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Q.
Which of the following is arranged in order of increasing second ionisation energy?
Classification of Elements and Periodicity in Properties
Solution:
Ionisation energy increases from left to right in a period. But oxygen after removal of first electron gets stable half-filled electronic configuration $(2s^22p^3) $ and thus its second ionisation energy is higher than that of fluorine. Thus, order of $ 2^{nd }$ ionisation energy is $ C < \,N < \,F < \,O.$