FAJAN RULE PDF

Jump to navigation Jump to search The Fajans—Paneth—Hahn Law also Fajans precipitation rule, Fajans-Peneth precipitation and adsorption rule, Hahn law of precipitation and adsorption, Fajans Law , in chemistry , is a rule governing how a small amount of one substance tracer is carried down to a precipitate of another substance present in much larger amount carrier by coprecipitation or adsorption. The rule states that: [1] [2] the lower the solubility of the tracer cation with the anion of the carrier, the greater the amount of the tracer carried down by the carrier through co-precipitates or adsorption; when the tracer substance forms a mixed crystal , then the separation by co-precipitation only weakly depends on the conditions; the tracer will adsorb on the surface of the carrier precipitate if the precipitate acquired a surface charge opposite to that of the carrier ions in the solution; and then the separation strongly depends on the condition of precipitation. The amount carried down is strongly affected by presence of complexing species regardless if it occurs by formation of mixed crystals or adsorption. The Fajans-Paneth-Hahn law is essential for understanding the behaviour of minute amounts of substances e. Note that the tracer is precipitated from the solution even when present at concentration far below its solubility limit. The law is also applied for separation of tracer substances by co-precipitation.

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This phenomenon of distorting electron cloud of the negative ion by the positive ion is called Polarization. Now, what inference can you draw from this? The negative charge will become less negative as some of the electron cloud has migrated towards positive ion and positive ion will also become less positive as it now has more electrons then it had before.

An ion of highly electropositive element will draw electron towards it with greater force and hence will create more polarization. As the charges will decrease on each ion, it will affect the degree of ionic or covalent character too. If polarization is less, the electron cloud will remain on respective ions. In this case, the molecule will retain ionic nature. However, if polarization is more, and the charges become less, the bond will become more covalent.

Small size of the cation The small cations have high electron density and hence tend to distort or polarize the electron cloud of the anion to a greater extent. Therefore, the compounds having small cations will have a more covalent character. Large size of anion Polarization increases with an increase in the size of the anion. This is because the electron cloud on the bigger anion will be held less firmly by its nucleus and therefore will be more easily deformed towards cation.

Thus, larger anion means higher polarisability and hence more covalent character. Anions such as I-, Br-, S2-, etc can easily induce covalent character. Take for example lithium compounds. The covalent character of lithium compounds follows the following order. Large charge on cation or anion Larger the charge on cation greater is polarizing power and larger the charge on anion greater is its tendency to get polarized.

Electronic configuration of the cation Cations with 18 electrons shell configuration cause greater polarization than the cations with 8 electrons shell configuration with the same size and charge.

This is due to the fact that in the case of 18 electrons shell ion, there are 10d electrons in addition to eight s and p— electrons. The d-electrons do not shield the nuclear charge effectively and therefore, they have increased effective nuclear charge. Consequently, these ions behave as though they are under the influence of greater charge and polarize the anion to a greater extent.

Therefore, they will have a more covalent character. By analyzing the points given above, you can easily find out whether the given molecule is ionic or covalent in nature.

We can analyze the molecule on the basis of the above rules and tell whether the molecule is ionic or covalent. All the Best! As we all know, practice is the key to success. Therefore, boost your preparation by starting your practice now.

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Fajans–Paneth–Hahn Law

This phenomenon of distorting electron cloud of the negative ion by the positive ion is called Polarization. Now, what inference can you draw from this? The negative charge will become less negative as some of the electron cloud has migrated towards positive ion and positive ion will also become less positive as it now has more electrons then it had before. An ion of highly electropositive element will draw electron towards it with greater force and hence will create more polarization. As the charges will decrease on each ion, it will affect the degree of ionic or covalent character too. If polarization is less, the electron cloud will remain on respective ions. In this case, the molecule will retain ionic nature.

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Fajan’s rule

That positive charge then exerts an attractive force on the electron cloud of the other ion, which has accepted the electrons from the aluminium or other positive ion. Two contrasting examples can illustrate the variation in effects. In the case of aluminium iodide an ionic bond with much covalent character is present. The large charge pulls on the electron cloud of the iodines.

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Fajans' Rules for Chemical Bonds

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