The correct order of second ionization potential of carbon. nitrogen. oxygen and fluorine is

(for second ionization potential,IE2 ).C−1s22s2p2          C+−1s22s2p1N−1s22s2p3          N+−1s22s2p2O−1s22s2p4         O+−1s22s2p3F−1s22s2p3          F−−1s22s2p4As for IE2 the electron in all the cases is to be removed from 2p orbital so it must follow the orderC < N < 0 < F (i.e. increase across a period). But in case of 'O' . the 2p-orbital is half-filled and is more stable as compared to others. So the orderbecomes : C < N < F < O .