Physical Chemistry

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11 Feb 2010 12:03:48 IST
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inert pair effect...please explain with diagram....
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inert pair effect...please explain with diagram....

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ronak's Avatar

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11 Feb 2010 12:43:07 IST
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 Inert pair effect is the tendency of the outermost s electrons to remain nonionized or unshared in post-transition compounds. The term inert pair effect is often used in relation to the increasing stability ofoxidation states that are 2 less than the group valency for the heavier elements of groups 131415 and16. The term "inert pair" was first proposed by Nevil Sidgwick in 1927.[1]

As an example in group 13 the +1 oxidation state of Tl is the most stable and TlIII compounds are comparatively rare. The stability increases in the following sequence:[2]

AlI < GaI < InI < TlI.

The situation in groups 1415 and 16 is that the stability trend is similar going down the group, but for the heaviest members, e.g. leadbismuth and polonium both oxidation states are known.
The lower oxidation state in each of the elements in question has 2 valence electrons in s orbitals. On the face of it a simple explanation could be that the valence electrons in an s orbital are more tightly bound are of higher energy than electrons in p orbitals and therefore less likely to be involved in bonding. Unfortunately this explanation does not stand up. If the total ionization potentials(see below) of the 2 electrons in s orbitals (the 2d + 3d ionization potentials), are examined it can be seen that they increase in the sequence:

In < Al < Tl < Ga.

by seeing u must be know the source!

edison's Avatar

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2 Mar 2011 15:53:48 IST
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 The tendency of the outermost s electrons to remain non ionized or unshared in post-transition compounds is called inert pair effect. The term inert pair effect is often used to the increasing stability of oxidation states which is 2 less than the group valency for the heavier elements of groups especially 13, 14, 15 and 16. The term "inert pair" was first coined by Nevil Sidgwick in 1927.

Physical Features of Inert Pair Effect

An important known fact is that the compounds present in the lower oxidation state are ionic, whereas the compounds present in the higher oxidation state are covalent. An alternative explanation of the inert pair effect was given by Drago in 1958 which attributed to the effect to low M-X bond enthalpies for the heavy p-block elements and it requires less energy to oxidize an element to a low oxidation state in comparison to a higher oxidation state.


This energy is to be supplied by ionic or covalent bonds, and if bonding to a particular element is weak, then the high oxidation state may not be accessible. As a result of this it has been suggested that the term inert pair effect should be seen as a description rather than as an explanation. The time when it was proposed, there were no known compounds with the intermediate, having +2 oxidation states.

Steric Activity of the Lone Pair

The chemical inertness of the s electrons in the lower oxidation state is not always stay side by side to steric inertness. A simple example of steric activity is that of SnCl2 which is bent according to the VSEPR model. Some examples are present where the lone pair seems to be inactive. They are bismuth (III) iodide BiI3 and the BiI63− anion.


In both of these examples, the central Bi atom is octahedral combined having little or no distortion, which is in contravention to VSEPR theory. The steric activity of the lone pair has been assumed long time before due to the orbital having some p character that is the orbital is not spherical in symmetry. More recent theoretical work in this regard shows that this is not necessarily the case always.

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28 Sep 2012 20:06:42 IST
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inert pair effect...please explain with diagram....

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