Fajan's Rule- Covalent Character in Ionic Bond, rules, factors and application

Kazmierz Fajan

Kazimierz Fajans (Kasimir Fajans in many American publications; 27 May 1887 – 18 May 1975) was a Polish American physical chemist of Polish-Jewish origin, a pioneer in the science of radioactivity and the discoverer of chemical element protactinium.

Fajan’s rule

This rule is used hey to find the covalent character in ionic bond

No bond is completely ionic or covalent

The electron cloud is deformed due to the polarization of anion occurs by the cation. So, there is sharing of electron but that is a property of covalent bonds only there for hey ionic bonds also have a little bit of covalent character in them

Factors and The Rule

There are 2 main factors that the determine the covalent character in an ionic bond

Cation: polarising power

Anion: polarization

Fajan's Rule states that if the polarising power of cation is more and polarization of anion is more then it has a higher covalent character.

Factors on which the polarizing power and the polarization depend

        1. Cation: Polarising Power

a.      Size of cation à small à polarizing power more

b.      Charge of cation à more à polarizing power more

c.      Pseudo Noble gas configuration  à atom has a pseudo noble gas configuration then it has a higher polarizing power so, higher covalent character.

 

2.    2. Anion: Polarization

a.      size of anion à large à Polarization more

b.      charge on anion à more à Polarization more

 

Q. Arrange NaCl, NaBr and NaI in ascending order of covalent character.

A. Since the Cation is same, we can arrange it in ascending order of polarization of anion. Size of elements increases down the group as number of shells increases. So Iodine has largest size and chlorine is the smallest. As size of anion à large à Polarization more

So order would be NaCl < NaBr < NaI

 

Application of finding Covalent Character:

The covalent character of a compound an be used to compare the solubility, melting and boiling points and the color imparting nature.

1.  Solubility is inversely proportional to Covalent character. So, the more the covalent character the les soluble an ionic compound is in a polar solvent.

 2.   M.P. and B.P. are also inversely proportional to Covalent character.

 3.  The Color imparting nature is directly proportional to covalent character.  This means If a compound has more covalent character it will impart more color and be darker in terms of color.

 

Another application, is the answer to the question: Why SnCl₄ is volatile and SnCl₂ is solid?

As Charge of cation à more à polarizing power more à Covalent character more. So, SnCl₄ is more covalent as in SnCl₄ Sn shows charge of 4+ and SnCl₂ is more ionic and it shows a charge of 2+. As ionic compounds are solid and covalent compounds are mostly liquid, therefore, SnCl₄ is volatile and SnCl₂ is solid.