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

(A) C < N < F < O

(B) C < F < N < O

(C) F < N < C < O

(D) C < O < N < F

Correct Answer: C < N < F < O

Explanation

Second ionisation potential is the energy required to remove an electron from a monovalent cation.

After first ionisation:

C⁺ : 1s² 2s² 2p¹ N⁺ : 1s² 2s² 2p² O⁺ : 1s² 2s² 2p³ F⁺ : 1s² 2s² 2p⁴

O⁺ has a half-filled 2p³ configuration, which is extra stable. Hence, removing an electron from O⁺ requires maximum energy.

F⁺ has one paired electron in 2p orbital, so removal is easier than O⁺ but harder than N⁺.

C⁺ has the least nuclear attraction among these cations, so its second ionisation potential is the lowest.

Therefore, the correct increasing order of second ionisation potential is:

C < N < F < O

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Updated for JEE Main 2026: This PYQ is important for JEE Mains, JEE Advanced and other competitive exams. Practice more questions from this chapter.

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