JEE Main · 2024 · 29 Jan 2024 · Shift 2 · Classification of Elements and Periodicity

First ionization enthalpy generally rises across a period and falls down a group. Between Al and Si (period 3) and C and N (period 2), the period-2 elements have the higher values, and within period 2, nitrogen has a stable, exactly half-filled $2p^3$ configuration. Removing an electron from this symmetric arrangement costs extra energy, pushing nitrogen's ionization enthalpy above what the simple periodic trend would predict — above even the period-3 elements listed here.

The order is $\mathrm{Al}<\mathrm{Si}<\mathrm{C}<\mathrm{N}$, and nitrogen has the highest first ionization enthalpy of the four.