ChemistryJEE Main 24 Jan 2026, Shift 2·Q11·MCQ·Moderate

Classification & Periodicity · Second ionisation enthalpy trend

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

(1)F $<$ N $<$ C $<$ O
(2)C $<$ O $<$ N $<$ F
(3)C $<$ N $<$ F $<$ O
(4)C $<$ F $<$ N $<$ O

We corrected a printing error in the original paper

Answer corrected; the source key is (2) — second ionisation enthalpies are C 2353, N 2856, F 3374, O 3388 kJ mol $^{-1}$ ( $\ce{O+}$ loses an electron from the stable half-filled 2p $^3$ ), so C $<$ N $<$ F $<$ O, option (3); the source's own solution also gives O $>$ F $>$ N $>$ C.

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See the full 24 Jan 2026, Shift 2 paper