What is Inert pair effect ? Explain in detail with examples... :]

Group 13 elements have ns2np1 electronic configuration. Hence they would be expected to be trivalent. In most of their compounds this is the case, however for the heavy elements lower oxidation states are more stable. This is explained by the s electrons remaining paired and not participating in bond formation. This inertness of s-subshell electrons towards the bond formation is called inert pair effect. This happens because the s orbitals are held closer to the nucleus, therefore the electrons present in s orbitals are held strongly by nucleus because of large electrostatic forces. Hence the energy required to unpair the s-electrons is high because of which they remain paired. 

for example:

1) In 13th group, thallium can exhibit +1 and+3 oxidation states but it is stable in +1 oxidation state only due to inert pair effect

2) In 14th group , lead shows both +2 and +4 oxidation states but it is stable in +2 oxidation state due to inert pair effect.

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