JEE Main · 2025 · 28 Jan 2025 · Shift 1 · p-Block Elements

Among Al, In, Tl, Ge, Sn and Pb, first ionisation enthalpy is highest for germanium — a p-block element close to the metalloid/non-metal borderline, with a small, well-shielded core — and lowest for indium, whose $I{E}_1$ (about 558 kJ/mol) is actually below thallium's (about 589 kJ/mol) even though thallium shows the stronger inert-pair effect in its chemistry; ionisation energy and oxidation-state stability are governed by different factors. Germanium's most stable oxidation state is +4 (it can show +2, but +4 is thermodynamically favoured, unlike its heavier congener lead), while indium, despite the growing pull toward +1 lower down group 13, remains predominantly trivalent, with $I{n}^{3+}$ compounds far more common and stable than $I{n}^{+}$. The element with the highest first ionisation enthalpy (Ge) is most stable as +4, and the element with the lowest (In) is most stable as +3.