JEE Main · 2026 · 08 Apr 2026 · Shift 2 · Classification of Elements and Periodicity

With $n=2$ the configurations belong to beryllium ($2s^2$), boron ($2s^{2}2p^1$), nitrogen ($2s^{2}2p^3$) and neon ($2s^{2}2p^6$). First ionization enthalpy broadly rises across the period, but boron lies below beryllium because removing boron's single $2p$ electron is easier than disturbing beryllium's filled $2s$ shell, while nitrogen's extra-stable half-filled $2p^3$ set raises its value sharply; the order is therefore boron $(800)<$ beryllium $(899)<$ nitrogen $(1402)<$ neon $(2080)\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. Hence $2s^2$ takes $899$, $2s^{2}2p^1$ takes $800$, $2s^{2}2p^3$ takes $1402$ and $2s^{2}2p^6$ takes $2080\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$.