JEE Main · 2026 · 24 Jan 2026 · Shift 2 · Classification of Elements and Periodicity

The second ionisation removes an electron from the singly-charged cation, so compare ${\mathrm{C}}^{+}(2p^1)$, ${\mathrm{N}}^{+}(2p^2)$, ${\mathrm{O}}^{+}(2p^3)$ and ${\mathrm{F}}^{+}(2p^4)$. ${\mathrm{O}}^{+}$ has the especially stable half-filled $2p^3$ configuration, so its electron is the hardest to remove, giving oxygen the largest second ionisation energy, just above fluorine; carbon's is the smallest and nitrogen's lies between carbon and fluorine. The increasing order is $\mathrm{C}<\mathrm{N}<\mathrm{F}<\mathrm{O}$.