Inert Pair Effect | p-block Chemistry | IIT Jee Mains & Advance, BITSAT | NEET, AIIMS and REE

The inert-pair effect refers to the empirical observation that the heavier elements of groups 13-17 often have oxidation states that are lower by 2 than the maximum predicted for their group.
For example, although an oxidation state of +3 is common for group 13 elements, the heaviest element in Group 13, thallium (Tl), is more likely to form compounds in which it has a +1 oxidation state.
The inert pair effect says that the ns2ns2 valence electrons of metallic elements, especially the 5s2and 6s2 pairs that follow the second and third row of transition metals, are less reactive than would be expected based on periodic trends such as effective nuclear charge, atomic sizes, and ionization energies. In, Tl, Sn, Pb, Sb, Bi, and, To some extent, Po do not always show their expected maximum oxidation states. Rather, sometimes they form compounds in which their oxidation states are 2 less than what would be expected.
The inert pair effect describes the preference of late p-block elements (elements of the 3rd to 6th main group, starting from the 4th period but getting really important for elements from the 6th period onward) to form ions whose oxidation state is 2 less than the group valency.
So much for the phenomenological part. But what's the reason for this preference? The 1s electrons of heavier elements have such high momenta that they move at speeds close to the speed of light which means relativistic corrections become important. This leads to an increase of the electron mass. Since it's known from the quantum mechanical calculations of the hydrogen atom that the electron mass is inversely proportional to the orbital radius, this results in a contraction of the 1s orbital. Now, this contraction of the 1s orbital leads to a decreased degree of shielding for the outer s electrons which in turn leads to a cascade of contractions of those outer s orbitals. The result of this relativistic contraction of the s orbitals is that the valence s electrons behave less like valence electrons and more like core electrons, i.e. they are less likely to take part in chemical reactions and they are harder to remove via ionization, because the s orbitals' decreased size lessens the orbital overlap with potential reaction partners' orbitals and leads to a lower energy. So, while lighter p-block elements (like AlAl) usually "give away" their s and p electrons when they form chemical compounds, heavier p-block elements (like Tl) tend to "give away" their p electrons but keep their s electrons. That's the reason why for example Al(III) is preferred over Al(I) but Tl(I) is preferred over Tl(III).
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