Inert Pair Effect

WHAT IS IT?

The elements after Au (gold) in the periodic table have as their most stable oxidation state one which is 2 less than the group valency. Although the common oxidation state for elements in group 14 is +4, most elements in the group can also exist in oxidation state of +2. Thus, Pb has as its most stable oxidation state the Pb(II) state, although Pb is in group 14. This is referred to as the ‘inert pair’, and is thought to be due to increased electronegativity caused by relativistic effects. 

REASON

The effect occurs when electrons are pulled closer to the nucleus, making them more stable and more difficult to ionize. In heavy atoms, such as those of Sn, Sb, Tl, Pb and Bi, some outer-shell electrons are not as well shielded as those in the inner core.  They are therefore sucked into the inner core of electrons and thus become inert. These metallic elements do not always show their maximum oxidation states, rather they form compounds where the oxidation state is 2 less than the expected group vacancy. 

The inert pair effect is obvious from the chemistry of the 13 and 14 group elements and beyond. The lighter elements of these groups tend to have an oxidation state of 4+, whereas the heavier elements prefer the 2+ state. Therefore, it is easy to convert  Sn(II) compounds to Sn(IV) and Pb(IV) compounds to  that of Pb(II). It can be established by the  fact that it is   possible to reduce a solution of Iodine to iodide by using a Sn(II)chloride solution, wherein, the Sn(II) ions are oxidized to Sn(IV) ions. In the contrary, Pb(IV) ions in solution are known as good oxidizing agents. For instance, PbO is much more stable than PbO2 which decomposes readily to PbO.