The correct order of C , N , O and F in terms of second ionisation potential is:

Chemistry · Periodicity of Elements and Periodic Properties

JEE Main 2026 — 24 January, Evening Shift — Question 58

The correct order of $\mathrm{C}$ , $\mathrm{N}$ , $\mathrm{O}$ and $\mathrm{F}$ in terms of second ionisation potential is:

Second ionization potential means removal of electron from $\mathrm{X^+}$ .

Electronic configurations after first ionization, we have

$\mathrm{C^+} = 1s^2 2s^2 2p^1$

$\mathrm{N^+} = 1s^2 2s^2 2p^2$

$\mathrm{O^+} = 1s^2 2s^2 2p^3$

$\mathrm{F^+} = 1s^2 2s^2 2p^4$

$\mathrm{O^+}$ has half-filled $2p^3$ configuration which is extra stable, so removing one electron requires maximum energy.

Across the period, effective nuclear charge increases, so ionization energy generally increases.

Thus overall increasing order is $\mathrm{C < N < F < O}$ .

Answer key and solution verified before publishing.

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Exam: JEE Main 2026
Paper: 24 January, Evening Shift
Subject: Chemistry
Chapter: Periodicity of Elements and Periodic Properties
Topic: Periodicity in Chemical Properties (Ionization Energy, Electron Affinity, Electronegativity)