The trend of attaining covalent character by ionic compounds as a consequence of polarisation may be generalized in terms of Fajan’s rule. According to this. Fajans’ Rule for the prediction of relative nonpolar character. Electrostatic forces in a crystal Learn Fajans’ Rule by Disclosing Covalent Characteristics in Ionic. Fajan Rule: Greater is the polarization, greater is the covalent character. | Online Chemistry tutorial IIT, CBSE Chemistry, ICSE Chemistry, engineering and.

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The fajand seen in the variation of MP and BP for the dihalogens and binary interhalogens can be attributed to the increase in the London dispersion forces of attraction between the molecules.

Fajans’ Rule – Disclosing Covalent Characteristics in Ionic Bonds

Fajans’ rule states that a compound with low positive charge, large cation and small anion has ionic bond where as a compound with high positive charge, small cation and large anion are covalently bonded. Here the significant difference between the cations is in their electronic configurations. When the melting points of two compounds are compared, the one having the lower melting point is assumed to have the smaller degree of ionic character.

They are derived from momentary oscillations of electron charge in atoms and hence are present between all particles atoms, ions and molecules. A cation having inert gas like configuration has less rulrs power in comparison to cation having pseudo-inert gas like configuration.

They can be summarized in the following table:.

bond – What is Fajans rule? – Chemistry Stack Exchange

Intermolecular forces are the attractive forces between molecules without which all substances would be gases. Both a fajjans b. Views Read Edit View history.

For high charge, small cation will have more polarizing power. In this article, we are going to discuss covalent characteristics in ionic bonds.


In reality, every bond has some covalent as well as ionic characteristics. Because if this electron cloud of anion is more diffused.

CHEM Polarizability

How do I ask homework questions on Chemistry Stack Exchange? From an MO perspective, the orbital overlap disperses the charge on each ion and so weakens the electrovalent forces throughout the solid, this can be used to explain the trend seen for the melting points of lithium halides.

The deciding factor must be the cation. If two cations have the same size and charge, then the one with 18 electrons in the outermost shell has greater polarising power than the other with 8 electrons in the outermost shell. Let us consider AlI 3 ; this is an ionic bond which was formed by transfer of electrons. On average the electron cloud for molecules can be considered to be spherical in shape.

The various types of these interactions span large differences in energy and for the halogens and interhalogens are generally quite small. It is found that the greater the possibility of polarization, the lower is the melting point and heat of sublimation and the greater is the solubility in non-polar solvents. Retrieved from ” https: When two molecules are close together, the instantaneous dipole of one molecule can induce a dipole in the second molecule.

The polarising power of the cation and the polarisability of the anion and hence the formation of covalent bond depends on:. This explains why for the common halides, iodides, are the most covalent in nature I – pm.

Leave a Reply Cancel reply Your email address will not be published. In general they increase with increasing atomic number.

As the electron cloud of the iodine nears the aluminium atom, the negative charge of the electron cloud “cancels” out the positive charge of the aluminium cation. We see that a similar situation occurs, but instead of iodine we now have fluorine, a relatively small highly electronegative atom.


This page was last edited on 18 Novemberat In this case, the aluminium ion’s charge will “tug” on the electron cloud of iodine, drawing it closer to itself. For help asking a good homework question, see: Larger the charge on the cation, greater is its polarising power.

Chemistry Chemistry Articles Fajans Rule. Since there are no free ions in covalent compounds to conduct electricity, they are bad conductors of electricity. Practise s of questions mapped to your syllabus. If the anions were different, then the answer could be affected by the variation of the anion.

Note that Fajans’ Rules have been largely displaced by Pauling’s approach using electronegtivites. To picture how this occurs, compare the situation 1 where the electrons are evenly distributed and then consider 2 an instantaneous dipole that would arise from an uneven distribution of electrons on one side of the nucleus. This results in synchronised motion of the electrons and an attraction between them.

Gas To Solid Is Called. When a cation approaches an anion, the electron cloud of the anion is attracted towards a cation and hence gets distorted.

This makes the anion easily polarizable.

What is Fajans rule? Hence covalent character increases. The ionic character arises from the polarizability and polarizing effects of H and I. Now, if we consider the iodine atom, we see that it is relatively large and thus the outer shell electrons fwjans relatively well shielded from the nuclear charge. Your email address will not be published.