Why is the electronegativity of indium greater than that of thallium?
One possibility which I had assumed was the poor shielding effect by the d and f orbitals which leads to an increase in the effective nuclear charge and hence the anomaly in the 13th group. But with thallium having a f orbital (which indium doesn't) which has a much poorer shielding ability than 'd', why does indium have more electronegativity?
Going down any group of the Periodic Table the general trend is that electronegativity decreases. The relative electronegativities of In and Tl are simply a reflection of this fact. That's the end of the story; you don't need to bring in anything more complicated.
You are correct that the 4f electrons are poorly shielding, and thus the 6p elements (Tl, Pb, Bi, Po, At, Rn) would likely be more electronegative than otherwise expected. But there's no reason why this should make them so electronegative so as to reverse the general trend. None of these elements are more electronegative than their 5p counterparts.
Stuff like these are one of few things in Inorganic Chemistry that have an explanation in High School(unless you are willing to touch research material). Getting to the point this is a result of poor shielding by electrons in the f orbitals. Shielding effect/capacity decreases in the order s>p>d>f.After Indium the electrons are put in s,d,f orbitals and the latter two have little effect in shielding, also considering that the size difference between the two atoms is very small the outer electrons of Thallium are attracted more strongly by the nucleus than in Indium and hence the electronegativity is higher in the former.
