JEE Main · 2026 · 04 Apr 2026 · Shift 2 · Classification of Elements and Periodicity

Each successive ionisation needs more energy than the one before it, because the next electron is pulled from an ever more positive ion of smaller radius and higher effective nuclear charge; the second ionisation enthalpy must therefore exceed the first ($+737\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$) and stay endothermic (positive). For magnesium ($1s^{2}2s^{2}2p^{6}3s^2$) both the first and second electrons leave the same $3s$ sub-shell, so there is no exceptional jump and the second value lies only moderately above the first, close to twice it. The only positive estimate larger than $737$ among the choices is the most probable value, and the second ionisation enthalpy of Mg is about $+1450\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$.