Second ionization potential (I₂) is the energy to remove the second electron.
After removing the first electron: C⁺ has 2p¹, N⁺ has 2p², O⁺ has 2p³ (half-filled, stable), F⁺ has 2p⁴.
C has lowest I₂ as 2p electrons are least tightly bound with lowest nuclear charge.
N has slightly higher I₂ than C due to increased nuclear charge.
F has highest I₂ due to highest effective nuclear charge.
O⁺ with half-filled 2p is relatively stable, making I₂ of O intermediate.
Order: C<N<F<O