Ionization enthalpy comparison: O vs F. The 1st IE of O is LESS than that of N (because O has paired 2p electron easier to remove). Similarly:

Anomaly: 1st IE of O < N due to paired electron destabilization in O (2p⁴ vs N's stable 2p³ half-filled). Same in 3rd row: S (3p⁴) < P (3p³). Similar in row 4: Se < As, Te < Sb. General: half-filled subshells are especially stable.