concept

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The stability of the lower oxidation state increases as we go down a group. This is due to an effect called inert pair effect. This effect is due to the poor shielding effect offered by the d-electrons and (if present) f-electrons.

Since they are poor shielders, they do not "shield" the outer electrons from the force of attraction exerted by the nucleus. This means that the outer electrons will be attracted stronger than expected. The two s-electrons cannot participate freely in reactions as they are the worst affected. Thus the most stable oxidation number comes down by two.

Puriyala naa sollu machan [4]

Well, saket. Sankara wanted us to explain what we have understood so that he can estimate upto what level he has understood. Else I could have easily given this link and told him to refer there http://en.wikipedia.org/wiki/Inert_Pair_Effect

oh, sorry for that, now i hav deleted that post.
by the way crystal field theory involves the interaction of degenerate d-orbitals with ligands so that they are no more degenerate and split into two different energy levels. these two energy levels decide many properties of coordination compounds like shape, magnetic nature etc.

Inert pair effect..(copied:P)

The term inert pair effect is often used in relation to the increasing stability of oxidation states that are 2 less than the group valency for the heavier elements of groups 13, 14, 15 and 16. The term "inert pair" was first proposed by Nevil Sidgwick in 1927.
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:
Al^I < Ga^I < In^I < Tl^I.

Al^I is Al⁺

The situation in groups 14, 15 and 16 is that the stability trend is similar going down the group, but for the heaviest members, e.g. lead, bismuth 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.

The high IP(2nd + 3rd) of gallium is explained by d-block contraction, and the higher IP(2nd + 3rd) of thallium relative to indium, has been explained by relativistic effects.
An alternative explanation of the inert pair effect by Drago in 1958 attributed the effect to low M-X bond enthalpies for the heavy p-block elements and the fact that it requires less energy to oxidize an element to a low oxidation state than to a higher oxidation state. This energy has to be supplied by ionic or covalent bonds, so if bonding to a particular element is weak, the high oxidation state may be inaccessible. Further work involving relativistic effects confirms this.In view of this it has been suggested that the term inert pair effect should be viewed as a description rather than as an explanation.

CFT is out of syllabus no , why you guys interested suddenly . read from J.D.Lee , it's fantastic. superb explanation.

y CFT out of syll ??

its in NCERT naa ?

are u sure its not there?

its a fantastic thing ... n one of the very rare topics which i love in chem [3]