Extra stability of electron pairs present in outermost s sub shell (ns) of post transition heavy metals available in p block of periodic table is called inert pair effect. Inert pair effect is observed with elements fourth, fifth, and sixth period under Gr-13, 14, 15 and 16 .Those elements are metal with higher atomic mass and electron releasing by nature. With increase of atomic number of elements of those specific groups, ns electrons become more and more chemically inactive. The inactiveness of ns electrons of those elements is considered as inert pair effect.
The inert pair effect is developed due to presence of (n-1) d electrons in penultimate shell. d electrons posses less shielding ability and support enhanced effective nuclear charge on valence electrons (ns2,np1-4). ns electrons being spherically symmetrical and closer to nucleus, experience more effective nuclear charge than np electrons. Therefore ns electrons are tightly held by nucleus and their ionization energy increases so much that it opposes release or hinder chemical interaction by those electron pair.
Inert pair effect is not active on 2nd, 3rd period elements under group 13, 14, 15 or 16 due to non availability of d electrons in penultimate shell. Inert pair effect is meaningless for group 17 elements due to their higher electro negativity character.
Inert pair effect increases along the group with increasing atomic number or shell number. It is due to decreasing shielding ability of inner d sub shell electrons. The shielding ability of electrons follows the order
ns > np > nd > nf and nd > (n+1)d > (n+2)d
Inert pair effect regulates the ionic size, Stability of oxidation states and oxidizing –reducing nature and other chemical properties of elements.
Except boron, Al, Ga , In and Tl of group 13 are all metals with general electronic configuration ns2np1.Those elements are available in both +1 and +3 oxidation states. The stable oxidation state of Al and Ga are stable with +3 oxidation state when In and Tl are less stable in +3 state and more stable in +1 oxidation state due to inert pair effect . The inert pair effect in those elements brings more stability for ns electron pair, enhances the ionization enthalpy of those electrons and restricts for chemical interaction.
Sn and Pb of group -14 are metals. Sn and Pb are available in +2 and +4 oxidation states. Sn is more stable in + 4 states than +2 states due to less prominent inert pair effect. Therefore Sn in +2 state (SnCl2) behave as a reducing agent. Pb is more stable in +2 oxidation state than + 4 oxidation state due to prominent inert pair effect. Therefore PbCl4 behave as an oxidizing agent but not SnCl4.
Sb and Bi of group-15 are metals. Those are available in +3 and +5 oxidation states. But both are stable in +3 oxidation states due to inert pair effect.
Te of group-16 is a metal. It is available in +4 and +6 oxidation states. The +4 oxidation state of Te is more stable than +6 oxidation state due to inert pair effect.
Inert pair effect is not observed with other elements of p block due to higher electro negativity character and nonmetallic behavior.