Question

Why is the electronegativity of indium higher than gallium?

Answer 1

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.