to the Pauling concept of electronegativity, a difference in electronegativity of 2.5 for sodium oxide indicates largely ionic bonding. In contrast, a difference of 1.5 for copper(I) oxide shows a solid covalent component to the bonding.

Example 3: KCl has a greater melting point than AgCl, although the crystal radii of Ag+ and K+ ions are nearly identical. Explain.

Answer: When two compounds have similar melting points, the one with the lower melting point is thought to have a lower degree of ionic character. As both are chlorides in this example, the anion remains the same. The cation must be the decisive factor. (If the anions were different, the answer might be influenced by the anion variation.) The electronic arrangements of the cations differ significantly in this case. 4d10, K+=[Ar] and Ag+ =[Kr]. This requires a comparison between a noble gas core and a pseudo noble gas core, with the pseudo noble gas being more polarising.

Example 4: Which compound should theoretically be the most ionic and covalent among the metal halides?

Answer: Technically, the smallest metal ion and the largest anion should be the most covalent. As a result, LiI has the highest covalent value. The most ionic cation and anion should be the largest and smallest. As a result, CsF should be the most ionic compound.

Example 5: Arrange the following items in ascending order of covalency:

• Sodium fluoride (NaF), sodium chloride (NaCl), sodium bromide (NaBr), sodium iod

• LiF, NaF, KF, RbF, CsF

Answer:

1. We will compare the anions as the cation is the same. The greater the size of the anions, the higher the covalency. Hence, the ascending order of covalency is: NaF <NaCl< NaBr< NaI

2. As the anion is the same in both cases, the comparison is between the cations. Smaller the cation, the higher the covalency. As a result, the order becomes: CsF

Conclusion

This article explains examples on Fajan’s rule. Based on certain factors included in Fajans’ rule, many compounds can be differentiated based on the nature of their bond. Compounds are never completely 100% ionic or covalent. This rule helps in identifying whether the compounds are predominantly ionic or covalent.